Suzuki TS250 - high idle

[Succubus]

I recently got my '74 Suzuki TS250 running. New top end, new lots of stuff. It runs great. Very quick. Lots of torque. One problem. When the engine gets to running temperature the idle won't come down all the way and instead stays very high. The throttle cable is new, slides freely and doesn't seem to be crimped anywhere. The carb slide appears to be coming all the way down, the air screw is at one and a half turns out. The idle screw is retracted all the way, but the idle stays up. It seems this must be a carb issue, but I'm out of adjustments to make. For what it is worth, the carb is not original. Original was 28mm, my carb is a 32mm. It still should adjust down, though. Suggestions?

 

[dirt bike dave]

A different slide with less cutaway might help.

But first I would try richening the air screw by turning it in.

 

[magneto]

Check for an air leak between the carb and the motor and around the crank seals.

 

[originaldirt]

Could be pilot jet is too small, if turning air screw in and out makes no difference. Also, could be a crankshaft seal leak, especially if they are original. To check pull the mag cover off and look for dampness. It should be bone dry behind the mag flywheel. Also, grab the flywheel with you hand and try to feel if there is any percievable movement that is not circular, lift up/down, push left/right, pull in/out. If there is even so much as a almost impreceptible "click" feel, you dry side bearing is going bad too, and probably sagging into the dry side seal. As for the "wet" side seal, it is most noticiable when bad as it causes excessive smoking, and "wet fouled" spark plugs.
OD

 

[magneto]

I see that you have a new top end. Check for airleaks around the cylinder base (and reed cage if it has one.) Spray wd40 around the mating surfaces while the engine is running slow, if the engine speeds up you got a leak.

 

[Succubus]

Thanks, guys. That's a few things to check. The thought of an air leak occurred briefly to me, but I thought I'd sealed it up pretty well and thought there might be something else to consider before I started going over everything again. I sealed the tach hole and replaced the left side oil seals and all the gaskets, but I didn't replace the case gasket or crank seals because I didn't split the case. The o-ring on the carb looked ok (it's a flange-type Mikuni), but it may not be sealing well despite how it looked. I replace it and see if that helps. I'll also check the play in the flywheel and pull it to inspect the crank seal.