i say this because its happened to me.
when you put the exhaust on. are you Sure you got both bolts THRU the holes on the exhaust gasket? on two occasions, while doing work on mine, i missed one of the holes and the gasket slipped down, opening it up to air. i caught it both times before i started the motor up, but had i note gone over them taking a good, carefull look over, i would of missed them both times.
may be worth a look.
otherwise, ya even on a well modified motor, 1 1/4 out on the low is usually a bit too much. not in every case, but in the majority of them.
check your linkages are correct. the idle screw is hitting the throtle arm? adjusting it makes no differance?
one last thing, i have a
668 here that worked great for ever. its actually the original
668 o got on my 06 rtr baja. just last fall. it started to idle high for some reason. couldnt figure it out and nothing seemed to make a differance.
so i yanked the motor expecting to find a bad gasket or some other airleak like from a hole in the input gas line. nope none of that, so i narrowed it down to the carb. looked it over but never seen anything funky. tossed on a 813 i had here. first time i tried one.
a couple weeks later, i started to check the carb over.
after a while i seen something i had never noticed before. in the throat of the carb (other wise known as the venturi), just to the lower side of the body, right where the throtle plate was almost closed, there was a bur of some sort. it looked like the plate, somehow pushed up a small piece of the wall of the venturi, and created a stop. so when the carb went back to idle, it would rest against this stop, instead of fully closing until it was on the idle screw.
weird one. not sure how that bur came to be. nothing in the motor or pipe???
maybe worth a gander.
only thing ive seen work for exhaust gasket is a Red High Temp silicone/rtv type caulk wingnut uses. it was firm, but somewhat plyable when dry. and really seamed to help and didnt appear to burn out.
i ll find out what it is when i can and let you guys know.
without the exhaust gasket, it can pull in additional air in the scavange phase of the pulse wave thru the pipe. the extra air ofcourse can/will increase rpms from running leaner.
also check the headgasket under the pipe. i just rebuit a 30.5 that didnt like the 1st 2 gaskets, but the head bolts were viberating loose and i didnt notice it until the idle jumped up and i got to looking close at why it went up.
