SOHC.co.uk Forums > Project Board
Doms CB550F1 project
K2-K6:
--- Quote from: DomP on October 02, 2024, 06:47:06 PM ---
--- 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
--- End quote ---
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:
--- Quote from: Sesman on October 03, 2024, 06:41:48 PM ---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.
--- End quote ---
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?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version