A call to all Carb experts

Hi all,

Having some trouble with my Yamaha XS750, the classic “it run when parked” story.
The bike had sat for 10 months or so, running perfect before, after a major rebuild, bored cylinders, new pistons and rings, complete head job, carbs rebuild and all.

Engine started right away but once I closed the choke the idle was very lumpy and the bike drove like a dog.
Took the carbs out, gave them a clean, no visible issues, put them back but same same.
Took them out again, did more of a clean and concentrated at the idle/pilot jet.

Now the idle is fine but a have a serious hesitation/misfire on low pm (off idle) when driving.
After 2.000 rpm works perfect and doesn’t miss a beat.
Carbs are Mikuni RS38, synched but pilot/air screws are adjusted by ear.
Puling the choke does not make any difference.

Any ideas besides the obvious take the carbs out again?


Have two 38 Mikunis on two 77 and one 79 XS 650.

When they sit the low speed jet for the idle circuit gets varnished. I have found a needle nose vise grip pulling out a single strand from a steel wire brush will get you a serviceable jet cleaner.

Other possibility is a pinhole or small crack in the slide diaphragm.

Mike’s XS stocks many carb parts. Good luck.

Thanks for the reply Jim.

I tried the trick with the very thin wire, cleaned every little hole and passage, mounted the carbs without the filter box, had a test drive and all was well… mounted back the filter box (big PITA) all well for the first ten minutes and then the problem returned… one cylinder is not idling… maybe some piece of dirt that’s dislodged and blocks some tiny hole? Don’t know but it’s very frustrating.

I guess I have to take the carbs out (again) to investigate further.
This time I am borrowing an ultrasonic cleaner to clean them.
Diaphragms are relatively new but will check them again.