RPM's not dropping / rising on it's own.

Since I had a bike mechanic rebuild the four carbs on my XJ650, after the engine warms up the RPM's will climb to 4000+ even though I'm not touching the throttle.
While riding, the RPM's will not drop when I release the throttle and clutch to change gears. While approaching a stop, the RPM's can climb so high that I have to kill the engine.
I've heard the problem could be a faulty TCI module, bad butterfly valves, or air leaking through the intake boots. The problem only manifests itself after the bike has had time to warm up. Idling is not a problem for the first few minutes, and then the RPM's start to climb out of control.

Any suggestions where to start troubleshooting? I can't ride the bike the way the engine is behaving now.

Tony

 

Simple answer is take it back to the “bike mechanic”...
As to the issue, forget the tci, you have an air leak, bad synch or bad mixture setting. The procedure for diagnosing and setting each of them can be found on this site, but really, I would ask your mechanic to go through all this. It’s not hard, but you have a bit of learning to do if you want to diy this.

 

Yup vacuum leak, or idle mixture. For sure take it back. Doing your own rebuild with a Haynes manual and YouTube (Mezzmo Engineering part 20 carburetor rebuild.. is very explanatory) next time around is both rewarding and educational. Even if you watch the video before taking it back, you will have a bit more knowledge . Synchronization of the carbs is challenging but once you get the hang of it it is easy. You'll need some gauges and a YICS blanking tool for the procedure. Once you get that baby rumbling smooth you'll feel like you're on top of the world. Xj4ever supporting vendor has a ton of advice. Either way all the best.

 

For the hanging idle related to idle mixture, is it too rich or too lean that causes the engine to idle high on its own? Having the same problem with mine, but before I go back through the carb rack, idle mixture seemed like an easy one to check off.

 

a lean mixture will idle high. Tweak the screws out a bit.

 

Thanks Simmy, I'll try that. I know has to be one of two things, and it should run right.

 

I"m betting the "bike mechanic" was not familiar with this carb set and/or did not spend any time researching the carbs.
They could have:
Not put the butterfly valves back in right either mixing the plates up or when installing the plates, not making sure they move freely and close enough.
Not put the springs back on that shut the valves back down.
Not used vacuum seals on the butter fly valves but used oversized O rings that is adding friction into the valves, not letting them spring back easily and all the way down.
Did something wrong when reassembling and the diaphragms are pinched or something.

Its been mentioned many times you can test for vacuum leaks with some carb start spray on the boots.

I have spent more time with 2 cycle engines and find a slightly rich idle works best. I'd move it till it is rich enough for the RPMs to drop, then turn it lean a tad. With 2 cycle RC airplanes, we set the mixture screws every day on the first flight.