non-GS: trying to sync, no vacuum on #3?

[Guest]

It's not a GS, but I know there are some all-around cycle gurus on this board. And besides, the sooner I get his bike fixed, the sooner I can get back to getting my GS back on the road.

The suspect bike is an 87 Yamaha Radian (YX600). Mikuni BS30's, just dipped and new o-rings. All cylinders get compression of 140-145 psi dry.

We put the carbs back on the bike, changed the oil and it started up. Idle is rough but stable. Cylinder 1 is pretty cold. It does warm up, but not nearly as much as the others. 2 and 4 are hot, 3 is very hot. Broke out a brand new Motion Pro vacuum set, which calibrated fine off #2 using the included calibration manifold. So we go to sync the carbs. We can't seem to get any vacuum off of #3 though. All the other tubes are about half-way at idle, but even when we open up the throttle there's no fluid in #3 at all. Switched the hoses for #3/#4, and then no vacuum shown in gauge tube #4 - so it's following the cylinder. We stuck a nail down in the vacuum tap for #3 with no resistance whatsoever - the length of the nail would put it right in the middle of the intake port. We used a propane torch to check for leaks around the intake on #3 but no effect.

So, what are we missing?

Thanks for reading/your advice.

[rustybronco]

The big question... is the vacuum port on #3 for the petcock and did did you plug off the vacuum port when you were doing the synch?

[mike_of_bbg]

Thanks for the reply. No, on his bike the petcock runs off the vacuum sync tap for #2. There's no separate port on the carbs like off #2 on the GSes.

[rustybronco]

the only thing that I can think of, is a leak between the carb and the head.

I doubt if propane enrichment will show up a leak. try the same thing with carb cleaner.

***edit*** did you bench synch them?

[mike_of_bbg]

No bench sync. Think #3 is so much more wide open that its relative vacuum is just too low to register?

[rustybronco]

No bench sync. Think #3 is so much more wide open that its relative vacuum is just too low to register?

That was the thought that I had in mind when asking about the bench synch, especially with # 3 running very hot.

[lurch12_2000]

You didn't mention the mileage and whether a valve adjustment has been done recently or ever done. Usually, I'd check the valve adjustments before chasing your tail with doing a carb synch. Sure, if the bike has been sitting awhile, then it's usually the carbs, but if those valves are way off then take care of them first.

[trippivot]

hot #3 pipe and no vacuum on #3 does not compute.

no vacuum can be:
a poppet valve not sealing shut
piston rings not sealing
vacuum port spigot not clear
throttle plate WAY out of idle position
exhaust pipe plugged
other vacuum leak bigger than the spigot
etc....

if it is hot there is combustion and if there is combustion there is vacuum
re check the simple stuff. the YX 600 is a simple machine so no Y.I.C.S. stick tool required to balance the carbs.

[mike_of_bbg]

Thanks again for the replies.

Well, that's what we were thinking too. The bike has ~40k miles, and last valve adjust was done ~36k, so it should be good there. Were there valve seating or ring issues I would expect lower compression on that cylinder - but it was right up with the rest of them.

I guess with the way the tool works, we're probably not looking at a no vacuum situation so much as a "much less than the others". I might have him try this weekend calibrating the gauge on #3 and see if he gets any vacuum at all out of it.

[mike_of_bbg]

I wasn't around to see it or help, but he said he pulled the carbs off, bench sync'd 'em, and it worked great from there. #3 was much wider open than the others, hence they had much more vacuum. As suspected. And the GSR crew comes through again.