Honda-SOHC

SOHC.co.uk Forums => CB750 => Topic started by: cbxman on April 26, 2022, 09:18:23 PM

Title: CB750 K1 Valve Timing Question
Post by: cbxman on April 26, 2022, 09:18:23 PM
Hi guys,

I have I have had my CB750 K1 since October 2021 and have been trying to tune it so that it idles and rides nicely….but it won’t.   I can get it to idle well ,or pick up on and off the throttle, but not both.  I am looking for your help……

I have checked the timing.  TDC mark is where it should be.

The advancer had sloppy springs, so I cut the springs, now it advances nicely to full at 2500 rpm without jumping about under the strobe.  (As recommended by HondaMan on .net)

Compression 160 psi on all within a pound or so of each other.

Sparks well in excess of 20kV on new TEK coils, resistor caps and non-resistor plugs. (NGK D8EA and ND X24ES-U).  Dyna S fitted…..same issues with the points plate.

Carbs stripped numerous times.

It had a Keyster kit installed, so I have re-fitted it with Keihin jets #40 and #120, with the original Keihin needles.  The only non-keihin parts are the float valves and the Needle Jets, which had been damaged somehow by a PO.

I checked the float heights out at 26mm, but I thought I would also check the fuel levels with the clear tube method and found that set at 26mm, the fuel level was really low.  Reading about this, I have found that the fuel level should be 1-2 mm lower than the bowl rim, so I have adjusted the floats to give me the necessary level.  This ended up with the floats being set at 22mm when measured the usual way.  I think these floats must have come from another bike as this early K1 should have brass floats and there are two distinctly different pairs of plastic floats in there…ho hum.

I Have changed the slow jets  to #35, #38 and the stock #40.  Mains #110, #115 and the stock #120. I have tried most combinations of those and with different needle heights.  Some just do not work well, but some combinations will work just like mentioned above.  Hard work changing needle heights!!  I can take carbs out in minutes now!

I had run out of ideas, so I had another think…. maybe I would check the valve timing.  Rather than removing the engine for a look see, I would try to measure what the cam is doing….and so ….entered the dark world of timing diagrams….EEK!

The WSM states Intake opens at 5˚ BTDC and closes at 30˚ ABDC.  Exhaust  open at 35˚ BBDC and closes at 5˚ ATDC.  I understand those, but I wondered how to measure them.

I used a degree wheel and set it up to TDC, but for convenience I used cylinder 4 as it is closer to the degree wheel.  I used dial gauges on the tappet screws to measure the closing and opening of valves.  The numbers I gathered made no sense to me, so I’m seeking the truth from this august body of SOHCers.

This is a diagram which I thought would help me figure out whether the cam timing is right or not.  I’m even more confused now.  There must be a procedure to take the book numbers and convert them to reality, but I can’t seem to figure it out.

Of course there are two strokes not shown (compression and power) hence why the valves are so close and out of their natural order, but I think it is simpler to present it this way?

Cheers,

Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on April 26, 2022, 09:34:38 PM
There may be a little information missing from original manual, in that the cam timing specified is listed as "at 1mm lift" in later publication.
It also gives figures for 0 lift in later manual, but those raw cam degrees look nothing like the ones you've stated.

Hopefully that may give you more to go on.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on April 26, 2022, 09:54:34 PM
There may be a little information missing from original manual, in that the cam timing specified is listed as "at 1mm lift" in later publication.
It also gives figures for 0 lift in later manual, but those raw cam degrees look nothing like the ones you've stated.

Hopefully that may give you more to go on.

Thank you K2-K6 for your quick response.  I have a genuine Honda WSM dated 1977 covering up to the F model, and have quickly looked through it,and only found the book figures in the spec section at the front.  I'll have another look.  Maybe you are referring to a completely different source?

Would 1mm lift make such a difference?  Would that be on the cam lobe, or the tappet position?

I must be measuring something really wrong to get such differing values to the book.....

I'm thinking it might be easier to remove the engine and take the cam cover off and have a look.

Cheers,
Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on April 26, 2022, 10:25:39 PM
I've a first iteration 1969 manual, also a last 1978 in which the two sets of information come from, Honda genuine manual.

No mention I can immediately find of where to measure (camshaft are often given timing criteria at a specific lift though as matter of routine)  I'm guessing that the original just omitted this detail.

The later manual gives timing for both, and so can be compared directly as follows;- intake opens @1mm lift 5 BTDC & 56 BTDC @ at 0mm lift. Quite a difference and likely to be causing your initial puzzlement.

It follows all the others with dual readings too to check either way (beginning of cam ramp and @1mm) and looks distinctly odd at those numbers if you compare to "normal cam protocol.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on April 26, 2022, 10:42:06 PM
I've a first iteration 1969 manual, also a last 1978 in which the two sets of information come from, Honda genuine manual.

No mention I can immediately find of where to measure (camshaft are often given timing criteria at a specific lift though as matter of routine)  I'm guessing that the original just omitted this detail.

