Honda-SOHC
SOHC.co.uk Forums => Anorak's Corner => Topic started by: K2-K6 on October 03, 2024, 09:16:11 PM
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To discuss and, hopefully inform, with more in depth discussion of carburettor function and how it relates to these SOHC bikes along with their setup.
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Great idea. How about people like yourself copy and pasting some of your previous helpful descriptive posts on the topic to this section Nigel?
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Great idea. How about people like yourself copy and pasting some of your previous helpful descriptive posts on the topic to this section Nigel?
;D literally at just this moment thinking the same to capture items already discussed.
I'm out today with some car bods but will get something done later.
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In response to the role played by idle jet function specifically around hanging idle effect
I'll try to pick the right wording to describe what their influence is.
Its situational rather than hard rev band or throttle position. This because the engine is responding to volume of ai, while the carburettor responding primarily to vacuum (the drop below atmosphere via venturi design) therefore it'll flow differently for different situations.
Example is if you rev the engine then drop the throttle instantly/fully, the air volume stops but the vacuum now climbs as the rpm decays according to flywheel energy. That provides a spike in fuel with the slide closed, making it temporarily rich which helps to quench combustion, finally to recover as these two influences equal out. All in a very short space of time. Failure to do that by having restriction in delivery jet, then it'll go lean, relatively, and promote "hanging" rpm.
Anther example .... if you're approaching a long sweeping bend and go down a couple of gears, throttle off to near closed, then say it's running 5000 rpm that'll be pulling tgrough the idle jets as vacuum is high, this facilitates it going immediately you then open the throttle, if it didn't do that you'd get a flat spot up there that's really annoying as initially it would be lean. So a much higher influence than most would suspect.
It's also why you get popping on overrun if there's an exhaust header leaking air into system. The fuel that's in excess from moving towards rich with throttle closed passes into the exhaust unburnt in some quantity, availability of leaked air then brings that back to combustible in the hot exhaust pipe, and so you get popping from air leak but supplied by excess fuel from idle jets working correctly.
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The effects described above you can observe, in vacuum terms, when you're running with vac gauges hooked up.
Running at idle, give the throttle a quick blip to lift revs to say 7000 rpm, watch the vacuum drop as the slide comes open, then when you drop it straight away the vacuum hits a high point as slides close with engine rpm still elevated as it's now drawing against a closed venturi.
These are some of the fundamentals that design of carburettor has to work round.
I've referred to fuel injection system sometimes, which may bring the question of "what's that got to do with our bikes ? " to which the answer is, the injection system has to get around the same fuelling idiosyncrasies (it's not working on vacuum to meter fuel ) as carburettor, but with the added detail that each "workaround" has to have a specific method and control regime to achieve. It just shows a bigger picture in examination of fuelling system and their strategy.
The combustion chamber "sees" only fuel air ratio from either system, being effectively blind to what's delivering that mix.
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This is not really in-depth more anecdotal evidence.
"I have little experience with no formal qualifications on bikes, with that disclaimer heres what what I think.
I'm often wrong with a relatively short experience on only two Hondas in the last four years.
I am in my seventh decade of experience on a variety of cars and at the limit of my memory from bikes during the 1960s & 1970s. I do eventually admit when I am wrong.
My perception now seen through the prism of old age & the many posts on this subject is roughly as follows.
These bikes are fitted with four carburetors that at a basic level have two fuel supplies to the engine.
1) The first is what I think of as the idle jet circuit - this supplies fuel not just at idle but at significant amounts as you increase the engine revs with a sort of reducing influence as the main jets take over.
2) The second fuel supply is the main needle & jets that operate by the sliders.
Assumptions - there are no air leaks on the induction side with standard air cleaners, original jets & needles in good condition. When hot the open & closing throttle cable are working & not binding. The ignition advance/retard is working as it should & timing is spot on at idle & full advance settings.
A hanging throttle is confusing as if the sliders fully close when you shut off the throttle the engine rpm should fall relatively quickly back to a normal idle as no fuel is being provided by the main jets as the needle is effectively shutting off the fuel supply.
Contrary to logic I believe a hanging throttle is caused by a lack of fuel or conversely too much air supply - a weak mixture at that point as the revs should fall instead they take a while to drop from say 2,200 rpm to 1200 rpm.
