'81 XJ650 High Idle Problems

Alright, I've been skulking around the forum for a few years after I bought my XJ65o for a whopping $400, and I don't like being that bike guy with an eternal project bike.

My issue: Bike starts up fine with half enrichment, after about 30 seconds the RPM starts to quickly rise, so I start to close the enrichment. It will idle at 1100 fine after it warms up from just running, but the rev down is slow, and if I rev it past 4500, it hangs at 4k. So obviously, stop signs were a little unnerving on my test ride to see if the issue would just shake out. If I back the idle adjustment screw out, it will start to settle down, but then proceed to die.

Things that have been done:

Valves in spec.

Carb rack broken down, cleaned, cleaned again, cleaned a third time, throttle shaft seals replaced and greased, all other seals replaced, new piston return springs, jets in correct spots, fuel levels in bowls correct, carbs fully seated in boots, clamps tight.

New carb-to-engine boots and vacuum caps.

I have vacuum synced the carbs and have the idle mixture screws set to 2 1/2 turns out.

New petcock.

Airbox was cracked and even broken in some spots, so I have to run pods, 114 main and 42 pilot. I know pods are not ideal, but I did buy ones that do not block any of the air ports, so there's that at least.

63.5mm overbore to fix a low compression issue, so that includes all new pistons, rings, and gaskets as well.

Brand new throttle cable (it has freeplay)

I have checked the carb and enrichment linkage, does not seem to be binding. (as far as I can tell, at least)

Throttle cable and enrichment cable are not swapped around.

Waved a propane torch around the entire intake and carb area like a madman, no change.

I only buy my parts from XJ4Ever.


Sages of the XJ forums, any Ideas? Something I haven't checked yet? Something I did check that is easily looked over?

 

I'll throw a few things out there, are the carb diaphragms in good condition with no splits or holes?
Are the diaphragm needles set at their right clip position?
Is the needle jet worn oval and/or seated correctly?
Is there any air leakage around the carb to head manifolds? You find out if there are leaks by firing the engine up and spraying something like WD40 or carb cleaner etc at the joint and see if revs rise.
Have you tried adjusting the mixture settings to a bit of a lower setting? This only needs to be about an eighth of a turn at a time.
I've run out of ideas at the moment but give us an update on the situation.

 

Agree with this. 2-1/2 turns is the starting position and in my experience always requires some tweaking.
Try going both ways until you find the sweet spot, as @FJ111200 stated - small tweaks at a time.
Some swear by the Colortune plug but I've never had any success. I found it too slow to react.
I just tweak all 4 the same until I get a steady idle, all while the mercury stick is still attached so I can balance them every time I change the mixture.
The lack of an airbox should not F it up at idle rpm. It will stumble at 5,000 rpm most assuredly though.

 

Have been fiddling with the mixture screws with no success, but it is definitely getting more air than it should be. Did the WD40 around the carb side of the boots, no dice.

I noticed that when the idle starts to climb, if I push down on the carbs, the bike settles down a little. Throttle return springs weak? (Maybe I need to wind the throttle shafts a few times?)

Going to make sure the bottom bolts on the boots are snug before I go breaking the rack for that, maybe they aren't sealing properly, going to spray the WD all over that afterwards and just deal with the smoke. Could be that my mating surface is messed up, could try thicker gaskets if that is the culprit.

 

You would have to remove the carbs to check but make sure the butterfly plates fully close with the snap test. One might be not aligned properly and binding.

 

I probably should double check those when I take the rack off. I am pretty sure I got them to close fully with no visible light before I joined them and bench synced them but, eh, I am prone to making silly mistakes like that, good thought.

 

Ideally you should have a small gap at the bottom of the butterfly valves when they are closed, you can use something like a paperclip slid between the carb body throat and the lower edge of the butterfly. That's what i've always done when bench syncing carbs. I think there is a tutorial somewhere on the forum. Whether it helps to stop the revs hanging i don't know but reckon it might help.

Here's the link > HOW TO: Bench-synch your carbs | XJBikes - Yamaha XJ Motorcycle Forum