Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Saesneg Shaun on September 04, 2024, 09:13:59 PM
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Hi all, I could do with some advice regarding another problem with the K6. When opening the throttle to accelerate the bike momentarily cuts out. Its much more than a flat spot as for a split second the engine completely dies making the forks dive before picking up and accelerating as normal. It only does this when its up to normal temperature and the hotter the bike gets the worse the problem.
I'm running Boyer electronic ignition with the original coils. New plugs and timing checked. Carbs have been cleaned and balanced, new needle valves fitted and float levels checked. Don't know if its related but the bike is (and always has been) a very poor cold starter.
I'm running out of ideas. Coil breaking down maybe?
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Does it seem to run lean?
White plugs,
What about R type plugs and resister plugs caps, are you running both?
Sent from my SM-A546E using Tapatalk
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Does it seem to run lean?
White plugs,
What about R type plugs and resister plugs caps, are you running both?
Sent from my SM-A546E using Tapatalk
Doesn't seem to be running lean looking at the plug colour. Air screws are about 1 1/8 out which I set during carb balancing procedure. Running NGK resistor caps and D8EA plugs.
Its usually a pig to start from cold but once started it runs fine until its hot.
I've cleaned and balanced the carbs numerous times now. I replaced the pilot with new genuine honda just in case and replaced all the fuel needle valves because 1 leaked. I previously had issues with wear in the point spindle but am now running electronic so I shouldn't be that either.
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Are you running an external fuel filter? As that sounds very similar to what my pegaso was doing years ago.
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Try running it with airscrews turned in 1/4 turn from where they are now (you can set them back if no or poor effect) to assess if that makes a difference in starting and running.
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Carb slide needles in the wrong position?
Set too low and giving too a weak mixture.
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Advance mech running freely?
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I always think poor cold starting indicates a lack of fuel so running a tad on the lean side. Other reason might be the ignition is a tad retarded.
If your E- ignition has separate adjustments for 1/4 & 2/3 might be worth checking the timing on both, as is the advancer unit if you are using the original mechanical unit.
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Thanks all, i'll try turning the screws in to richten it up a bit. I was told too lean and the revs hang when I blip the throttle and too rich revs will bog down then recover. I'm running the stock airbox & filter and the advance mech is replaced with the boyer kit.
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I always think poor cold starting indicates a lack of fuel so running a tad on the lean side. Other reason might be the ignition is a tad retarded.
If your E- ignition has separate adjustments for 1/4 & 2/3 might be worth checking the timing on both, as is the advancer unit if you are using the original mechanical unit.
Boyer system has no adjustment between the two sets to be moved. Effectively, the backplate is set up geometrically in manufacture to fix them at exactly 180 degree apart, no seperate spacing needed to cause problems.
In addition, they fire all four plugs at every event of timing ..... at every 180 degree increment that passes the trigger point.
Also, there's no "advance" as such. They set the trigger point at maximum timing point (hence installation notes to check it above specific rpm) then they delay the timing point in software based on engine rpm to give a retarded effect for low crank speed, this detected from pulse count register as you crank it.
Also facilitates "multi spark" at low rpm firing to try and mitigate plug fouling etc and provide more effective cold start.
Its a pretty comprehensive system solution to replace points with.
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There is an old post on this problem by Johnwebly? - turned out to be a couple of loose wires on the Boyer unit. I can't post the link using my phone but it's there if you search the term Boyer.
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Thanks all, i'll try turning the screws in to richten it up a bit. I was told too lean and the revs hang when I blip the throttle and too rich revs will bog down then recover. I'm running the stock airbox & filter and the advance mech is replaced with the boyer kit.
It sounds like the needle and main jet settings are substantially in correct range, as that's more the influence on plug colour from decent running.
If it just gets a little lean as you open the throttle slides, that suggests a deficit in slow speed circuit to stoke it up into response. The air screw adjustment should show if that's true or not.
In essence, if it does clean it up, then that suggests it's "asking" for a step up in volume of idle jet to bring it in competent range, at which point the airscrew set point would look more normal in comparison to manual.
Interesting to see if that tweak will show any desirable response.
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It's a long shot this is the link about a Boyer wiring issue.
.http://www.sohc.co.uk/index.php/topic,5861.msg29390.html#msg29390
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It's a long shot this is the link about a Boyer wiring issue.
.http://www.sohc.co.uk/index.php/topic,5861.msg29390.html#msg29390
Thanks Ted. I will take a look tomorrow. Unfortunately the predicted monsoon arrived today so I didn't get to play with the bike.
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If it's a 750K6, @Hondaman on the sister US SOHC4 forum, has explained a few times that turning in the idle screw weakens the mixture, as it isn't an air screw, it is metering the flow of already mixed air/fuel. So unscrew to richen.
