OzVMX Forum
Clubroom => Tech Talk => Topic started by: 1983_Enduro_YAMI on September 15, 2016, 05:42:18 pm
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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???
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its either lean or rich on the needle jet.....is my guess
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How does it go with the original carb on? as nothing wrong with those carbs. Had an "86 brand new & by memory the carb on that had an oval shape bore which i thought unusual at the time but worked well. Best 125 i ever had straight out of the box & "85 model was good too.
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Yep, sounds like the needle needs work.
If it revs (more) cleanly in neutral but is a problem when riding, then it's probably too lean.
If it is worse in neutral, then it is probably too rich.
The fact it's a round bore carb in place of the original oval bore means that stock jets are only a starting point.
The KX125s from the mid 80s had great power deliveries - what you describe is not how they're supposed to behave.
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OTH, the lack of a mixture screw on the original oval bore carbs is a huge PITA. Otherwise I agree totally.
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The carby is damaged(I don't know how).....Bike was not being look after before. Jetting properly an issue too.... but only thing that comes across my mind would be the stuck open "KIPS" system....It's pretty common if you don't clean and inspect them.
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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.
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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.
I kept this on my computer straight away...I will follow this to re-jet the bike. Much appreciated :)
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im thinking your power valves are more likely the issue in hindsight as it was very common for "unloved bikes" to gum them up puttering around the back paddock until some one "opens er up" and strips the gears off the valves. check some more obvious things like blocked muffler, airfilter isn't collapsing and filling the carb up with bits of foam or rats nest?
You should strip and clean the carb as it may be filled with moss or cement from old fuel. new plug and check the plug lead as well.
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I bought a brand new Vm34 round slide to replace the old oval 34mm.....She bogs down in the mid range but i'm not sure what's the one size down of 6FL52 needle..i will be still taking the KIPS out for inspection (just in case) but i definitely need a bigger pilot jet....
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I now see from your first post NEW CARB :o.. sorry, that one went thu to the keeper. no need to strip that down... ::)