Vacuum leak with no vacuum leak.

XJ650J acting like it has a vacuum leak, but has no apparent vacuum leak. Starts, idles up to about 3500, backs down to about 2k. Then revs back up when throttle is given more gas. and runs around 3500. When I back down the idle, it dies, yet a slight increase will get it to run around 3500 again. I've replaced all vacuum lines, rebuilt the carbs, with new diaphragms at 200+ bucks a pop and synced. I've sprayed carb cleaner around the whole engine, where a vacuum leak may occur and it doesn't cause any difference in rpms. Honest to God, I've spent hours working on this, brought in my buddy who's a freaking genius when it comes to bikes and bike engines. He's stumped, I'm stumped and my bike is about ready to be ran into a stump. Any ideas would be greatly appreciated.

 

Please provide the history of any work that has been done to the carbs or valves so that diagnosis is easier....

 

Complete rebuild on carbs, set valve clearances to spec.

 

Where did you get an idea to spay carb cleaner around? Use an unlit propane torch.

 

Got it from this forum sektorgaz. But I'll keep in mind the unlit propane torch. Thanks.

 

throttle shaft seals ?

 

Throttle shaft seals, float levels, sync way off, valves not in spec; any or all take your pick. Maybe even just rock-hard vacuum spigot caps.

 

Intake manifold boots - clamps
YICS bolt came loose??
+1 on throttle shaft seals & propane check.

 

airbox to carb boots on properly too?

 

Polack, throttle seals are brand new
Fitz, float levels are set, carbs have been bench synced
TimetoRide, intake boots are new, and clamps are tight
Still not running right, going to re adjust valves, and re=sync carbs Wednesday.
Guess what it boils down to is all of the above suggestions are great and have been done. So it must be operator error, gonna start from scratch and see what happens.

 

i dont know alot,,but when my bike was doin that, i did what you did, i know this probably isnt what your problem is but check to make sure your main idle screw isnt binding on the spring,,dumb thing stumped me for a week!

 

They're either in spec or they aren't. If you had them in spec they shouldn't have suddenly changed.

 

Frankenbiker

One of your previous messages: "Complete rebuild on carbs, set valve clearances to spec."

and a more recent one:"...Still not running right, going to re adjust valves, and re=sync carbs Wednesday..."

Why on Earth would you need to readjust valves if they were in specs not long ago???

 

Long shot but maybe the air jets under the diaphragms are installed incorrectly? I know at least one of the service manuals has them wrong in the diagram. Off the top of my head, I think that the smaller orifice one should go towards the rear of the carb.

 

what did you use to bench sync your carbs? they could be open to far maybe

 

Guys, weds. probably not going to do a valve adjustment as it was just done last week. I'm also going to check my diaphragms to make sure there are no pin holes again. Yet they're brand new also.
Like I said, its probably human error on my part, but I wanted to get ideas in case it wasn't. You guys are freaking awesome, and I can't tell you how much I appreciate the advice, and the knowledge you all bring to this forum. I'll keep you all posted.

 

Another "trouble spot" that Rick-O often points out - -
the intake boot-to-head gasket goes bad.
Just throwing you another bone.
.
Could you have "Botched" the valve adjustment??
the shim supported by a grain of sand, not seated correctly, numbers not tallied right ??

 

Franky being the weird bike that he is decided to function properly. He's idling fine and has all the giddy up he had as a lad. Don't know what the hell the problem was or still is but lying dormant waiting to strike again, but until it does. If it works, I'm not going to fix it.

 

When you rebuild your carbs did you perform a clunk test? Do the pistons fall without any resistance when you lift them? A quick polish of the bores with ScotchBrite pads can make a big difference. It is possible that some of the pistons are stuck open and causing the engine to stay at higher rev/idle. In addition, you shouldn't be able properly vacuum sync the carbs unless you can get it to idle around 1200rpm.

http://xjbikes.com/Forums/viewtopic/t=3 ... art=0.html

 

He's got a balky float.

 

Kinda what I thought Fitz, as a matter of fact, we took the carbs off again and found that one of the pistons was slow to respond or fall as the case may be, so I'm hoping that this was the problem. Like a stated earlier, its running great now, and can't think of anything else we neglected.