So how does the engine manage to maintain this hanging throttle?
I think it is as a result of a tad too much air passing through the intake under the sliders when the throttle fully closed, this draws fuel via the idler jet circuit as well as an unknown amount from the main jet in some situations.
This might be as a result of 1.2.3 or all 4 sliders being too wide open in the throttle closed position.
It's possible to think you have the air screw spot on when you do the bench sync by reducing the fuel flow by opening the air screw you can get an a false idle. When the engine is up to full running temperature this in my view points to the slider gaps being just that bit too wide. So it's all good when warm but after a fair run the idle gets too fast.
If when the main throttle stop screw is completely undone the sliders are fully closed then as no petrol can get through it will of course not even idle but stop.
I think the trick is getting the least amount of slider gap yet maintaining a tickover contolled by the air screw adjustment only - on a engine that is up to temperature.
I'm waiting for incoming - my shields are up !
Just edited some poor spelling & sentence structure for clarity
Modify message
« Last Edit: October 02, 2024, 03:41:04 AM by McCabe-Thiele (Ted) »
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This is really confusing me,
But that's easy,
I recently did some work on my 500,nothing to do with the carbs,
And fitted new carb to head rubbers as the old ones were hard,
Since then, I have had a hanging throttle,
It runs well when cold, but as it warms up ,if I blip the throttle it zooms up to about 4,000rpm,
It can be slowed by pressing the large nut in the linkage,
But why?
I haven't touched the settings,
Help and advice needed
I have the carb sync gauges connected, all read the same,
Sent from my SM-A546E using Tapatalk
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My suspicion would fall on what's been changed could be air being sucked in between the carbs & the engine I guess you have thought of that ?
Did you remove the alloy part of the inlets or just change the rubbers?
What large nut are you reffering to - the main throttle adjustment screw?
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My suspicion would fall on what's been changed could be air being sucked in between the carbs & the engine I guess you have thought of that ?
Did you remove the alloy part of the inlets or just change the rubbers?
What large nut are you reffering to - the main throttle adjustment screw?
I renewed the 4 rubber tubes between the carbs and manifolds,
The 4 large nuts,are In 2 pairs between carbs 1 and 2,and 3 and 4,
Part of the actuation mechanism,
Sent from my SM-A546E using Tapatalk
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It must be something to do with Herefordshire air John
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My suspicion would fall on what's been changed could be air being sucked in between the carbs & the engine I guess you have thought of that ?
Did you remove the alloy part of the inlets or just change the rubbers?
What large nut are you reffering to - the main throttle adjustment screw?
I renewed the 4 rubber tubes between the carbs and manifolds,
The 4 large nuts,are In 2 pairs between carbs 1 and 2,and 3 and 4,
Part of the actuation mechanism,
Sent from my SM-A546E using Tapatalk
If pressing down on the Brass coloured nuts reduced the idle speed then assuming your throttle cables are adjusted correctly and the cable is allowing the throttle the mechanisms to close to their bench set position then that appears to be an issue with the slider gaps being to wide imho.
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This is really confusing me,
But that's easy,
I recently did some work on my 500,nothing to do with the carbs,
And fitted new carb to head rubbers as the old ones were hard,
Since then, I have had a hanging throttle,
It runs well when cold, but as it warms up ,if I blip the throttle it zooms up to about 4,000rpm,
It can be slowed by pressing the large nut in the linkage,
But why?
I haven't touched the settings,
Help and advice needed
I have the carb sync gauges connected, all read the same,
Sent from my SM-A546E using Tapatalk
When the throttle is closed, can you "find" any slack in the exposed cable where it arrives at the carburettor quadrant ? Indication of the twistgrip and cable being not the problem.
Something very small, the liner inside the throttle cable where if turns out of the twistgrip, the inner cable eventually cuts through that 90 degree turn making it non linear in operation.
With the cable removed from bike, and all the inner pulled out the twistgrip end, you can usually pull that little liner out of the cable cover to inspect it. If it's cut through , it makes them quite variable in both actuation and return function.
Honda cables were particularly good in their materials, not so much in reproduction stuff though.
