SOHC.co.uk Forums > Project Board
Doms CB550F1 project
DomP:
--- Quote from: K2-K6 on October 02, 2024, 06:12:01 PM ---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 ?
--- End quote ---
Only time I'm getting no firing on a cylinder is because the plugs soot up
K2-K6:
--- Quote from: McCabe-Thiele (Ted) on September 30, 2024, 02:05:18 PM ---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
--- End quote ---
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:
--- Quote from: davidcumbria on October 03, 2024, 08:15:22 AM ---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.
--- End quote ---
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):
--- Quote from: davidcumbria on October 03, 2024, 08:15:22 AM ---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.
--- End quote ---
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version