It still feels alien to me (I'm more used to Amal carbs). But, that's his advice.
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If it's a 750K6, @Hondaman on the sister US SOHC4 forum, has explained a few times that turning in the idle screw weakens the mixture, as it isn't an air screw, it is metering the flow of already mixed air/fuel. So unscrew to richen.
It still feels alien to me (I'm more used to Amal carbs). But, that's his advice.
Thanks it is a K6, good to know as I thought it was the other way around.
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If it's a 750K6, @Hondaman on the sister US SOHC4 forum, has explained a few times that turning in the idle screw weakens the mixture, as it isn't an air screw, it is metering the flow of already mixed air/fuel. So unscrew to richen.
It still feels alien to me (I'm more used to Amal carbs). But, that's his advice.
Thanks it is a K6, good to know as I thought it was the other way around.
I'd be careful with that "information" as the carburettor layout shows nothing to differentiate it's idle circuit from any of the others with, what we understand as, conventional response.
Its been debated on here and US site without a verified conclusion. No logically observable working through has been forwarded, as far as I can see, to say that statement is correct.
Clearly, the PD type later carbs, along with CV type fitted after SOHC fours has the opposite response to turning the screws. This is very clearly differentiated by having the adjustment screw on the engine side of venturi as it's location. That's logical and clear in it's layout.
There's nothing on these K6 carbs to give that effect. Unless someone can adequately demonstrate it with HC and CO meter during adjustment, then it seems false.
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If your bike has the four silencers then I would be guided by the change in exhaust temperature when you make the adjustments.
Just using your hand at the tail pipe when the bike is up to running temperature will give you an indication of how the screw turn affects the mixture.
If you open the screw by a quarter turn it will norally result in an increse in exit gas temperature as it runs leaner.
Turning the screw in should richen the mixture resulting in a cooler exit gas temerature.
If the reverse happens then that would indicate it's not an air flow screw but a neat fuel one. Sorry if I'm stating the obvious.
This US video is basic but it has helped me get my head around adjusting these screws.https://www.youtube.com/watch?v=q9A2TL9RvwQ&t=338s
I'm not advising this a way to set your mixture but to help you work out what turning the screw does on your bike.
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The missing part for me though Ted is that these two design are fundamental in their differences, not just an interpretation of what it does.
Most FUEL adjustment screw type have a fixed air intake size in the carb entrance, with no modulation, then the adjustment happens on engine side of carb venturi by attenuation of the fuel passage on its way to entering the carb throat.
The air adjustment type (usually on these SOHC) up until fitment of PD carbs, are all AIR adjustment type. They are different, by design, in their application. Its not really an interpretation topic, they are designed as different system type.
The contrary claims are just that. With no (obvious and available) evidenced material to show why they should be considered differently when their design says otherwise.
If there's something available to show this, then that would be of benefit to the forum.
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I agree with you completely Nigel - my logic with the post about the temperature of the tail pipe gases is that it will confirm what most of us know is true & discredit the US idea that its a neat petrol flow screw rather than an air screw that reduces the draw of petrol via the jet. It makes you wonder if they meant to refer to PD carbs.
I have not been on the US sister sohc site much though I have been on some US websites for old Mercs & Nissan 4x4's. I have found the US sites occasionally full of inaccurate information re-inforcing my mental Redneck Sterotype.
I would add that when our old Merc W124 TE 4-matic broke down as Wendy was returning home one dark winter night a few hundred yards from home I turned to the US site for a diagnosis. I was in bed at gone midnight on my laptop looking for the likely cause. (the engine cranked over but would not fire - I knew it was not the OVP relay as that was new) At 1.30 am I got dressed, walked down to the vehicle, lifted the hood, removed a plastic cover on the bulkhead, with a spanner I tapped the relay as per the post advice. The car started straight away as per the US blogg advice. I repaired the relay as per the site advice to keep us mobile. Purchased a new MB relay for about £25-30.
My experience undermined my prejudices no end.
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I would love to get to the bottom of this. Being used to an Amal's airscrew, and also having always understood if the screw is before the slide (on the airbox side of the carb), then that's air adjustment, if after (cylinder inlet side), then it is fuel, I was expecting the screw to work consistent with K2-K6's comment.
The advice seemed so odd, I wrote separately to Mark (Hondaman), to check this advice. He was certain. He explained the screw goes into a section of the carb which already has air and fuel mixed, so opening the screw richens the mix entering the cylinder. As he has perhaps 50 years experience on these Hondas, I took this as good.
