Vacuum leak on a xj650 1981

I recently got a Yamaha xj650 1981 and it wasn't running for years got it for 250$ and a title!

The engine had perfect compression and the previous owner told me the only issue it had was carbs, so it came with 2 pairs.

Yesterday I cleaned out the carb and installed and the bike actually started it was running fine when all hell broke loose, typical vacuum leak symptom where it would rev up all the way to 5k and stay there. Now, I have no idea the the leak is! i would spray carb cleaner on every single boot and nothing would change, i even replaced the intake boots and did a smoke test that showed no leaks, i deleted the vacuum hose and just capped it off but it still had a air leak somewhere.

Has anyone had this issue before? maybe the carb it's self is damaged somehow?

 

i assume the throttle stop screw is backed off, and the throttle cable has free play on it?

 

Expanding on what bensalf hinted on the throttle cable.
Its interesting how so many times, people focus on vacuum leaks and its really something else.
I"d place a mark on the side of the butterfly axle on one side carb when its at idle. Go ahead and push on the throttle cable to make sure its fully down.
Now, when it revs up, and you lower the throttle, make sure its rotated back. I had a run away idle when relocating a throttle cable and it was just the cable pinching and not allowing the butterflies to move back to idle.
If you had a vacuum leak, your letting in more air. To go to 5K you also need to have enough fuel to hold that. So your mixture would have to be VERY rich for the vacuum leak to do that IMO.
I ran a couple carbs w/o the vacuum nipples on and it did not rev any where near 5K.

 

So I've put a dot on top and was reviving it and it wouldn't run away this time i think (bc it was cold) and on choke it does go all the way up to 5k

I uploaded the video to youtube so here is the link

https://youtube.com/shorts/L6ezAVXe9DI?si=SG7r9LUZKvPV_cf3

Wow I found where the leak was
So for anyone interested that's what a vacuum leak is like this whole time i was spraying the carb into the boot not the boot to the engine it's self

https://youtube.com/shorts/4obfy2XvwGM?si=XBN1PCCcur2R6JVo

 

Are your new intake boots Chinese ones off EBay? Is the bottom bolt tight?

 

Video show huge gap at bottom of boot, bolt not tight at all.

 

Or is the bolt too long?

 

So it seems like the threads of the bolts were gunked up i cleaned them and was able to get the screw more in but now it just spins in place. there is still a little gap. i'm thinking of putting JB Weld Extreme Heat putty in the gap? and yes they were about 40$ on ebay

 

Good find, Well that is a big vacuum leak but as I mentioned, you don't have a 5K run away. I still suspect you have a throttle cable sticking. Since that's obviously not the stock bars, you or someone else changed them. So the stock throttle cable is very long and Look at how its routed and make sure there are no sharp curves. Sync, fuel mixture, vacuum leaks, sticky CV piston in the carb can all play on a slow deceleration to idle. You can pull the intake boots loose and push back and then lift the CV cylinders up and they should drop right back down If not then you need to clean/wet sand the carb to let it slide better. I've done that on a bike where I just pulled tank, and removed the tops of the carbs while still on the bike. You may find that on these old bikes, there is more than one thing that needs correcting.

 

Well the throttle isn't sticking that was already proved with the video the runaway actually happened on choke which makes sense. also The guy gave me the oem handle bars, should i install them back?

 

oh and the cv are fine i tuned and cleaned the carb to perfection

 

I would fit a slightly shorter bolt before I would use heat putty, will probably just fall out with the vibration of the engine. People on the forum have had problems with Chinese intake boots cracking but since you have them fitted might as well keep them on. If the bolt is spinning is the thread in the head stripped?

 

So i've just put the putty combination of jb weld everywhere and it ran really good for like 20 seconds and idk what happened but it started doing it again, i guess the putty cracked or something. i'm gonna go to home depot later to get shorter bolts, or is there another place? Also I have no idea if the bolt is stripped i'll check that out later too the bike decided to do a massive red line roar at 11 pm so i had to wrap up

 

oh and there was a dead cylinder on the left and after the putty it became the best and the one on the right became dead... Also will the airbox boots affect anything?

 

Airbox boots need to be like the engine boots in that you have no air getting in where they are clamped to the carburettors and fit into the air box joint.

 

oh so that's what it probably is i'll see if some electrical tape will help tommorow thank you!

 

Carburettor dotted rib should be against the boots.

 

If the bolt is spinning, the threads are NG. Best fix is a pain-in-the-neck Heli-Coil.

 

I don't think electrical tape will help you need to do the repairs necessary correctly. Bodge ups won't give you a reliable machine, not being nasty just trying to help.

 

Alright so, today I got all the boots on and sealed them properly one airbox boot had to be electrical taped but the bike is running perfectly on like half choke, I got it up to 70 mph on a highway after it wasnt ran for like 3 years. There are some misfires on the right side that was actually the good side so i guess ill test it with a smoke machine see where its leaking. But for now I think im gonna richen up the mixture and see if it will run off choke, is there a guide to using the mixture screws? also I did use the heli coil those are really usefull!!!

 

You said the run away happened on choke (or after you turned choke off?) its really easy to see if the choke (fuel enrichment) is truely turning off cause when its fully off and the plunger is down, there should be a small gap between the fork that pulls the plunger up and the plunger.
When I put standard bars on my xj, I disconnected the choke cable and ran it differently I also did that with the clutch cable.

 

So it's running lean because of the air leak and when the choke is all the way on it has the run away. Right now the bike will run perfectly on like half a choke, i tried making the mixture more richer but it didn't do anything