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I recently picked up a 94 ktm 300 exc supermoto (yes it’s a 94, newer plastic). It has the stock pwk39 carb, fmf gnarley and good dual stage reeds. I’m at 2000ft elevation.



It runs really poorly and I have tried many jetting combinations. Stock jetting is 180 main, 50 pilot, n85a needle.



Most recommend 172 main, 45 pilot, and n85d needle in second from top position, 40:1 mix, 1.5 turns on mixture as a starting point.



The bike as I got it had a 148 main, 58 pilot, and n85d in middle position, no Air filter



Which seems too lean up top and too rich everywhere else, it would run strong in high rpms but would spit and sputter everywhere below and was basically unrideable. I added a notoil air filter and have tried close to a dozen jetting combinations with no luck, in fact it may be worse. I tried the recommended jetting and it was too rich, I’ve been slowly leaning it out with no luck. Now it will start and idle but as soon as I hit the midrange it bucks badly and won’t even reach the power band.



I currently have a 158 main, 38 pilot, and needle in second from top position. Plug is rich. Something isn’t adding up, it shouldn’t need to be jetted this lean. The odd thing is in neutral or with the clutch pulled in it revs very cleanly all through the rpms, but under load it bucks like crazy as soon as I reach mid rpms. I’ve tried middle position on needle and it didn’t help. I also adjusted the float correctly. I’m running a br8eg plug, which is for newer 300s because the original is discontinued. Can anyone shed any light? There is not a lot of info online about the pre-internet KTMs. I have already tried removing the breather cap and have changed the peacock. Thanks so much!



-Calvin





92303
 

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I recently picked up a 94 ktm 300 exc supermoto (yes it’s a 94, newer plastic). It has the stock pwk39 carb, fmf gnarley and good dual stage reeds. I’m at 2000ft elevation.



It runs really poorly and I have tried many jetting combinations. Stock jetting is 180 main, 50 pilot, n85a needle.



Most recommend 172 main, 45 pilot, and n85d needle in second from top position, 40:1 mix, 1.5 turns on mixture as a starting point.



The bike as I got it had a 148 main, 58 pilot, and n85d in middle position, no Air filter



Which seems too lean up top and too rich everywhere else, it would run strong in high rpms but would spit and sputter everywhere below and was basically unrideable. I added a notoil air filter and have tried close to a dozen jetting combinations with no luck, in fact it may be worse. I tried the recommended jetting and it was too rich, I’ve been slowly leaning it out with no luck. Now it will start and idle but as soon as I hit the midrange it bucks badly and won’t even reach the power band.



I currently have a 158 main, 38 pilot, and needle in second from top position. Plug is rich. Something isn’t adding up, it shouldn’t need to be jetted this lean. The odd thing is in neutral or with the clutch pulled in it revs very cleanly all through the rpms, but under load it bucks like crazy as soon as I reach mid rpms. I’ve tried middle position on needle and it didn’t help. I also adjusted the float correctly. I’m running a br8eg plug, which is for newer 300s because the original is discontinued. Can anyone shed any light? There is not a lot of info online about the pre-internet KTMs. I have already tried removing the breather cap and have changed the peacock. Thanks so much!



-Calvin





View attachment 92303
Hi Calvin. Not sure most of the crap on internet is much help to a sick motorcycle. I suspect you may have trouble in power valve or possible restriction in exhaust. Stock and some aftermarket silencers use a spark arrestor screen and if it becomes blocked or otherwise fouled screen it will make your motor weak from mid to top end. Stuck power valve will do this also. The newer PWK Kehins use a jet holder block o ring that can degrade with ethanol fuels also and run rich at any jetting. You should be aware if you run ethanol fuels your plug will never burn perfect even when motor is jetted perfectly. Ethanol burns richer and gives plug a black ring inside. Best to always avoid ethanol fuel if possible. Have a look at your power valve and exhaust and check back if needed. Good luck
 

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Hey thanks! I just repacked the powercore (no silencer) so I know it is flowing properly. I actually checked that o-ring today. It looked fine but I added a little rtv. It actually seemed to help a little. It’s still bucking and lurching at high rpms but now I can “work through it” and get to the powerband. I’m currently running a 158 main and it’s still too rich, is it possible that I need such a small main or is that an indication of something else being wrong? Also, a couple drops of oil leaked from the air box after my last ride. Could that indicate a problem with reeds? I’m not discounting the possibility of a power valve issue.
 

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Could be a bad Reed. Can sometimes hear an exhaust sound from airbox with a bad reed petal but the symptoms are usually low speed sputter and mid to high flat/blubbering feel. Best to have a look at reeds and power valve. You don't want to use rtv(silicone) on any fuel system part with liquid fuel or vapor. Hydrocarbon will attack it. Keep after it. You are getting closer to the problem
 

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At times it is easy to spend hours trying to adjust a bike to run properly when the problem actually is in another part. In this case it sounds like you have spent hours trying to sort the carbi out. I tend to agree with @augiedoggie and recommend cleaning the powervalve bits. Being a 1994 bike also makes me wonder if some of the parts might be worn past their best and as a result you will never get it right withouth fitting new bits. Running the bike w/o an airfilter would not have helped and I suspect that you have a worn carbi/reed/bore/piston and/or rings as result of grit going through the engine. Only taking them apart will tell you what state they are in.
 
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