The later manual gives timing for both, and so can be compared directly as follows;- intake opens @1mm lift 5 BTDC & 56 BTDC @ at 0mm lift. Quite a difference and likely to be causing your initial puzzlement.

It follows all the others with dual readings too to check either way (beginning of cam ramp and @1mm) and looks distinctly odd at those numbers if you compare to "normal cam protocol.

Interesting. I’ll take a closer look in the wsm.

The camshaft installation instructions are pretty clear, so unlikely to be timed incorrectly.  I don’t suppose that many people look to do this type of check. The tuners will of course be dealing with non standard cams anyway..  the engine is quiet when the idle is set right, so I don’t think it is knackered.
Cheers,
Jerry
I’ll
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on April 26, 2022, 11:11:16 PM
If it's the K1 from Farnham, I was looking at that and got stuck, literally, when we couldn't turn the airscrew to adjust idle mixture.

Sounded one on the nicest 750 I've heard for a while when running, appears to be in very good condition mechanically.

Thinking further about cam timing, think convention has in imperial given timing at 40thou lift (39.37mm) as recognized flow is considered from that opening to understand cam performance. 

Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: Lobo on April 27, 2022, 01:25:51 AM
This is all too technical for me, but having recently had to buy a ‘new’ OHC for my K2, have become aware that there were many Honda tweaks to the lobe heights and timings over the years, and not necessarily accompanied by a part # change.

Whether or not this is relevant I don’t know….
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on April 27, 2022, 09:52:15 AM
As long as you're using the numbers with the lift set as they are intended, then it'll serve well to establish if cam is correctly timed.

The reason for the difference is that the @ 1mm numbers describe in reality what performance (in terms of rpm, powerband etc) the cam is going to give, with the absolute (start and finish of approach ramps) showing details of peripheral geometry more related to how much acceleration load the valve gear can realistically sustain, plus cylinder leak down (below that 1mm there's not much absolute flow) outright compression, cylinder efficiency directed toward reducing compression resistance etc.

For example, a lighter valve train can sustain shorter ramps as it's less likely to give valve float (valves bouncing in excess of cam lobe peak) and more a feature/advantage of 4 valve heads and engines like Ducati desmodromic actuators in using shorter more staccato geometry.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on April 27, 2022, 07:12:28 PM
Nigel,

Really helpful. Thank you.

So if I set the valve lash to 1mm and then measure from the valve stem as before, that should set the threshold I think you are describing.

You're right about the bike it was Andrew's. The stiff airscrews were due to the ill fitting Keyster kit which I think enlarged the threads.  I have added some small 'O' rings to the airscrews as the original screws are quite sloppy in the carb.It does seem to have evened up the action of the airscrews but that wasn't going to fix it.

I'm pretty sure that the engine has been removed for some work, as there are witness marks on the frame, hence wondering whether the cam was put in a tooth out or something.

I have looked in the WSM and there are variations in the valve timing, but not by much.
K1, 2, 3, 4 has Inlet opens 5˚ BTDC, closes 30˚ ABDC, exhaust opens 35˚ BBDC and closes 5˚ ATDC.
K5   supplement does not state the timing
F    has as above order 5˚, 35˚, 35˚, 5˚
K6   supplement does not state the timing
K7  has as above order  0˚, 40˚,40˚, 0˚
F2  has as above order  5˚, 40˚, 40˚,5˚
K8  has as above order  0˚, 40˚,40˚, 0˚
F3  has as above order  5˚, 40˚, 40˚,5˚

The 'tuners' here would probably make some sense of it.

All to do with emissions I suppose, but they keep changing their minds!

Cheers,
Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on April 27, 2022, 08:17:10 PM
"So if I set the valve lash to 1mm and then measure from the valve stem as before, that should set the threshold I think you are describing."

Yes, I see that as correct as the dial guage will pick up immediately the 1mm gap is closed to give you the book spec stated. Hopefully you'll see a conformance (or at least valid statement) of if it's right or wrong without having to remove the motor. 

The timings usually vary for torque point in rev range generally.  The F2/F3 (same motor) have raised red line, bigger valves, stronger valve springs, shorter final drive gearing than the others.

Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on April 28, 2022, 12:41:34 PM
Hi Folks,

I have set a lash of 1mm on the intake and exhaust valves.  I used a dial gauge, but I am aware that there may be deviation due to the arc of the rocker and the end of the tappet screw's flatness (gauge's nose is not long enough to reach the spring top). So I expect there to be some slight mis-measurement.  The ramps can be quite sharp so there maybe a tiny a discrepancy in setting lash might give a significant alteration of the measured open time.  This is what I think may be happening in this chart.

The opening ramp and closing ramps are likely different rates so the variation from stock maybe due to the lash again.

I may run another test, but with 0.75mm lash and see what that looks like.

What do you think?

