SOHC.co.uk Forums > CB500/550
Revs rising on tickover since fitting new fuel tap -weird
MrDavo:
I think messing with the float heights is the culprit. The fact it happens on the side stand points to this also as leaning the bike will interfere with the fuel levels - a carb where the level is marginally high can overflow once tilted, the mixture goes crazy and so do the revs.
When I first got my CL450 barn find running the revs were all over the place at tickover.The only way I sorted this out (I now have a rock solid even 1000 rpm tickover) was to exactly follow the factory manual procedure for setting the float heights, which included making a cardboard float height template and taking the carbs off to hold them vertically. It was a pain in the butt to do, but the results were 100% better than my previous trial and error guesswork.
I'm not sure of the procedure for your bike, but the factory manual will have what you need to know.
K2-K6:
I've got the floats being tipped at an angle as self compensating, which comes from them essentially having float mass distributed to either side of the jets. In effect it "should" neutralise the level as aggregated normal when leaning the bike to either side.
Changing float height though will change the jetting at idle, you'd have to reset idle airscrew and subsequently the main tickover position.
It shouldn't vary from standing upright to leaning on side stand though. Some variable seems to be there.
As already mentioned, check the operating slack in throttle pull cable to make sure you've not missed something fundamental.
McCabe-Thiele (Ted):
--- Quote from: deltarider on October 20, 2020, 03:43:18 PM ---
--- Quote from: Macabethiele on October 20, 2020, 01:07:11 PM ---... rebalance the carbs.
--- End quote ---
Not necessary as balance hasn't changed.
--- End quote ---
I beg to differ if you change the float level on one carburettor that will alter the mixture on one cylinder slightly hence some imbalance. Unlike some multiple carburettor car engines there is no balancer manifold arrangement. IMHO.
deltarider:
Mixture doesn't matter much as all pistons are on one and the same crank, so they're all forced to go down, creating underpressure, even if mixture is not equal. Long time ago, I've experimented with this and never saw the vacuum change a bit, not on a 4 cyl. bike.
K2-K6:
--- Quote from: Macabethiele on October 20, 2020, 06:33:28 PM ---
--- Quote from: deltarider on October 20, 2020, 03:43:18 PM ---
--- Quote from: Macabethiele on October 20, 2020, 01:07:11 PM ---... rebalance the carbs.
--- End quote ---
Not necessary as balance hasn't changed.
--- End quote ---
I beg to differ if you change the float level on one carburetta that will alter the mixture on one cylinder slightly hence some inbalalance. Unlike some multiple carburetta car engines there is no balancer manifold arrangement. IMHO.
--- End quote ---
I'd hope to offer a persuasive argument to the contrary. It's because there is no "balancing" manifold on these engines that the setting of the slides has to be made for air volume parity, but not for tickover.
Slide parity is an absolute for maximum engine demand, with tickover running a byproduct of having that set correctly.
It's not the method to "set" idle parity from cylinder to cylinder though as it has nothing near the fine resolution to do so. That is (given in the workshop manual right from the first 750 sohc) the idle airscrew, which does have the resolution to incrementally adjust each cylinder to match.
It's so often given on many forum that a carb synchronisation could cure something or other, but the only thing it can cure from a technical perspective is the slides set wrong in the first place.
So if the "sync" was correct, then it will still be correct.
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