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This is really confusing me,
But that's easy,
I recently did some work on my 500,nothing to do with the carbs,
And fitted new carb to head rubbers as the old ones were hard,
Since then, I have had a hanging throttle,
It runs well when cold, but as it warms up ,if I blip the throttle it zooms up to about 4,000rpm,
It can be slowed by pressing the large nut in the linkage,
But why?
I haven't touched the settings,
Help and advice needed
I have the carb sync gauges connected, all read the same,
Sent from my SM-A546E using Tapatalk
When the throttle is closed, can you "find" any slack in the exposed cable where it arrives at the carburettor quadrant ? Indication of the twistgrip and cable being not the problem.
Something very small, the liner inside the throttle cable where if turns out of the twistgrip, the inner cable eventually cuts through that 90 degree turn making it non linear in operation.
With the cable removed from bike, and all the inner pulled out the twistgrip end, you can usually pull that little liner out of the cable cover to inspect it. If it's cut through , it makes them quite variable in both actuation and return function.
Honda cables were particularly good in their materials, not so much in reproduction stuff though.
The cables are fine,
I even use the "closer" cable to try to drop the revs,
Sent from my SM-A546E using Tapatalk
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A classic fault finding split would seem necessary to progress.
Mechanical restriction, giving non reliable return .... or .... mixture derived lean running that promotes raised rpm with throttle closed.
Obvious to say, but something must have changed to observe a clear resulting effect.
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This is not really in-depth more anecdotal evidence.
"I have little experience with no formal qualifications on bikes, with that disclaimer heres what what I think.
I'm often wrong with a relatively short experience on only two Hondas in the last four years.
I am in my seventh decade of experience on a variety of cars and at the limit of my memory from bikes during the 1960s & 1970s. I do eventually admit when I am wrong.
My perception now seen through the prism of old age & the many posts on this subject is roughly as follows.
These bikes are fitted with four carburetors that at a basic level have two fuel supplies to the engine.
1) The first is what I think of as the idle jet circuit - this supplies fuel not just at idle but at significant amounts as you increase the engine revs with a sort of reducing influence as the main jets take over.
2) The second fuel supply is the main needle & jets that operate by the sliders.
Assumptions - there are no air leaks on the induction side with standard air cleaners, original jets & needles in good condition. When hot the open & closing throttle cable are working & not binding. The ignition advance/retard is working as it should & timing is spot on at idle & full advance settings.
A hanging throttle is confusing as if the sliders fully close when you shut off the throttle the engine rpm should fall relatively quickly back to a normal idle as no fuel is being provided by the main jets as the needle is effectively shutting off the fuel supply.
Contrary to logic I believe a hanging throttle is caused by a lack of fuel or conversely too much air supply - a weak mixture at that point as the revs should fall instead they take a while to drop from say 2,200 rpm to 1200 rpm.
So how does the engine manage to maintain this hanging throttle?
I think it is as a result of a tad too much air passing through the intake under the sliders when the throttle fully closed, this draws fuel via the idler jet circuit as well as an unknown amount from the main jet in some situations.
This might be as a result of 1.2.3 or all 4 sliders being too wide open in the throttle closed position.
It's possible to think you have the air screw spot on when you do the bench sync by reducing the fuel flow by opening the air screw you can get an a false idle. When the engine is up to full running temperature this in my view points to the slider gaps being just that bit too wide. So it's all good when warm but after a fair run the idle gets too fast.
If when the main throttle stop screw is completely undone the sliders are fully closed then as no petrol can get through it will of course not even idle but stop.
I think the trick is getting the least amount of slider gap yet maintaining a tickover contolled by the air screw adjustment only - on a engine that is up to temperature.
I'm waiting for incoming - my shields are up !
Just edited some poor spelling & sentence structure for clarity
Modify message
« Last Edit: October 02, 2024, 03:41:04 AM by McCabe-Thiele (Ted) »
While I can see the reasoning, I think this deserves expansion to understand.
The amount of fuel added to the air going under the slide at idle us adjusted by the routine that Honda gives for setting airscrew idle mixture. In other words it can't be wrong if its set correctly.
It can go wrong if set badly, or IF the idle circuit, both jet and screw controlled air passage, change after its been correctly set.
In effect, you can't get too much gap under the slide as it should always be proportionately accompanied by the correct amount of fuel. This is exactly what the idle circuit is for.
