Author Topic: CB750 K1 Valve Timing Question  (Read 1726 times)

Offline cbxman

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CB750 K1 Valve Timing Question
« 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
CB750 K1 1971 Wisconsin, USA
Suzuki GT550 J 1972 Michigan USA
CBX1000 A 1980 Canada
CB1300 A5, 2005 UK

Offline K2-K6

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Re: CB750 K1 Valve Timing Question
« Reply #1 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.

Offline cbxman

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Re: CB750 K1 Valve Timing Question
« Reply #2 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
CB750 K1 1971 Wisconsin, USA
Suzuki GT550 J 1972 Michigan USA
CBX1000 A 1980 Canada
CB1300 A5, 2005 UK

Offline K2-K6

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Re: CB750 K1 Valve Timing Question
« Reply #3 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.

Offline cbxman

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Re: CB750 K1 Valve Timing Question
« Reply #4 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
CB750 K1 1971 Wisconsin, USA
Suzuki GT550 J 1972 Michigan USA
CBX1000 A 1980 Canada
CB1300 A5, 2005 UK

Offline K2-K6

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Re: CB750 K1 Valve Timing Question
« Reply #5 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.

Offline Lobo

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Re: CB750 K1 Valve Timing Question
« Reply #6 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….

Offline K2-K6

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Re: CB750 K1 Valve Timing Question
« Reply #7 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.

Offline cbxman

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Re: CB750 K1 Valve Timing Question
« Reply #8 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
CB750 K1 1971 Wisconsin, USA
Suzuki GT550 J 1972 Michigan USA
CBX1000 A 1980 Canada
CB1300 A5, 2005 UK

Offline K2-K6

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Re: CB750 K1 Valve Timing Question
« Reply #9 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.

Offline cbxman

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Re: CB750 K1 Valve Timing Question
« Reply #10 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

CB750 K1 1971 Wisconsin, USA
Suzuki GT550 J 1972 Michigan USA
CBX1000 A 1980 Canada
CB1300 A5, 2005 UK

Offline K2-K6

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Re: CB750 K1 Valve Timing Question
« Reply #11 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.

Offline cbxman

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Re: CB750 K1 Valve Timing Question
« Reply #12 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
CB750 K1 1971 Wisconsin, USA
Suzuki GT550 J 1972 Michigan USA
CBX1000 A 1980 Canada
CB1300 A5, 2005 UK

Offline cbxman

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Re: CB750 K1 Valve Timing Question
« Reply #13 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

CB750 K1 1971 Wisconsin, USA
Suzuki GT550 J 1972 Michigan USA
CBX1000 A 1980 Canada
CB1300 A5, 2005 UK

Offline K2-K6

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Re: CB750 K1 Valve Timing Question
« Reply #14 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.

 

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