Runaway idle

I've got a 1982 Seca 750.

Ok... I've been working on this bike for a couple months now. Something is not right with the fuel system I think.

I've completely rebuilt the carbs and even replaced the throttle shaft seals. I thought I had a vacuum leak so I replaced the intake manifold gaskets. The carbs are sync'ed.

I can get it to idle... but sometimes the idle climbs by itself and sometimes it won't settle down again until I shutdown the engine. I'm including a link to a YouTube video I just made that shows the behavior. Any suggestions?



Thanks,
Rustee

 

lube and adjust the throttle cable
check to see if the linkage is hanging up
looks like the throttle blip caused it to runaway
cable routing?
remove the throttle cable from carbs then manualy pull linkage up.

and my favorite choke and throttle cable reversed on carbs?

was idle set properly ?

 

Cable routing is good. It's not getting hung up. In fact I pushed down on the throttle linkage while it was in run away idle mode to make sure the butterflies were closed.
If I hit the kill button while it's in runaway idle, it shuts off. Wait 2 seconds and then hit the start button, it will start normally again. Then another throttle blip causes the run away idle again.

 

when it's running slow, it's not firing on all the cylinders. then you bump the throttle and wake up the others and the idle goes way up.
get the idle high back then back down the idle knob to get it to want to slow down, now turn the mixture screws out a bit until that high idle starts comming back. repeat as necessary.
you can find the missing cylinders if it's cold and never gone to high idle by checking how hot the pipes are, be careful.

 

I played around with the idle screws as suggested by Polock and the bike is running well now. However I don't understand how it got fixed... when it had the runaway idle the pilot screws were turned out 2-1/2 turns on all 4 carbs. Now - after adjustment - they are turned out 5-1/2 to 6 turns. I checked the main and pilot jets which are 120 and 40 which is stock I believe. So the bike runs well now but I don't understand why the pilot screws are out so far.

 

When you replaced the gaskets did you see any small cracks on the mating face of the manifolds? I had a similar problem on my Xj750rl. A small smear of RTV will fix what the gaskets can’t do alone.
The other possibility is that when you did the throttle shaft seals the butterflies did not sit flat when reassembled. Loosen the screws and back the idle screw out so that they align true in the bore.

 

The extra richness is compensating for a vacuum leak. Find the leak. The fueling will still be lean at some point above idle, and that's not a good thing.