I intend discussing in carb thread, when I get to this bit
That Honda routine specifically targets fuel air ratio delivered by the idle circuit against the set position of the slide. If that setting then becomes inappropriate, the mixture, for that cylinder, moves out and away form how you've set it and usually lean with the fuel jet orifice being compromised. So you do get too much air, but not from overall position of slide, but failure of delivery fuel jet to maintain flow at defined rate.
To make clear , and I'll shout this bit :) THERE'S NO REQUIREMENT TO SET THE AIRSCREW EQUALLY ..... THE HONDA METHOD VERY SPECIFICALLY TARGETS PARITY OF COMBUSTION AND ACCEPTS DIFFERENT SETTINGS FOM CARB TO CARB ON THE SAME ENGINE.
If you carry out the setting routine to the manual description, it will "show" you if there's any impairment in that specific circuit by needing a setting that just doesn't fit the expected.
It does this by you making observations of the fuel air ratio as you turn the screw and listen to the combustion.
That must be one of the most under used parts of their manuals .... but so important in setup.
Everyone goes straight to carb synchronization, missing the substance and reasoning contained within this little routine that they give.
(Copied over from another thread to facilitate here)
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I must be missing it but I can't find the air screw adjustment procedure in the shop manaul
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I must be missing it but I can't find the air screw adjustment procedure in the shop manaul
Unsure of location in manual, but usually it's within a carburettor strip and rebuild section as it deals with optimisation after reassembly. Thats rather than service section.
I haven't viewed all of the manuals, but it's definitely contained in the mix I have.
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To describe the idle setting procedures is worthwhile here to give widened overview.
Caveat:- it's necessary to ensure all the other influences are correctly in specification to make this adjustment valid, ignition, sync, plugs, fuel supply etc. A refined "bench" synchronization is absolutely ideal for this purpose as it introduces no extraneous tolerance of gauge or method into the assembly of carburettor component.
Starting with a base setting of installation in adjustment screw, that 1 1/2 type setting, gives competent running to begin.
Then it requires (working one at a time, for each individual carb) to adjust the screw in giving weaker mixture until you here that cylinder start to falter with some missing beat/firing. This should be indication of the mixture now approaching and then breaching stoichiometric balance. This happens when fuel molecule mix with air/oxygen at 14.7 parts O2 to 1 part fuel. Below which leaves fuel unburnt, above which stops reliable combustion from taking place because there's not enough fuel to burn in tbat mix.
It's effectively using you and the observance of that cylinder combustion as a O2 sensor that you'd expect in a fuel injection setup.
At that point, it directs to then bring the mixture back towards richer mixture by observing a 100 rpm drop in crankshaft speed. This is now the reference point for that cylinder.
Now to correct the overall engine speed if its too high or low with the master idle control screw (the one that winds the whole rack of carb slides up and down) then repeat the same setting process for each successive cylinder to give quantified position on all of the carbs/cylinders.
This routine has now calibrated all of your idle circuit,,taking into account the hardware (bore in carb, jets, screw, vacuum available, fuel content etc.
Its likey, but I've not measured it, that the resulting fuel air ratio at idle will be between 12.7 & 13.7 ~1 ratio that gives the conditions for smooth idle. Aso the ability to bring stable return to idle when throttle is released.
If the normal adjustment on screws on each carb doesn't yeald satisfactory response (usually within 3/4 to 2 3/4 turns out) that shows that something inside that idle circuit design is not as it should be.
Within these idle routing and components, something is causing the irregularities that you are trying to now adjust into range. Usually without successful outcome.
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I must be missing it but I can't find the air screw adjustment procedure in the shop manaul
Unsure of location in manual, but usually it's within a carburettor strip and rebuild section as it deals with optimisation after reassembly. Thats rather than service section.
I haven't viewed all of the manuals, but it's definitely contained in the mix I have.
I found an old post of yours where you explained it very nicely so I'll put it into practice.
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This is really confusing me,
But that's easy,
I recently did some work on my 500,nothing to do with the carbs,
And fitted new carb to head rubbers as the old ones were hard,
Since then, I have had a hanging throttle,
It runs well when cold, but as it warms up ,if I blip the throttle it zooms up to about 4,000rpm,
It can be slowed by pressing the large nut in the linkage,
But why?
