QuestionMy machine is driving me nuts. It doesnt idle very well. The worst though is it will start racing- like really fast. I can feel the play in the cable which suggests that the slide is still down, but the engine is doing 5000rpm. Turning the idle screw out will bring it down but slowly. Then it wont idle at all. If I turn the idle adjust screw up to keep it ideling, it will soon begin zooming. It takes many turns on the idle adjust to get it back down and then it dies. It will idle ok if I use the throtle to keep it right where it should be.
So,,,I checked for air leaks- none. I filed the manaafolds flat. I've removed and cleaned that carb several times. Blew out all the jets, replaced the needle and seat as well as the air adjust screw. The float floats and is set right. The slide slides easily. The main jet needle is new.
Im ready to try a new carb if I can find one.
Also, I just bored it and a new piston and rings. I think I may have forgot a rubber o ring on reassemley. I got the one around the bottom rignt head stut, but without a picture, dont know if there should have been another one. Should o ne have also gone between the cylinder and case joint?
Wow, you are great. Glad I found you.
Thanks for your help.
AnswerCliff, you are making somewhat of a classic beginner's mistake. Blame the carburetor before you finish checking the ignition system.... The next error is to work on the engine w/o a shop manual... :>)
Crazy fast idling can come from a bad or mis-routed throttle cable, especially when you turn the handlebars back and forth. The throttle cable should have just a bit of slack at all times, when the throttle is released.
Be sure that the cable is retracting inside the throttle control area, too. You seem to indicate that it is okay, but metal strands inside the cables can break loose and hang the slide in some cases.
After that, assuming that you got the cam timing in okay and the valve clearances are set at .002" COLD on both intake and exhaust sides, take the spark advancer apart or at least check it for smooth action in advance/retard functions. The point cams often get sticky on the shaft and hang there and/or the advancer springs get stretched or weak and that keeps them from staying retarded at idle. When the spark advance is gone past the F mark, the idle speed will start rising and the faster it goes, the faster it goes.... Sometimes, you can tighten the ends of the springs a bit with needle nosed pliers. Set the spark timing at the F mark alignment, using a 6v test light or volt meter.
Float level is 19.5mm... if too low, it will lean out the motor and cause the idle speed to rise, like an air leak. Check with the float just touching the needle tip enough to close the valve from leaking. Don't force the spring-loaded needle tip down. Make sure all O-rings are good at the carb body and manifold connections. Idle speed screw setting is about 1 1/8 turns out from lightly bottomed out.
Go to BikeBandit.com and search through the microfiche illustrations for correct sealing of the cylinder and cylinder head. There is the oblong sealing rubber surrounding the camchain tunnel on both ends of the cylinder, plus an O-ring round the cylinder stud where the oil comes up to feed the cam and valves. I chose a 1979 CT90, as an example, as all the OHC 90cc motors are the same in that regard.
Bill Silver
