Carburettors with more depth of discussion

[DomP]

I must be missing it but I can't find the air screw adjustment procedure in the shop manaul

[K2-K6]

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.

[K2-K6]

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.

[DomP]

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.

[Johnwebley]

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,





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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,



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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

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[K2-K6]

Ooooh, that wouldn't help  if there's error there John.

Definitely best to check and eliminate a fundamental like that.

[McCabe-Thiele (Ted)]

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.

[K2-K6]

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  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 ?

[Johnwebley]

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

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[Johnny4428]

Glad you have sorted John!

[Johnwebley]

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

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[Johnny4428]

Considering it’s probably one of the most used classics on this forum (unless i am mistaken) it’s probably not too bad!😂

[K2-K6]

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   

No harm done anyway.

[K2-K6]

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."