SOHC.co.uk Forums > CB500/550
Revs rising on tickover since fitting new fuel tap -weird
McCabe-Thiele (Ted):
Okay, I follow your reasoning but am I wrong in thinking that a change in float level on an individual cylinder will also affect the mixture at idle as the fuel pressure head to the idle jet will have changed - thus making it run richer or weaker thus affecting the idle speed especially if more than one cylinder has been affected. I see the logic that once running on the main jet the effect would be less.
The post did not mention how many float levels were adjusted but it appears to be plural.The consequence of this I would have thought could result in an increase in overall engine rpm. The consequence of an increase in mixture/ rpm on one or even two cylinders would then affect the air flow through the remaining cylinders would it not? (assuming no air leaks)
I'm starting to get lost in my own logic - I accept that sometimes in defiance of all rules engines can do some strange things. I particularly liked the idea that on it's side stand all carbs would be equally affected by a change in float level. The post is a little ambiguous as it starts to read that the new fuel tap caused the increase in idle speed whereas it was more likely that it was a result of changing float level(s).
Back in the early 1970's the Vauxhall Viva Brabham conversion kits were plagued with hot engine idle speeds as were the early MG Maestro models - in both cases it was caused by fluctuating inlet manifold temperatures. Both manufacturers cured the issue by introducing heated inlet manifolds!
K2-K6:
"Okay, I follow your reasoning but am I wrong in thinking that a change in float level on an individual cylinder will also affect the mixture at idle as the fuel pressure head to the idle jet will have changed - thus making it run richer or weaker thus affecting the idle speed especially if more than one cylinder has been affected. I see the logic that once running on the main jet the effect would be less."
Yes, exactly. If a float level in altering is the cause, then it follows that it's this that should be corrected. That's if it's been altered away from spec.
If it were the float "out of spec" in the first place, and now correct, then it would have been the airscrew was set to accommodate that, and should now be reassessed as to it's position in now working with a within specification float height.
As you write though, if individual floats are changed it will consequently then shift the idle mixture, for that cylinder. The amount on these engines are very small, any (however small they are perceived) shifts will change that cylinder firing. The carbs are, when components are in good order, deadly accurate, but of course you have to diagnose the right thing to change.
deltarider:
It's the piston going down that is causing the underpressure in the intake manifold. That individual piston - or rather the composition of the mixture in its cylinder - doesn't have much say, as it is forced down anyway by the other three pistons it shares the crank with. But... I'll welcome findings by you when they prove different from what I have seen back then.
paulbaker1954:
Thanks for all the replies and physics theories, interesting reading
All sorted now by rebalancing carbs and resetting idle screws
Ticking over lovely again 👍👍👍
McCabe-Thiele (Ted):
--- Quote from: paulbaker1954 on October 22, 2020, 08:12:58 PM ---Thanks for all the replies and physics theories, interesting reading
All sorted now by rebalancing carbs and resetting idle screws
Ticking over lovely again 👍👍👍
--- End quote ---
I'm curious as to what you did in what order. I understand adjusting the idle air screw but what did the rebalancing consist of - was it adjusting an individual slider height?
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