Cheers,
Jerry

Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on April 28, 2022, 01:38:45 PM
As I understand it the 1mm is at valve rather than anywhere else, so direct onto valve spring cap.

The reason it's usual to work there is that the valve is not deemed to flow enough volume to influence peak power characteristics until it passes that point. It's much more common on USA V8 when they routinely choose camshaft timing to consider it this way too as they have the option of changing rocker ratios in addition.

The ramps do vary, but for different reasons. This is used to soften or shorten the abruptness of the whole valve train to stay within durability limits on road engines (often compromised on race for pure power) also it can affect efficiency as follows;- the K7 to F2  shift the absolute degree for exhaust more the the 1mm setting, this can either promote flow starting at the margins or delay it to reduce the active compression ratio and make the motor softer with less pumping loss (bleeds just a little of the absolute compression away and reduces running resistance) to help with economy.

That last bit about bleeding out compression is what they do in "Atkinson" cycle engines, leaving the intake valve open too long rejects some of the intake mixture back out the inlet to lower the final compression at low speeds. Toyota do this on the Prius to get soft starting, quiet running, increased efficiency when used in combination with electric torque.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on April 28, 2022, 07:29:21 PM
Nigel,
Thank you for your continuing insight into this.

I have run some readings with the lash set to 1.25mm.

I will try to bracket these readings with one at 0.75mm to see if I can find a trend.

Cheers,
Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on April 30, 2022, 06:11:51 PM
Nigel,

Well.  I have learned a few things!!.   Measuring valve timings requires very accurate Lash adjustment.  Setting the lash to exactly 1mm took quite some time.

Testing Cylinder #4 at the start was fortuitous, because if I had started with cylinder #1, I would have found that the timing was spot on and started scratching my head again and looking elsewhere.  However after checking cylinder #3 and finding that was spot on too, I can now say that cylinder #4 definitely has a problem.

I have decided to remove the engine and have a look.

I expect it will be expensive, whatever I find... :-[

Thanks for your help

Cheers,
Jerry

Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on April 30, 2022, 09:52:05 PM
It'll be interesting to hear what you find, strange indeed to get a difference there.

Speculative, but one of the two prominent owner induced problems with a 750 (final drive chain break to damage cases, oil supply to head area) may have given cause for someone in the past to "repair" it.  Could only guess at the cam being refinished if damaged to alter just one set of lobes.

I saw it running with the two previous owners (UKpete, Andrew) and don't think it's been worked on in the UK, perhaps much earlier in it's life previous to import.

Hopefully your investigation will get to the bottom of it.

Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: Oddjob on May 01, 2022, 04:37:35 PM
Could you not try another cam to see if the results change?
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 01, 2022, 08:31:13 PM
Hello Nigel,

Just had the motor out and the cam cover off.   I was expecting a horror show, but I was pleasantly surprised that it looked OK in there.

There is some slight scuffing on one journal and small bit on one of the caps.  A little bit of chipping on one cam lobe, but not number 4.

I measured the cam lobes.  According to the book, they are all undersized a bit, but actually all very similar.  Your point about re-facing the cams may be true.  They look very serviceable.  However, this does not explain the timing issue. If it was reground, then number four may have been ground off alignment.

I'm not sure where to go next....

Lobo,

Thanks for the idea.  It seems that 750 cams are in short supply, so unless someone has one to lend me, then I would be forced to buy one from eBay....

What I might do is re-assemble. it all and check the timing again with cam cover off.  It should be easier...

Here are some photos

Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 01, 2022, 08:34:22 PM
Ho Hum.  It says five photos per post....only one seems to attach.  Last one too.

Ah...

Need to select "more attachments"
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 01, 2022, 09:37:17 PM
I like your engine lift method Jerry.

Slightly puzzling, and doesn't look bad at all in there. The scuffing is very minor and something that is not unusual as the valve springs push the cam down, along with the cam chain running in tension that way too. Ultimately looks like oil film performance compromise at some point.

Any pictures of the rocker cam follower pads ? A change there in radius can cause a timing shift too as it picks up the lobe earlier or later.

Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 02, 2022, 08:30:33 AM
Nigel,

The rocker cam faces look ok.. I'll post some pictures shortly.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 02, 2022, 09:33:09 AM
Nigel,

Some photos of the follower faces.

The transverse line on each of the valves are smooth to the finger nail touch.

Some of the tappets have irregular tips.  Not all just some, one photo shown.

I have noticed that there is wear on the No 3 & 4 exhaust rocker shafts.....  Could this be it?  Shame they can't be rotated, or can they be swapped inlet shaft for outlet shaft?

