Doms CB550F1 project

[DomP]

If you start it from cold, run it 30 sec, switch off and then check the header pipes for temperature (basic spit on your thumb and touch will do) what have you got ?

Any cold ones  ?

Only time I'm getting no firing on a cylinder is because the plugs soot up

[K2-K6]

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

A good synopsis Ted. I think some parts need further discussion to clearly establish exact relationship and effects.

Probably another thread would be a good idea, else it gets buried in more general information like this.

[davidcumbria]

I have come to the view that with the twists and turns it may be nigh on impossible to clear a partially blocked air screw circuit so it doesn’t work as intended. Playing tunes with smaller pilot jets in affected carbs might be the solution as it was for my sooty number 1 cylinder. This could play into the hanging idle scenario.

[K2-K6]

I have come to the view that with the twists and turns it may be nigh on impossible to clear a partially blocked air screw circuit so it doesn’t work as intended. Playing tunes with smaller pilot jets in affected carbs might be the solution as it was for my sooty number 1 cylinder. This could play into the hanging idle scenario.

This is ultimately the cause as I understand it. Thats dependent on how "hanging" idle description is observed and applied though. IF there's another cause possible, ignition advance, plugs, mechanical impairment of cables etc, that has to be eliminated before this status can be emphatically linked to fuelling.

With the idle circuit being the only fuel supply and metering route at closed carburettor slide position, then it must have a reciprocal relationship with the air going under the slide. Failure of that relationship in a stable accumulation of fuel as the slide moves up will give the variance we see in mixture, leading to below or above target air fuel ratio given.

[McCabe-Thiele (Ted)]

I have come to the view that with the twists and turns it may be nigh on impossible to clear a partially blocked air screw circuit so it doesn’t work as intended. Playing tunes with smaller pilot jets in affected carbs might be the solution as it was for my sooty number 1 cylinder. This could play into the hanging idle scenario.

That's an interesting observation David that makes complete sense, a practical solution. After I had cleaned my carbs in the hot ultrasound bath I casually squirted some releasing fluid through the idle jet followed by some canned compressed air a mate gave me. I can see a situation where a part of the passage might be reduced in size by corrosion that will not budge by normal cleaning.

[K2-K6]

If you start it from cold, run it 30 sec, switch off and then check the header pipes for temperature (basic spit on your thumb and touch will do) what have you got ?

Any cold ones  ?

Only time I'm getting no firing on a cylinder is because the plugs soot up

Are you absolutely certain, by doing this test it will at least show a confirmation.

The effect, if one cylinder is not running reliablly when throttle is closed, is that the main rpm adjustment is made to effectively accommodate only three cylinder pulling the crank around, making it too high IF four are running when you drop the twistgrip, to later go to three and have corresponding drop in rpm .... can be determined as a hanging idle.

The reason I check for this is that many which I look at have this fault with the owner unaware that it existed.  Then you're back to cleaning idle circuit passages etc 

[DomP]

Ah I see what you're saying, I'll give it a go this weekend, I do sometimes get a bit of a pop that sounds like a small backfire which this could explain.  I have fully striped and cleaned the carbs with the original keihin jets for a 550f which Ken helped me out with when one was a bit crushed, every jet was clear and carb cleaner is spraying through the passages.  I didn't however use an ultrasonic cleaner

[Sesman]

Appreciate the “hanging idle” discussion, but does this explain sooty plugs? I thought the idle jets played an insignificant role above 1500 rpm or so.I appreciate there is an element of transition phase.

[K2-K6]

Appreciate the “hanging idle” discussion, but does this explain sooty plugs? I thought the idle jets played an insignificant role above 1500 rpm or so.I appreciate there is an element of transition phase.

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.

I do think it needs more dedicated thread that can be referenced rather than difficult to search within another topic.

[McCabe-Thiele (Ted)]

Why don't you start a new post yourself under tricks & tips or do you want me to do an opener?

[K2-K6]

Why don't you start a new post yourself under tricks & tips or do you want me to do an opener?

I don't mind Ted, conscious of not wanting to muck up Dom's project thread,  but obviously it'll be useful to have discussion containing this topic.

I'll start one in the anorak section and see how we get on.

[DomP]

Im now beginning to realise how much I don't know or understand about carburetors,  thanks again chaps, I need to read that all over again slowly to try and get my grey matter around it all.  Much more going on than I thought.

[Laverdaroo]

Phones always on Dom if you need confusing or just need a rant or even just to offer a theory mate
I know how frustrating it can be buddy


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[Sesman]

Appreciate the “hanging idle” discussion, but does this explain sooty plugs? I thought the idle jets played an insignificant role above 1500 rpm or so.I appreciate there is an element of transition phase.

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.

I do think it needs more dedicated thread that can be referenced rather than difficult to search within another topic.

Err….sooty plugs?

[DomP]

Phones always on Dom if you need confusing or just need a rant or even just to offer a theory mate
I know how frustrating it can be buddy


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Thanks Roo, you're a gluten for punishment mate😂