Almost there, but bike gallops and stays there!

Hey all!

It's been several years since I've posted but I got the bug again and am trying to get the bike back on the road (or really on the road for the first time I guess).

Anyways, I have pretty much stock 1983 KZ750 L3.

This is what I've done:

Opened and inspected carbs - they were all almost pristine inside. No need for cleaning.

Synched carbs - they were pretty badly out of synch, so I did so at around 5k RPM

Adjust float levels - they're now within spec.

Replaced engine-side boots with new from Jeff@Z1 (thanks!)

Replaced all vacuum plugs with new squishy ones!


With all that done the bike will start pretty well but it jumps in RPMs right away to 5k or so, even with the choke off. If I pull on the brake and release the clutch a bit to put load on the engine it will bog down to idle speeds and I can let up on the clutch and it will stay there. However, if I blip the throttle again the RPMs will jump back up to 5k and stay there. It also won't stay at idle for more than 30 seconds or so, it will generally start creeping back up to 5k.

I suspect a vacuum leak so I did the WD-40 check, but it's kind of inconclusive. I don't think the boots are the problem, but I think the throttle shaft might have a vacuum leak.

Is this galloping a typical symptom of a vacuum leak? If so, has anybody else had any problems with the throttle shaft? How do you fix that? I have access to a machine shop and can machine new brass bushings or put in new O-rings.

Please help if you have any advice! I've been so close with this bike for so long and I just want to get it on the road...

I guess it could be a vacuum leak on the engine side of the carbs (intake manifold area) that provides enough vacuum to start the carb piston creeping up which would accelerate it and keep the RPM's rising as the piston raises up.

Are you sure you have all the vacuum fittings accounted for? Sometimes the one that runs the petcock gets overlooked.

Also: are the rubber seals around the pistons OK?

Binding on the throttle shaft won't cause the speed to creep up and runaway, it will just stick and stay at one place.

I think the vacuum leak is more likely. Maybe a PM to Wired George, he is the carburetor expert.

I'm taking shot at this but have you cleaned and lubed your ignition advancer? Might be worth a try.

I have not checked/lubed the advancer unit. It would be a quick thing to try, but I'm not too suspicious of that since I doubt it would make the bike creep up in RPMs from idle. I'll give it a shot.

Also, bountyhunter, I may have misspoke. I actually didn't mean that the throttle shaft is sticking,but that there MAY BE a vacuum leak around where it rotates in the carburetor body.

Thanks!
Dave

Twist the throttle and release to see if the common shaft returns to full closed position. Sometimes the throttle cable can have a couple of strands that break and hang up inside the cable sheath, or the cable sheath has been kinked. I had a high idle problem that was solved by getting a new cable.
Just one more thing to check out.

Yes, the throttle does return to closed.

inspirado wrote:

Also, bountyhunter, I may have misspoke. I actually didn't mean that the throttle shaft is sticking,but that there is a vacuum leak around where it rotates in the carburetor body.

Thanks!
Dave

I suppose that could be the vacuum leak in question?

Do you think mentioning what type carbs are being discussed might help with a solution?

Not exactly sure how to find out, but they're the stock carbs that come with the bike. Mikuni Constant velocity. I'll comb the manual and try to find out when I get to work.

Alright manual says Mikuni BS34. Hope that helps.