Cheers,
Jerry

Title: Re: CB750 K1 Valve Timing Question
Post by: Oggers on May 02, 2022, 12:13:56 PM
I have an issue with erratic idling on my old Velo - which has a slide carb. The cause here seems to be worn slide or throtlle body. There is a small amount of play if you wiggle the slide which then effectively acts as an air leak. Not sure if yours has slide carbs...Another possibility is the old chestnut of a more obvious air leak somewhere. The classic is the small nozzles for the carb synchronization/balancing on the carb outlets - if you have them -  are letting in air. The sealing caps can come off. Another is the flange faces, another is the vac take off etc etc.   
Title: Re: CB750 K1 Valve Timing Question
Post by: Bryanj on May 02, 2022, 01:26:29 PM
No vac take off, the sync ports are sealed with screws and never seen honda slides that worn
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 02, 2022, 02:17:35 PM
I have an issue with erratic idling on my old Velo - which has a slide carb. The cause here seems to be worn slide or throtlle body. There is a small amount of play if you wiggle the slide which then effectively acts as an air leak. Not sure if yours has slide carbs...Another possibility is the old chestnut of a more obvious air leak somewhere. The classic is the small nozzles for the carb synchronization/balancing on the carb outlets - if you have them -  are letting in air. The sealing caps can come off. Another is the flange faces, another is the vac take off etc etc.

Oggers,

Thank you for your suggestions.

As Bryanj suggests.  These carbs show no wear at all on the sliders.  The inlet manifold rubbers are recent and are soft with the bands fully tight. Balance port is sealed with 'O' rings rather than aluminium washers.

Cheers,
Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 03, 2022, 08:13:33 AM
Rockers are interesting as wear pattern looks pretty even, a test may be to drag some "catchy" fibre material across the surface (like a traditional yellow duster) to see if the surface picks up any fibre and give indication of surface characteristics.

Some of those adjuster screws look a bit rough from manufacturing as far as I can see. Looks like an inclusion in one of them, low risk as they don't actually have to do much.

Rocker shaft wear, obviously ideal if it wasn't there and may be one reason why there's difficulty in setting the lash as you want it. Between this and the screw tips would seem to make it tricky.

To see if the cam lobes are out one to another you'd need to put the degree disk onto the cam direct and measure that way. Must admit I've never measured a cam to that level and so wouldn't know if there's potential production variation in samples.

The inclusion on one cam lobe looks the most concerning longer term, doesn't make that much difference as it's polished away a little, but think you be speculating at longer term prognosis. 

Brutal overview, needs to be kept in perspective though as it's really quite good condition in there, is that it's at some point run with compromised oil condition (likely fuel dilution from short runs or long change interval, possibly both) as the wear surfaces appear consistent with that.

As I mentioned earlier, it does sound very good mechanically when running in comparison to others and so the above I see as very critical but overall it's not a showstopper.

Perhaps given the cam concern it would be ideal to use another cam ultimately. Can't emphasize enough that it's a very good example overall.

Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 08, 2022, 07:38:06 PM
Thought it may be worth adding something about carb setup and how they derive the "correct" idle jet size.

The manual has routine for starting with airscrew at a baseline then running it and shifting the airscrew further outward to make it leaner, this until it's heared that the cylinder you are working on starts to falter as it goes too lean. Then bringing the idle speed back downward by 100 rpm when turning the airscrew inwards to make it run slightly richer than that peak rpm just as it falters.

Then you reset the main idle speed control (master adjust for carb rack) to bring rpm in reasonable range overall. Repeated for each cylinder, you'll then have set the mixture for those idle jets in use.

After that, if you put vacuum guages on and run at the above setting it should now target 20/22 cmHg as ideal tickover vacuum.  If this reading is too high, that indicates that you need the throttle slides too low (causing too much resistance) to control the engine with the jets you've used. It follows that these are too small a jet, giving lean mixture and needing extra air control to stop idle running away.

Equally, if Hg reading is too low it would mean the idle jets are too large and causing rich mixture, then too high a slide position to compensate with additional air to prevent stalling.

It's a circular loop, but should give you the correct fuel metering to run competently.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 09, 2022, 09:26:19 AM
Thought it may be worth adding something about carb setup and how they derive the "correct" idle jet size.

The manual has routine for starting with airscrew at a baseline then running it and shifting the airscrew further outward to make it leaner, this until it's heared that the cylinder you are working on starts to falter as it goes too lean. Then bringing the idle speed back downward by 100 rpm when turning the airscrew inwards to make it run slightly richer than that peak rpm just as it falters.

Then you reset the main idle speed control (master adjust for carb rack) to bring rpm in reasonable range overall. Repeated for each cylinder, you'll then have set the mixture for those idle jets in use.

After that, if you put vacuum guages on and run at the above setting it should now target 20/22 cmHg as ideal tickover vacuum.  If this reading is too high, that indicates that you need the throttle slides too low (causing too much resistance) to control the engine with the jets you've used. It follows that these are too small a jet, giving lean mixture and needing extra air control to stop idle running away.

Equally, if Hg reading is too low it would mean the idle jets are too large and causing rich mixture, then too high a slide position to compensate with additional air to prevent stalling.

It's a circular loop, but should give you the correct fuel metering to run competently.

Nigel,
Thank you for the info.