This is such a fundamental for tuning these bikes. I'm sorry if I have caused confusion. Does anyone have a diagram of these carbs which might put this to bed?
Ted, I can't really tell any difference in temperature at the silencer. I guess one of those snazzy electronic sensors would, but my hand isn't sensitive enough.
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I asked this:
"Is this true for my K6, I should be adjusting the air screw clockwise (inwards) to make the idle mix leaner?"
Here is the relevant part of Mark's reply to me:
"Yep, turning inwards on all the SOHC4 carbs will make it 'leaner' by restricting how MUCH of the fuel-air
mix is pulled up the tiny jets during the intake stroke. It is already aerated, sitting inside the tiny chamber around the emulsifier tips of these little jets (for about 1.5 seconds before the air gets away from the little drop of it). "
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I thought it's called an air screw for a reason - it lets in air to reduce the flow of the petrol from the jet in the carb bowl. Interestingly the 500 and 550 have a completely different design of air screw. My 500 now has the screw with all the holes in it not the solid brass type.
When I had a cooler front pipe on the 500 I found that screwing in the air screw for the relevant carb made the pipe as hot as the others. I'm talking front pipe being hot here not exit gas at the tail pipe.
I think Bryan will know the answer from his workshop experience.
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If the "header" pipe was cool from not firing (insufficient fuel mix to ignite) because of impaired idle jet circuit, then it will pick up to operating temperature by forcing it rich with a closed air passage.
Basically as you close the air screw down it will just force it to suck what it can get through partly blocked fuel jet. Obviously a weird set point but it can bring regular firing to that cylinder instead of none.
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OK so to be clear I need to turn the air screw IN to richen it a bit?
I'm thinking of getting a cheap Amazon temperature gun for checking the header temps. Anyone used one for this?
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I'm convinced that on my 500 air screw turning out makes it leaner.
I can't comment with any authority on your model of 750.
We need Bryan or Graham/Julie to say with certainty it might be useful to include your carb model type number that is cast on the flange end near the inlet manifold.
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OK so to be clear I need to turn the air screw IN to richen it a bit?
I'm thinking of getting a cheap Amazon temperature gun for checking the header temps. Anyone used one for this?
Yes, turn it IN for richer.
If you wanted to "guinea-pig" it .... then turn then to only 1/2 out total from closed and run it, then to 2 1/2 turns out and run it like that. Should answer the mixture question by showing what happens.
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I'm convinced that on my 500 air screw turning out makes it leaner.
I can't comment with any authority on your model of 750.
We need Bryan or Graham/Julie to say with certainty it might be useful to include your carb model type number that is cast on the flange end near the inlet manifold.
It's an air screw on all Keihin CB SOHC 4'S carbs, except the Keihin PD carbs. Turn the screw in for less air = richer, turn the screw out for more air = leaner.
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As Julie notes ... they are called AIR SCREW ADJUSTMENT in original CB750 workshop manual.
Further ... the fault finding table in assisting diagnosis of idle problems is consistent in description of faults and detection with how we understand it here, that its adjustment of air in modulation to effect a change in mixture.
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OK. I will follow that advice from now on. I'm sorry I took us off at a tangent. Thank you.
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I do think its worthy of debate here ... thank you to the contributing debaters in following this topic through.
If a statement of this type can't be usefully examined, cross checked and verified ... then really its probably myth in quality.
Things like this must be able to withstand reasonable scrutiny to hold a validity in making adjustments to these carburettor and engines.
There is ... in official workshop manual of 750 ... an engineering drawing, in section vertically through the carburettor and with noted key parts.
Subsection of that drawing is an expanded view of the idle circuit junction/interface showing decent enough detail.
It's also split in vertical plane (important here) and not derived into plan view, usually signified in technical drawing by 45 degree extraction notification to make clear that has been accepted. It's important here as that expansion diagram could be inadvertently "read" as an airscrew illustration if it was indeed plan view. Its not.
What that shows is a side view, sectioned vertically of the idle jet, installation and emulsifier tube in situ. This has an air passage disecting it around the emulsion tube height. There's no modulation drawn for adjustment of that fuel supply route, at all. It's just plain through from idle fuel jet orifice, up the tube and through the emulsifier toward venturi.
If that drawing was mistaken for an adjustable compartment (the sectioned jet lookin like a needle type adjustment screw, because it's section illustration has similar form) then someone could make the incorrect interpretation, based on that detail, that it is a different component.
I'd be honestly surprised if it could be shown that this design could work in tbe opposite of that which we understand it.
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Generally speaking the Coils tend to fail as the bike gets hotter. Worth checking them out.
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Always fascinated me this discussion on mixture. When on tickover or very low revs how much of the air goes past the the carb sliders and how much goes via the air mixture screw (not much)? Where does the air that passes the screw feed in? Still have an open mind.
