Fine tuning my 1981 xj650

Purchaced this xj650 pretty recently, just been going through it fixing envious things. Lastly I pulled the carbs, cleaned em, replaced a couple a jets, made sure eveything was stock, set the floats to stock height, bench synced em and through them back it. Then replaced the plugs, and synced them. The bike ima running really well now, but does was taking a little while to warm up (idle withong dying after 30 seconds or so). However, every time I pull the plugs they're nice and white (lean) and not evenly so. In decendig order of leanness is goes 4 1 3 2. No vacuum leaks.

Any ideas?

 

you didnt mention anything about the pilot mixture screws.

you take them out when you cleaned the carbs?

 

Sorry, yes I did, and they are currently backed out 5 half turn (2.5turns) which is stock.

 

Factory settings are slightly lean (on purpose). Set the idle air screws out another 1/4 turn and re-check. Most of us end up with the screws 2 3/4 to 3 turns out.

 

I've played with them to no avail. Could this potentially be a valve shim issue? I've been hesitant to bust the engine open as I don't want to replace the gasket.

 

If the gasket isn't leaking you can re-use it. If the valve clearences aren't in spec you're just going to be chasing your tail trying to get it to run correctly.

Kindly post up pics of the plugs if you can.

 

After a tune and rejet at the stealership my bike isn't where it should be. They had the mixture screws set to 4 turns out, which was causing it to bog down at full throttle, and when warm the idle would sit at 2000rpms regardless. I now have the mix set to 2 1/4 turns and it seems to be better, but still slight hesitation at full throttle, especially around 4500rpms. Not sure what the jet sizes are now, but is it possible that the bike needs to be just 2 turns out? I don't want to run it overly lean.

 

A stumble at 4500 may be a sign of needle requiring to be raised up. Either by shimming or lowering clip on needle retainer.

 

Absolutely. Have you checked the valve clearances?

Regardless of the idle set screw? Sounds like a sync issue if you can't get below 2000 RPM at idle with idle adjustment screw backed out. Does it run smoothly at 2000 RPM?

 

Runs well at 2000.

 

Is backing out the pilot mixture screw to different levels (say 3 turns and 2 3/4 turns) to compensate for differing leanness between the cylinders a bad idea?

 

Sure. That's called "mixture tuning" and the most efficient tool for the job is a ColorTune.

But it's also NOT a "sync." While it contributes to synchronization, a vacuum sync is still necessary.

Once the valves are in spec.

 

Where's the best place to get valve shims?

 

PM Hogfiddles. He runs a shim pool.