Topic: CB750K1 two cylinders not firing at low RPM but fine when RPM's come up.

[SoCalHarley]

I have a K1 that I did a cafe build about 5 years ago and in the process rebuilt all of the carburetors, ignition etc.  There has been limited usage of the bike due to moves etc.  The bike easily starts and revs up but the right two cylinders lag firing until the RPM's come up.  At higher RPM the bike runs perfectly and makes great power with no missing.  I am thinking it is a carburetor issue likely due to possible varnishing from sitting and lack of use.  What would keep the carbs from performing at low rpm but work perfectly at higher rpm and under throttle.  Thanks for any direction and where to start. 

[seanbarney41]

plugged pilot jets

[ekpent]

And if they are plugged you can get to them without pulling the carbs but its a little fiddly. Love those snap on bowls of early 750 carbs.

[SoCalHarley]

Thanks for the help.  I have a diagram and the shorter of the two is indicating the "slow" jet and the taller is the main jet.  I am assuming the pilot jet the slow jet?  What do you suggest the best way to clean them?

[BenelliSEI]

Correct. Soak them in a bit of carb clean and blow with air until you can see through. Don’t be tempted to ram something through. That can destroy the shape and they not longer atomize (spray) properly.

[SoCalHarley]

Well that was it.  But now I have a low rpm back fire I bet as a result of trying to push a pin in there to clean them out.  I may have damages them....  Would that cause the backfire?  Anyway where do you guys recommend a place to buy them from?  Thanks again

[Don R]

 It could be rust on a valve seat from sitting. It should wear off with a little running.

[SoCalHarley]

Drained the gas and put in new.  Definitely better and not really all that bad.  It didn't do this until I messed with the pilot jets so was thinking maybe this was it.  I'll take her out and run her for a while and see how it goes.  It runs great except for that bit of popping.  I think I may still get a new set of pilot jets just to be sure. 

[TwoTired]

I would do the entire 3K mile tuneup before changing parts.  Amazing what that will fix.

Cheers,