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There is a small brass orifice below the slide on filter side of carb
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Here is an observation I've made on my 500.
When the engine is started from cold (Summer time) I do not like to use the choke so it's soon off. I made the error a couple of days ago of screwing down the main throttle bracket screw 1/4 turn to make it idle when only part warm. As I rode off it started popping and banging like crazy. I returned home thinking something was very wrong - by this time it was well up to normal running temperature so the idle was 2k plus hanging throttle. Returned the throttle stop screw to idle again at 1100-1200 rpm as engine was warm. Blipping the throttle now no popping or banging, no hanging idle. Went back out for ride all was good. Using the choke plus it's fast idle from cold is different - no popping or banging. The only difference I could work out was the hanging throttle here was caused by the sliders being prevented from closing to the correct position.
This has led me to believe that hanging throttle issues are as a consequence of the sliders not being sufficient closed in the shut throttle position. If the sliders are in the sufficiently closed position any idle speed adjustment should be made via the air screw only.
I think the trick might be knowing what the sweet spot is for "sliders are sufficiently closed". Open for debate
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To this day Mark Paris is still insisting screw in for more air, screw out for more fuel. Anectodal and personal experience on 750's and all other non PD SOHC /4 carbs, is that out gives more air, in gives more fuel. If we had set the screws on our own builds and bikes as per Marks advice, the bikes wouldn't have run. Regular riding of all our bikes by ourselves and all our customers bikes, prove we have done it correctly.
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Sounds as if Mark Paris is getting confused between when an engine is running rich and when it is running weak.
If he has those concepts reversed then he is correct.lol
Back in my youth I used to think a hot tail pipe exhaust meant it was running too rich my Dad put me right it took me a while for the penny to drop. After all too much fuel it would burn hotter no?
He was right of course when it's rich the exhuast gases are cooler at the tail pipe - in a way it defies logic if you ignore what's taking place inside the engine and how the petrol actually burns.
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Haven't been out much on the bike but got the chance to ride today. I tried turning the air screws in a bit but no difference to the bogging down unfortunately. Slightly richer means it won't idle cleanly unless I up the tickover speed to nearer 2000rpm so i'm guessing 1 1/8th turns out is about right. Also tried spraying the inlets with brake cleaner but no air leaks either.
The I rule out the more i'm beginning to think its electrical and the boyer ignition is to blame. Its happening between 2000-3000 rpm in all gears.
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On the other hand you may have the same problem that I had.
Needle jets worn and guttered out, giving a massive flat spot when accelerating.
Honda part number 16012 392 004
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On the other hand you may have the same problem that I had.
Needle jets worn and guttered out, giving a massive flat spot when accelerating.
Honda part number 16012 392 004
I hadn't considered that. The bike has 33k miles on it but their probably the originals in there. Is there any obvious visible signs of wear I can look for or just replace?
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If the needle jets are worn,it's likely to make it to rich,
Try dropping the needle by one groove, see what happens
I assume you have the standard air box and filter?
Sent from my SM-A546E using Tapatalk
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Have you eliminated the possiblity that its coil or plug lead related rather than suspecting the Boyer unit?
As has been said if it was worn needles that would make it run richer then you might expect that to be reflected in the spark plug colouring. Are you using resistor plugs or just resistor caps. Are the spark plugs relatively new?
If you have the points base & advancer might be worth returning the ignition back to standard temporarily - easier than having to take the carbs off. (Never seen a Boyer Unit so not sure they are)
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Have you eliminated the possiblity that its coil or plug lead related rather than suspecting the Boyer unit?
As has been said if it was worn needles that would make it run richer then you might expect that to be reflected in the spark plug colouring. Are you using resistor plugs or just resistor caps. Are the spark plugs relatively new?
If you have the points base & advancer might be worth returning the ignition back to standard temporarily - easier than having to take the carbs off. (Never seen a Boyer Unit so not sure they are)
Yes, I checked them all and trimmed back and loose ones. I checked the connections on the boyer ignition and just to be sure the kill switch and ignition switch. I'm running resistor caps and non-resistor NGK plugs.
Unfortunately I had issues with the points cam running out of true hence going to boyer electronic.
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On the other hand you may have the same problem that I had.
Needle jets worn and guttered out, giving a massive flat spot when accelerating.
Honda part number 16012 392 004
I hadn't considered that. The bike has 33k miles on it but their probably the originals in there. Is there any obvious visible signs of wear I can look for or just replace?
The originals in mines had done 23k.
Held up to the light along with the new ones you will probably easily see the wear difference.
Good luck.
Skoti