I have seen that sequence, but not noticed the 20/22 Hg figure.  I have been wondering whether there is a balance between the slide opening height and the airscrew and whether it needs to be raised a little more.  Lifting the slide inevitably increases the revs, then having to reduce the revs by slowing it down with the airscrew seems counter intuitive, but maybe that is where I'm going wrong.  The fact that the other cylinders are trying their best to spoil your fun does not help.  If the plugs were taken out of the other three cylinders so only the one carb is influencing the revs would help, but I expect that would not work due to all the drag from the non-working parts...
I will have another go and try with higher/lower slide settings.

As for the camshaft.  The one from twowheelspares was very nice. Having installed it seems that it might be from a later bike, as the valve timing was different.  However, these readings were done after I had re-faced the tappets.  This must have been influencing the timing readings from the old cam.  The tappets from cylinder 4 were quite badly deformed.  It is much easier to set the correct lash now.  I will probably replace all the tappets if they start going out of spec soon.  I'm not sure how much hardening those faces have.

I have also ordered four new brass floats from CMS, so I have all the same floats.  I can then start off from the WSM setting of 26mm and work from there.  The current floats are not from this bike, I'm sure.  Plastic floats came in with the K2...I think.  These are from two different bikes as the valve tangs have to set at different heights.  I have tried to compensate for these differences by using the "clear tube" method.  However there seem to be differing settings depending on who you listen to.  Some say 1 to 2mm and some say 3 to 4mm, some say from the gasket face...which one, body or bowl.....some say from the bowl top. I have managed to get them the same.  So hence the new floats.

Another point...I believe the 7A carbs were from the early K1 production (this bike November 1970) and it should have the brass floats and the hollow tipped airscrews.  Mine had the solid tipped ones.  I do have some hollow tipped airscrews from a CB500-4 which I had many years ago, so I will put the carbs back as Mr Honda decided they should be......

Probably another false assumption.

Cheers,
Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 17, 2022, 10:00:16 AM
Hi,

Just an update on my struggle with this 750.

After fooling around with jets, settings, advancers, camshafts, needle heights etc………

I did install some new brass floats.  I set them at 26mm (some say 24mm).  The clear tube method indicate about 4-5mm below gasket.  I barely had to adjust the float tangs at all.   I don’t think this has any bearing on the problem though as the fuel height was no different.  It could be an issue with the angle the tang presents to the float needle, just a guess.

I believe that I may have sorted the problem….

Although there were some camshaft issues, I don’t believe that it was bad enough to cause the fuelling problems I was having.  I always felt it was a mostly a carb issue.

Along the way, I have found that the condition of the tappet ends were bad enough to give erroneous cam timing results.  I refaced the tappet screws and it made the cam timing results much closer to the spec.  The main thing is that it was so much easier to adjust the clearances.  However all the minor improvements I have done may have influenced the final result a little, but to me, It was something that Nigel (K2-K6) mentioned to me in a PM…. He pointed me to the Honda carburation setting section of the WSM, which loosely described the set up procedure. It was not very specific on the exact process.  Nigel suggested in Reply #25

“The manual has routine for starting with airscrew at a baseline then running it and shifting the airscrew further outward to make it leaner, this until it's heard that the cylinder you are working on starts to falter as it goes too lean. Then bringing the idle speed back downward by 100 rpm when turning the airscrew inwards to make it run slightly richer than that peak rpm just as it falters.

Then you reset the main idle speed control (master adjust for carb rack) to bring rpm in reasonable range overall. Repeated for each cylinder, you'll then have set the mixture for those idle jets in use.”

It made me think that I may have been starting the procedure with too much idle set, rather than starting low and adjusting the airscrews to improve the rpm. I went round the carbs with this in mind and ended up with airscrews out further that I had managed before.  I knew that with the airscrews set out would allow the throttle response to good.  So with a good idle with a hot engine, I took it out for a ride……really good throttle response on-off the gas and when I  stopped it would idle nicely….Result.

So it seems to have the been the mental approach to adjusting the carbs rather assuming only one way to do it.

Could say that the 750 WSM is geared for qualified Honda Service guys, rather than amateurs like me.  Maybe if I had the Haynes manual.it might have helped.

I would not say I had any real experience with carbs, in spite of quite happily stripping and refurbishing a six-pack of carbs from the CBX and getting it to run perfectly with just following the set up procedure…

The CBX1000 WSM is very specific regarding tolerances and torque settings, etc making it really useful to the enthusiast….

Big thank you to Nigel for his guidance.

Cheers,

Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: Bryanj on May 17, 2022, 01:14:44 PM
Haynes is worse as its basically the Honda one with bits missing
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 22, 2022, 11:14:09 AM
Good results from your work Jerry, and always nice to feel progress with the path you take to optimise setup.

Interesting when you can put a couple of minds to work on a problem that it may bring a different facet to the overview, along with generating a potentially different route through to explore and evaluate. 

