1979 CB750, Carb, float height, run on

[nighthawking]

Hey all, odd issue I think is carb related. Bike pops occasionally at fast idle, but general runs well with stock air and 4 into 2 with basically no baffles. Mains 68 and 105. At speed, say 30mph, if you let off the throttle, pull in the clutch completely, revs go down to idle fine. Still coasting, when you brake to stop, engine revs up as if it is getting fuel somehow, and you have to let out the clutch to bring the rpms back down and you can sit at the stop in idle. I thought it was float height too high related, but pulled the carbs and found the floats too low at 20mm or so and not all the same. Is the float height measured at the highest part of the float from the body? Has the run on happened to anybody? It is not cable related as this happens going in a straight line. Thanks.

 

[Travis]

Yeah that sounds like a weird problem. Usually you see it the other way around where the idle increases when you pull the clutch. For the float height, You measure from the top of the floats to the flange of the body. 15.5mm. I guess it's been about a week since you posted so have you figured anything out?

 

[nighthawking]

I set the floats at 15-16mm, they were around 19-21mm. Waiting for fuel screw O-rings and will re-install. What do you think about the 105 jet with the 4 into 2 hardly any baffle exhaust?

 

[Travis]

No idea about the jetting. Sorry!

 

[K2750tracy]

Check for any vacuum leaks around the intake boots and the carb butterfly shafts. vacuum increases at idle and a leak will pull more air into system actuating the cv slides and increase fuel flow. just delt with this on a gl1000, found leak at 2 of the shafts. spray starting fluid around these spots with a tube attached to spray tip to pinpoint the location. be carefull around your paint. by the way cv carbs suck.

 

[Travis]

Wow I just re-read the original post and realized I completely read it wrong. So my first response doesn't really make sense because what your bike is doing it not weird.. I thought you were saying the revs increased under load and decreased at idle with the clutch pulled. I agree with K2750tracy that it's probably a vacuum leak. I've had a few bikes act the same as yours and it's almost always either the throttle shaft seals or the carb holders.

 

[nighthawking]

Thanks, while I've got the carbs off I'll do the boots. Do the rubber plugs in the carbs have anything to do with it?
It seems as if it is a sloshing gas thing, as the idle at speed is ok. It only surges when you hit the brakes to stop.

 

[K2750tracy]

they should still be plyable but not soft.

 

[dirtdigger]

The rubber plugs you are referring to are the plugs that are over the main jets correct? The do need to be pliable and they need to fit snug. If they are hard and not sealing it will cause rich condition and erratic fuel mixture.