I just bought a 85 KX125 recently, i replaced the old stock carb with a new mikuni Vm34 with stock jetting....i Got it going and went for a ride. When i was revving the bike in neutral, It's bogging down between 1/2-3/4 throttle and also the bike was hesitating when i snap the throttle....took about 5 secs to the powerband....Is it symptoms of vacuum leak or is it what a 125 suppose to be???
I found this advice on the net ages ago and always keep a copy as it tends to help....
Pilot jet/mixture screw:
If you can start it cold with no choke, the pilot circuit is too rich.
If, when warm, the idle seems to hang when blipping the throttle, the pilot circuit is too lean.
The best way to set a pilot circuit is with an RPM guage. Warm the bike up and turn the mixture screw to where you get the highest RPM. If it's below 3/4 turns, or above 2.5 turns, change the pilot jet and try again.
On a 2-stroke - You should be able to ride in 3rd gear, throtlle BARELY cracked open, and it should cruise smoothly. If it sputters and crackles, the pilot is too rich. If it bogs, the pilot is too lean.
Needle Clip:
On a 2-stroke - Riding in 3rd gear, with a warm engine and the throttle BARELY cracked open, roll the throttle to 1/2. If the bike sputters and crackles, and you feel like you have to keep rolling on the throttle to smooth it out, the needle is too rich. If, on the otherhand, you get the dreaded 'buhhhhhhwaaaaa', the needle is too lean.
Main jet:
On a 2-stroke - Riding in 3rd, with the throttle BARELY cracked open and cruising along, whack the throttle wide open. If you end up with a set of handlebars impacting your nose, or you loop out, the main is perfect! If it crackles, smokes, and won't get 'on the pipe' quickly, then the main is too rich. If it gives a 'buuuuhhhhwwaaa' sound and feels like it's sucking for air, then the main is too lean.
