Is vacuum sync the solution? 1981 xj650

Hi there.

I have askd around before about my bike on this forum but now I have a peculiar problem with my 1981 xj650.

It runs fine and starts easily when I am riding it.
Idle is another story though. I only responds to throttle in a normal way if I turn the main idle screw all the way out. If I set the idle so that it keeps itself running with the main idle screw the engine sometimes revs itself up and does not rev down in a normal manner if I blip the throttle.

Valves are in spec.
Carbs are cleaned.
throttle and "choke" cables are lubed and fine.
No vacuum leak between head and carbs.

I have only benchsynced the carbs since I have not yet gotten a hold of a four gauge vacuum sync. My question is what do YOU guys believe is the problem and what step do you suggest as a next step.


Thanks a lot!



Ivar Nelson

 

Try Vacuum syncing first. But your symptons tell me there is a vacuum leak, have you checked the throttle shaft seals ?
I would remove the carbs and clean them really well, take everything apart to clean. Replace throttle seals while its apart,

 

Thanks for the tip.

The carbs have been thuroughly cleaned and all rubber parts (except diaphragms) have been replaced.
I am absolutely positive there is no vacuum leak.

Will order Carb syncers and colortune tonight.

 

Update: still waiting for my carb syncers and colortune.
However, I have been riding my bike a bit to work.

It has a peculiar stuttering sound that seems to be emminating from cylinder #3.
Other than that my problem lies in the fact that if I try to use the main carb syncscrew at all the engine will idle erraticly and only slowly lower its rpm after I blip the thorttle.

I did a compression test today and #2 and #3 showed 142psi and 140psi and while #1 and #4 showed 150psi. When I lubed #2 and #3 they showed around 160psi.
This suggest some kind of problems with the rings right? When reading other posts I figure that my numbers are still relatively good, or at least nowhere near as bad as the ones in other posts.
I think I will have to wait until I can get the bike properly synced before I can investigate this problem further.
Any other ideas, opinions or such? I am really grateful for anything.
btw, carbs are rebuilt and valves in spec.

 

The sync and possibly a mixture adjustment will get you dialed in.

The compression numbers look fine. I wouldn't worry about that.

 

yes compression is good. I know you mentioned there's no vacume leak between the manifolds and head but how do you know that for sure? if the bike doesn't have alot of rust I wouldn't worry about the manifold bolts breaking off in the head. you can make those gaskets easily with gasket paper. As far as I know, a bench sync and correct mixture screw will get the bike going with no Real compaints. the real vacume sync just makes it Awesome with that new bike feeling. I just posted to previous thread 2 mins ago about what to look/hear for when colortuning/adjusting the mixture screw