Revs rising on tickover since fitting new fuel tap -weird

[paulbaker1954]

Just fitted new fuel tap and did some fiddling with float heights

Now when I put bike on side stand revs are rising to around 2k

Anyone any theories?

[deltarider]

... and did some fiddling with float heights

Why?
Higher revs ai idle usually indicate leaner (less rich) running.

[Bryanj]

Or incorrect cable routing pulling tight

[McCabe-Thiele (Ted)]

Change in float level will alter the mixture - might need air screw adjustments /  rebalance the carbs.

[deltarider]

...  rebalance the carbs.

Not necessary as balance hasn't changed.

[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)]

...  rebalance the carbs.

Not necessary as balance hasn't changed.

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]

...  rebalance the carbs.

Not necessary as balance hasn't changed.

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.

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.

[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)]

Thanks for all the replies and physics theories, interesting reading

All sorted now by rebalancing carbs and resetting idle screws

Ticking over lovely again 👍👍👍

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?