RPMs jump to 3500 when hot

I have been working on a 1982 XJ750 Maxim for the past month and took it for the first test ride this weekend. It starts up great and idles just above 1,000 RPMs. After about 10 minutes of riding the RPMs jumped up to the 3,500 range when in neutral or when I pull in the clutch. I made sure all the carb boot clamps are tight. Anyone have any insight on why this may be happening? Thanks for the help.

 

I could be a few thing. Have you synced your carbs yet?

 

Yes. I had the carbs rebuilt and sync'd.

 

could be a few things, it's missing on one cylinder when it's cold as it warms up that cylinder starts firing.
you say you had it synched, maybe they synched a cold motor.
or just set the idle when it's fully warmed up

 

Did whoever did the sync check the valve clearances first?

I suspect they sync'ed a cold motor that has a few tight valves.

 

I believe they were sync'd to a cold motor and no, the valves have not been checked. Should I check the valves and then have the carbs sync'd to the motor when it is fully warmed up? If I adjust the idle to the higher RMPs will it be bogged down too low when it's cold? Thanks for the quick replies everyone.

 

Get the valves in spec ...... top priority

 

From EVERY XJ FSM there is:

 

Understood. Thanks for putting me in the right direction.

 

I had that problem today. Put a new vacuum line on from carb 3 to the petcock and the engine liked to race away to 2500. Put the old one back on and it idles fine Only thing i could figure is the new one was a bit larger inside diameter and wasn't getting the right vacuum.

 

No, it didn't seal at the intake manifold spigot properly and you caused a vacuum leak.