1988 Suzuki lt250f idle problems

[mvtowns]

I just recently picked up this lt250f quadrunner. When i first got it it was hard to start. adjusted valves a bit and now it starts easy. When it is cold it idles just fine. after it warms up the idle races way high and stays high. If i put it in gear it will idle fine. I have noticed after it is warmed up and i get the idle to come down, it will have a little miss like it is backfiring through the carb. The carb is brand new 30mm. I can not adjust the carb after it warms up. nothing seems to work. I took the intake boot off and checked for cracks, resealed and put back on. I also took the pitcock off and tore apart and cleaned real good. I am not sure what the deal is. Also what are the valve clearances supposed to be? Thanks much!

Also when I turn on the light, it is bright at idle and dims when i hit the throttle. Idk if that can be fixed...

[oxidized_black]

if it were me, and i be way off base here, but i would get a voltage meter and check your coil, voltage regulator, stator and pick up coil. if the bike is running under minimal load and your lights are good, there is obviously no generation happening when you rev it and your lights dim. something is robbing the power. make any sense? i would think maybe more stator and pick up coil though ... JMO

[Ajmboy]

I agree with oxidized_black, check the output on idle after it warms up.

[XWrench3]

check the intake manifold between the carb and engine. i have seen several of these crack, some times radially (which makes it very hard to diagnose without removing it). a cracked intake manifold will allow extra air into the air/fuel stream, and the engine will run erratic. an easy way to check is to spray water over the manifold while the engine is running. if the engine rpm increase or decrease rapidly (the engine may very well stall), you have an intake leak someplace. i used to use a flammable substance, which would tell me much faster, and with a lot less mess. but i would not recommend it to a novice. i repaired bikes and quads for more than 35 years. so i knew where, and not to spray things. you could very easily set your machine and or garage on fire by doing that.