The manuals are originally fairly sparse in content and may be a little influenced by naive translation that may miss some of the original nuances of the factory intention when first written.

Experience, in a technically unrelated field, of working with instruction brought me an approach in which we could question the manufacturer about what the manual was asking us to do (not the accuracy but the intention of what the process was trying to accomplish) and looking at my view of these bike manuals originally this has influenced my reading of them more recently. 

Some parts being outright setting in absolute terms for obvious reason, this section being more method to achieve rather than absolute. 

I think that section on carburettor setting is a very, very underestimated little gem of a routine that's often not even considered in many discussion of these bikes.
Effectively it's (with no more than a screwdriver as only tool) getting the user to bring each cylinder to a stoichiometric (all fuel burning all oxygen) place to determine the running characteristics of that cylinder, then bringing it back from this point to set a reference mixture for that cylinder.  In other words it's calibrating the system you have in place, but without any expensive equipment. 
The idle jet size is selected on altitude/barometric conditions first, then the airscrew is used to dial in this choice.
The Hg reading you end up with will actually tell you if the jet size is wrong to start with as too high or low will support a shift in that jet to normalize the range you are working with.
A sort of "squaring the circle" type equality in which you are after a perfect circle of settings, but maybe end up with it egg shaped if you didn't conclude all of the steps accurately.  In that scenario, it works reasonably but still with niggling compromise.  When its fully resolved, all the characteristics fall into a good place.

It's this setup that ultimately controls slow running refinement and not the "carb balance" routine as often given on internet discussion. 

The point of overall Hg figures is to place the slides in the correct position, too low to control runaway idle speed, and they'll give to much vacuum when you close the throttle especially on overrun. The effect of this is;- as you roll it of for example coming into a corner it will start to hold the slides hard against the carb bodies, making it difficult to cleanly and very gently bleed the throttle back in again (those comments about stiff throttle activation have their basis in here) as it's both hard to move and runs slightly lean giving rise to a flat spot hesitancy just as you try to bring the torque back on.
Getting it set as intended brings a smooth linearity to this response in setting it much more like the factory would have done for a new bike.

Each time I read through that section of manual it adds something to my understanding of it, so very simple in appearance but with some very astute logic built into it.
A difference from early to later Honda wsm are little statments that this routine is giving you a accurate setting for your carbs and jets to be recorded as part of bike setup especially after a carburettor rebuild to correct metering at that condition. 

The method is a bit like a course of antibiotics in that you needed to complete the entire process to bring correct resolution. As above, I think it's a very neat and forward thing routine contained in there.
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 22, 2022, 03:14:28 PM
Just thought of another key aspect of the carb idle setting routine.

Although written in the sixties, it will take account of fuel difference including any formulation changes and ethanol content.

It's using the fuel you have in use to properly calibrate the whole low speed running ability on these engines.

Very clever  ;D
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 23, 2022, 11:24:06 AM
Nigel,

Thank you for your in-depth analysis of what is going here.

Yes, the procedure should make allowances for varying qualities of fuel, bearing in mind these bikes were sold all over the world.

Interestingly, Hondaman on .net has written about how the ethanol content, that Mr Honda was probably not planning for, has affected the way ethanol alters the burn rate and temperature.  He suggests that the main jet emulsifier holes which have two different diameters, the smaller ones should be drilled to the size of the upper ones....I think.  Opening the holes up allows more air to the mixture without reducing the fuel rate through the main jet....I think.....

As I am lucky enough to live in an area where the premium fuels are slated as having no ethanol in them, I will reserve judgement.

Anybody seeing this information should check very carefully before altering their emulsion tubes.  They seem to be quite scarce items.

Cheers,

Jerry

Title: Re: CB750 K1 Valve Timing Question
Post by: SteveD CB500K0 on May 23, 2022, 12:08:34 PM
Where in Suffolk are you Jerry?
We are all on the way.

Actually, I’m there already as mother-in-law lives in Woodbridge.

Berkshire Superunleaded is 5%


Sent from my iPhone using Tapatalk
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 23, 2022, 04:15:22 PM
Steve,

I'm very near Needham Market.

Jerry,
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 23, 2022, 09:34:19 PM
Jerry, do you have a link to the hondaman thread about ethanol?  I'm interested in reading it but couldn't readily find it on that forum.

Thanks, Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 24, 2022, 09:48:32 AM
Nigel,

http://forums.sohc4.net/index.php/topic,164119.0.html

This is a long thread.  Checkout reply #27, then go on to #55, then #60.

All very interesting, particularly the arguments over the what the airscrew does!

Also mods to Emulsifier.

Cheers,
Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 24, 2022, 11:21:21 AM
I see what you mean on airscrew direction  ;D I'm with MRieck on this one, turning outward makes leaner mixture on these carb.