I haven't touched the settings,
Help and advice needed
I have the carb sync gauges connected, all read the same,
Sent from my SM-A546E using Tapatalk
When the throttle is closed, can you "find" any slack in the exposed cable where it arrives at the carburettor quadrant ? Indication of the twistgrip and cable being not the problem.
Something very small, the liner inside the throttle cable where if turns out of the twistgrip, the inner cable eventually cuts through that 90 degree turn making it non linear in operation.
With the cable removed from bike, and all the inner pulled out the twistgrip end, you can usually pull that little liner out of the cable cover to inspect it. If it's cut through , it makes them quite variable in both actuation and return function.
Honda cables were particularly good in their materials, not so much in reproduction stuff though.
The cables are fine,
I even use the "closer" cable to try to drop the revs,
Sent from my SM-A546E using Tapatalk
Just an update,
Have adjusted airscrews to 1 1/2 ,
still the same,
But spits back through the carbs when cold,
I had worked on the head, cam etc,
So may have not timed the cam accurately
Will lift the lid and check
Sent from my SM-A546E using Tapatalk
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Ooooh, that wouldn't help :o if there's error there John.
Definitely best to check and eliminate a fundamental like that.
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I suspect if you were a tooth out on the Cam it would be very obvious.
You may already know there is a realtively easy way to check valve timing iirc Bryan posted on the subject on my 400 project.
Bryan has explained it reproduced below - with plugs out.
Get all the tappet covers off, as the mark approaches 1-4 TDC or T watch the valves on number 1 and number 4 cylinders, one of them will have both valves going down/up, the other will have them both up, the one with both up is the one to do. If you grab a tappet adjuster you can rattle the one on compression stroke, if it's number 1 (and it's a 50/50 chance of it being that) then after doing the tappets turn the crank (clockwise, always clockwise) then as 2-3 T mark comes up you do number 2 cylinder, turn again to 1-4 and do number 4 and lastly turn again to 2-3 T and do number 3 cylinder. The firing order is 1-2-4-3.
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They'll "run" and without contact I think, with 1 to 2 teeth out in my understanding.
Not that you'd want to do that, just that it could be done without catastrophic contact of the wrong components.
Quite possible and needing a check definitely to avoid the resulting running shift that's in place if that were the cause.
Additionally, if tbe cam were to be run advanced the effect should be to move the torque higher at the expense of crap idle symptoms ;D so, a possible cause that needs to be eliminated.
Classic fault finding ..... ask what was changed between working and not. Carb rubbers to head were, but have been checked for leaks. Carburettor setup, no. Is there something else that could plausibly have changed that can affect it, yes. How do we now check and verify that ?
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Problem solved
The cam was one tooth advanced
Silly me
Running sweetly now, but it has some new carb rubbers
Lessons learnt
A simple error can result in a red herring looking for a fault that does not exist
Sent from my SM-A546E using Tapatalk
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Glad you have sorted John!
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Glad you have sorted John!
Thank you, so am I
Just intime to enjoy a sunny afternoon ride out
PS,it needs a good clean (https://uploads.tapatalk-cdn.com/20241010/fb24fbc560ef76a307eca30fc8ddc10d.jpg)
Sent from my SM-A546E using Tapatalk
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Considering it’s probably one of the most used classics on this forum (unless i am mistaken) it’s probably not too bad!😂
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Good you've got it sorted John and easy to get something wrong too.
I check these things so many times (cambelts too) that I start to question myself and if you're seeing something right or wrong as you keep looking at it again and again after rotating the engine :)
At least we know the affect that's resulting from this definitively now for the forum ;D ;D
No harm done anyway.
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Copied here from another thread as it adds to this topic.
"As Ted has noted it in another thread, thought I'd add something to the check list.
When it's "hanging" try gently bringing the choke into operation and see if that's going to slow down the idle.
But also don't forget the basic reality check of all cylinders firing, indication of operational idle fuel jets etc (even if they've been cleaned) do this by stating it cold, run 30 sec, then switch off. Now check exhaust headers for temp and any cold cylinder. Cheap,and easy to do, a cold one and it'll never have competent idle running, whatever you do or adjust.
As noted in thread too with John's experience, they'll run with camshaft timed out by a tooth, but you'll get problems again getting it to run correctly."