Engine races on start up

Alright first some background. The bike is a 1981 KZ750 4 cylinder with Keihin type carbs. The bike sat for a long time before I bought it a few months ago. I took the carbs off and they were filthy. I finally got them cleaned up, with many tips from you guys here, I used several cans of carb cleaner and brasso.(thanks again for that tip) When I reinstalled the carbs it all seemed great at first. There were no leaks and fuel flowed to carbs great. After some tune up stuff, new plugs, plug wires, air filter, new vacume hoeses, and oil change with new filter, the bike actually started up for the first time in all these years but then raced like the throttle was wide open.

After shutting the bike down I checked the throttle and it was closed so I backed off of the idle adjustment screw until it was completly lose. Now the throttle cable does not seem to work well such as when you twist the throttle and let go it stays until you twist it back. So I know I need to lube the cable or replace it but the throttle was closed when the engine raced. I tried to start it again with the choke opened this time thinking it should be warmed up from the racing. It wouldn't start right out until I closed the choke and then it started and raced again. I opened the choked and it calmed down for a second and died.

Being that this is the first bike I have ever owned or worked on I am just guessing at things but if this was a car or truck I would think that maybe there was a vacume leak. But I replaced all of the vacume lines before I installed the carbs. So I am thinking that I must have messed up a setting or something, inside of the carbs. On the one hand I am happy to hear this engine running but on the other hand I am frustrated that it acts like it wants to blow itself up. I am afraid to let it run long enough to use a spray to check for vacume leaks.

Please any one have any suggestions?

Off the top of my head, vacuum leak sounds appropriate... or carbs jetted incorectly.

Not sure exactly what model carbs you have, but it could just be your idle input is STUCK to high. I had a problem like this once, and it was the throttle rocker bottoming out on the edge of a ring clamp from thecarb holders.

Check carefully at the carbs to make sure the throttle rocker gets all the way down to its natural position without interference.

Maybe the throttle slides are sticking/stuck in the carb bores...

Just some ideas.

Goodluck!

Your carbs are different than the Mikunis on my KZ650, but here's my suggestion. I assume the bike won't start until you give it full choke. If that's true, then if you start it with full choke and it races, instead of pushing the choke lever all the way down just push it 1/2 way or so to see if the revs come down. If you can do this without the engine dying, use the choke lever to control the idle until the engine warms up. At that point you should be able to push the choke all the way down and adjust the idle with the idle control knob. Then spray carb cleaner where the carb holders attach to the head to see if the holders are leaking. Before I replaced my carb holders, my engine would rev way up when I started it using the choke and I had to be quick to bring it under control using the choke lever. With the new holders this problem disappeared. Ed

does the twist grip snap shut hard? your bike has two cables and they could be adjusted wrong.

Actually I only have the one cable coming from the twist grip. And it doesn't return on its own at all. you have to twist it back to the closed position.

I'll check those out but I also noticed while checking things out that the oil fill sight glass was showing over filled so I drained the oil and sure enough it smelled like gas. So I am thinking that somehow there is so much fuel going into the cylinders that it is leaking past the rings to the pan. Again this is just guessing.

Your right it won't start unless at full choke, and dies if I open it all the way. I am going to try the suggestion of slowly opening the choke to try and control the fast idle and see if I can get it running slow enough to spray some cleaner around and see if I can find a vacume leak. I hope that is all it is cause I really don't want to open up the carbs again if I don't have too.

the 750's with Keihins have two cables. look close at the spot where the cable attaches to the carbs. there should be two pulleys. if not your bike may have different carbs.
because the carbs don't snap shut start there and find out why they don't.

There is only one cable comeing from the throttle control to the carbs. I got the name of the carbs from the Haynes book I got for the bike. Here is a picture of the carbs on the bike. If they are a different type I would sure like to know.

Let me try that picture again

those look like Keihins to me. you made me drag out my kawasaki factory manual. your Keihins do only have one cable. so start there and get the twist grip to snap shut. could be a rusted cable, pinched, kinked or routed so when the bars are turned the cable is getting pulled. also see if the twist grip is dirty inside or the end of the bar is rubbing on the twist grip. jam the grip on the bar with the screws loose then push the grip out a tad and retighten the screws.

While we're on the throttle there is something on the throttle that doesn't seem like it belongs. It is a plastic piece of somekind that doesn't seem to do nothing. any ways here is some pics of it maybe you can tell me something about it.