It's opposite on PD and many CV type as the screw sits between the venturi and the inlet valve in comparison to older type slide carb with screw next to air filter.
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 24, 2022, 12:03:54 PM
Two tired #62 post is absolutely correct, and checked with exhaust gas analysis too  :)

Amazing they are all missing the Honda setup that brings combustion parity (at tickover)  to the engine via idle circuits and not through absolute setting of carb synchronisation.
Title: Re: CB750 K1 Valve Timing Question
Post by: Oddjob on May 24, 2022, 01:28:06 PM
They both can't be correct though Nigel, either it's leaner going in or leaner coming out. My understanding was that going in makes it richer but that's directly contradicted by Marks statement and also by Hondas reps during that time if I've read right, as in #70.
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 24, 2022, 01:59:01 PM
They both can't be correct though Nigel, either it's leaner going in or leaner coming out. My understanding was that going in makes it richer but that's directly contradicted by Marks statement and also by Hondas reps during that time if I've read right, as in #70.

Yes Ken, he's wrong as most of us understand it. There's contradiction in the explanation of how a jet flows by reducing below atmospheric in venturi that allows fuel to flow through the jet to the lower pressure area (correct) but that's ignored in explanation of this interaction.

If you had no air at all, then the idle jet would be full flow as soon as you started the engine, and giving no gradient of fuelling at all.
Bringing air into that stream (between float bowl and carb throat) allows air volume to mitigate that full fuel flow as the vacuum would rather suck air (less resistance) than fuel.
Passage more open  (screw out) just allows more air and less fuel. More closed and vacuum portion to fuel line increases and sucks more fuel in proportion to that restriction. 
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 24, 2022, 04:41:58 PM
Not being an expert at all.  See my bumbling earlier in this thread.

The theory may be fine if you are familiar with fluid dynamics and can understand it, but simple test surely is to check plug condition against an airscrew setting.  that should settle it....

Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 25, 2022, 09:21:38 AM
Yes and easy to check. Turning in until it stops the cylinder just goes far too rich and black smokes like it's on choke (unburnt HC) and the opposite as you back the screw out.

The others on thread do offer a more scientific basis with anecdotal supporting analysis though. All academic though as thread age is older in that case.

Jets are more akin to siphon though rather than anything more sophisticated. Try sucking liquid up with a straw, then put a hole in the side of the straw, you'll just get air bleeding in if air enters above liquid level.

I don't understand the reasoning behind changing emulsion tube holes in response to ethanol content. They are just there for atomisation of fuel droplets in the mixture (left as they are would seem reasonable)  and doesn't doesn't deal with combustion characteristics that seem associated with ethanol mix.

Essentially "emulsion" arrangement helps produce a stratified charge to burn effectively.  Meaning, each fuel droplet would be surrounded by 14.7 parts oxygen content air, the more evenly and homogeneous this is the more consistent the burn rate across the combustion chamber. It wouldn't appear to need any shift there to accommodate it, and certainly would need some very concise observations to understand the effects.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 25, 2022, 02:48:47 PM
Nigel,

It seems that the air entering the throat by way of the twist grip is the air that sets the 14.7, demand from the needle jet. I guess the emulsion tube makes the petrol flow out of the needle jet in smoother and, as you say, homogeneous way.  However enlarging the emulsion tube holes even ever so slightly must add air to the mix, which I guess means leaning the overall mixture.  Mark suggests that ethanol in the fuel makes the combustion burn cooler.  So maybe the extra leanness is compensating for the coolness and makes the combustion run hotter,   maybe ????

Perhaps a reduction in main jet size is too crude for such subtlety?

Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 25, 2022, 03:42:57 PM
Obviously it's a complex topic, interesting piece for discussion

"Ethanol molecules include oxygen atoms, whereas gasoline molecules don't. That's part of the reason why ethanol has less energy than gasoline. Another effect of the oxygen from ethanol is that ethanol blends tend to run "leaner" than pure gasoline because there is more oxygen available in the fuel-air mixture. If your engine is not able to compensate by reducing the incoming airflow, the resulting combustion conditions in the engine cylinder may be less than ideal. Newer vehicles are generally designed to take care of this automatically, but older engines may need a bit of manual adjustment to get the air-fuel mixture just right.

Some people have reported engines overheating when ethanol blends are used, suggesting that ethanol burns "hotter." This is a bit mysterious since ethanol contains less energy per unit volume than gasoline, and the flame temperature of ethanol is more than 40°C cooler than gasoline. The most likely explanation is related to the air/fuel ratio. Most engines are designed to run with an excess of fuel relative to the amount of air (a "rich" mixture); experience has shown that this leads to higher power output and cooler engine temperatures. When ethanol blends are used, newer engines are equipped with sensors to adjust the air/fuel ratio automatically. Older vehicles and small engines may not be equipped to do this, resulting in a "leaner" burn that may increase engine temperatures and/or reduce engine power. A simple adjustment to the fuel system to "richen" the mixture can often fix this problem."

This effectively contradicts adding more air.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 26, 2022, 10:14:04 AM
Nigel,

Interesting stuff, so why open the emulsifier holes.  That would make sense of Mark's explanation of adding extra air with the airscrew which enriches the idle mixture.  However, we know that adding extra air via the airscrew to the idle jet actually leans it out.

Maybe the difference between how an idle jet works and how a main/needle jet works is the answer.

Somewhere, the emulsion tube function is explained, maybe in that same thread.  The lower holes are under the fuel height in the bowl, the upper (smaller ones to be opened up) are above the fuel height.  This would imply weakening the mixture if we are observe the equivalent in the idle circuit.  Marks explanation of making the fuel bubble up faster would make sense in making it richer.....

Confused

Jerry

Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 26, 2022, 01:23:56 PM
Nigel,

Just had another look at an emulsifier.  The small holes are at the bottom, not the top as mentioned in my last post

Sorry,

Jerry
Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 26, 2022, 02:09:56 PM
I strongly suspect that Kiehin (and subsequently Honda) have the original arrangement as taper characteristics to track main jet volume flow throughout it's range to bring consistent aeration to the fuel delivery. I'm highly suspicious of uncontrolled actions to change things like this without the "director" understanding why it was this way in the first place.

I like the outlook and involvment mark has in these bikes, but things like this need to withstand significant scrutiny to offer validation.  I can't see the logical path to that decision (perhaps I've missed something) as the percentage change doesn't involve a step change, and if it did then shifting atomisation seems an odd answer. 

The statement of leaner mix making more temperature is fundamentally untrue and backed by research of far greater depth than we can bring here and shows to me a naive setting off point for further analysis. This a interesting topic in itself.

For interest, the clip statement a couple of post back about ethanol is from here https://extension.psu.edu/fuel-ethanol-hero-or-villain which appears to offer reasonable balance in topline discussion of that fuel supply.

Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 26, 2022, 04:39:37 PM
For interest, the clip statement a couple of post back about ethanol is from here https://extension.psu.edu/fuel-ethanol-hero-or-villain which appears to offer reasonable balance in topline discussion of that fuel supply.

Nigel,

Interesting article.  It is light on the downsides, like for "older machinery".  We all know how corrosive the use of ethanol can be and this must be our main worry.  However, I don't see that we have a great deal of choice.

Mark is suggesting a way of compensating for the "Leanness", but of course if you end up using variable E numbers, or none, then you may run rich.  I think in the states they are stuck with E10, so it makes sense to re-tune the engine somehow.

I have read that some people make a weak two stroke mix in their tanks, which is claimed to reduce the corrosive element of ethanol fuel.  Not sure of this as I am not a chemist.

Cheers,
Jerry


Title: Re: CB750 K1 Valve Timing Question
Post by: K2-K6 on May 28, 2022, 08:24:49 AM
The combustion element, as with other effects, will obviously split opinion. I just can't see a difference of any significance that needs reacting to and principally because these era carburettor fed engines essentially run rich in greater error from the outset.

I agree with tentative exploration of running two mix "light" to asses if it affects/improves your own storage stability if using E fuels.

Two aspects that I feel could be considered. One is some residual oil may help in protecting the exhaust interior of these expensive systems, especially when used sporadically. Two stroke exhaust will generally not corrode internally from this effect, that could offer something significant to owners. The water vapour output from burning petrol has a sulphuric acid content (ultimately from sulphur content of fuel) which is the primary cause of corrosion within these mild steel systems, and something to consider at least.

Fuel stability is another aspect. It seems to me that premixed fuel does improve in keeping and corrosion effects in systems that use it, this is experience and not from outside reports. I have a number of smaller engines that seem to benefit in this way. One a Honda generator 4 stroke that has significant periods of non use that I've had oil mix in for about 8 years and never had to clean the system in any way. Usually run the carb dry with fuel off before storage, then it just starts again on re use.  It's been with a outlying type of oil though that's not in general use. From this I've been running same at 100:1 mix ratio in a petrol car to guage the effects there too, and with no detrimental characteristics in approx 10,000 miles. Notably this starts impeccably and runs without compromise, it's 32 yes old and  170,000 miles status.
Anecdotal, yes, but maybe of interest in using E 10. I use std fuel from local supermarkets and never enhanced product, they are high throughput though which can have a different effect on their own storage I understand. 
I've set some long term samples in glass jars too, this to try and judge longe term keeping dropout, water accumulated etc. Certainly in sealed containers the E fuel seems to keep without problems, but also with the oil appears less likely to give the micro corrosion on some carburettor components (that crystalline type accumulation that effectively impairs jets etc) and could be worth experimenting with for others.

Nigel.
Title: Re: CB750 K1 Valve Timing Question
Post by: cbxman on May 28, 2022, 05:07:24 PM
Nigel,

Interesting.

I think I might try a 100:1 mix with two stroke oil in the tank and have ride before I put her away in the autumn for the long winter snooze.  Same with the CBX.  I have a Suzuki GT550, so I have 2T oil handy.

Cheers,

Jerry
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