YZ'er got a minor orange bug, so hunted craigslist for a cheap 125. Replaced everything that needed replacing or was missing. Cranks right up, immediate throttle is good. Wide open screams as power valve opens. But 1/4 to 3/4 throttle there is nothing. Runs like a thumper with the timing way off. Tries to go, but never catches up. More like not getting gas as opposed to too rich gurgle. No wire chaffing. Float level looks good. New plug and air filter. Removed carb and cleaned with cleaner/air. Jets/needle are stock #. Moved clip up and down. Moved power valve timing up and down. Reeds are not cracked. No obstructions I can see. Revs up just fine on the stand! I'm at a loss. Should be a needle problem, but isn't. Once you take off under load - nada. When I run through 1st, 2nd, 3rd while on the stand it makes almost the same 'missing' sound. Back in neutral and it revs fine. Anyone with helpful experience would be a blessing. Yamaha/Honda have never done this to me. Next step will be to kick it over off the stand (Well, not over too hard. Has new grips too.), spit on it, and leave it there until I feel like taking it to a shop and pay...yes I said Pay for a fix.
Since it was run without an air filter from the p.o., maybe do a compression check (worn rings) or pull the top end to check piston clearance/ ring gap/ cylinder condition. Make sure there are no air leaks around the intake manifold. Make sure the power valve functions properly and isn't stuck open. Then again, it may just be low speed jetting. I know, lots of maybe's here. What year 125?
Last Edit: Mar 6, 2017 19:35:50 GMT -5 by johnnyb300
Didn't pull the head/cylinder yet since I don't have gaskets, but compression is 140. (it's a 2005) Sprayed ether all around intake, carb, reeds and never sped up, so I figured no leaks. As you rev up, power valve timing marks move up and down, and ultimately into the site window. So that seems to be functioning normally as well. I had a racer tell me it was the slow jet. I said no, it's the needle. He said, NO - it's the low speed jet/air screw or passages. Clean them again. Better yet replace them no matter how well you think you cleaned. So I'm doing that as well seeing as it's now two against one. Checking again. Thanks
If you're confident with all else, jetting is the easiest to fiddle with. Everybody has an opinion on what jets to use for what model. I don't pay any attention to them. For KTM, the manual has always worked for me. I attached a link to a manual pdf for the 2005 ktm125. The manual has a jetting chart toward the back for both the SX (PWK 39 carb) and EXC (PWK 38 carb) models. I buy all my ktm parts from Rocky Mountain atv. Usually a 2 day delivery via UPS standard. I hope this helps. file:///C:/Users/John%20B/Downloads/59885.pdf
Post by gregtowers on Mar 29, 2017 15:03:43 GMT -5
Replaced ring. Compression is good. Verified coil. No messed up wiring from alternator. Removed power valve, cleaned and adjusted. Works nice. Checked reeds again for cracks. Cleaned carb AGAIN. All new gaskets, 0-rings. Replaced all jets even though looked clean to me. Including air screw. Starts first kick, revs up good on the stand (still). Newly packed silencer sounds great. Jump on it and go: idle/off idle moves ok, just on the gas all the way through maybe 3/4 throttle feels like I'm towing a huge dumpy fish wife. Around 3/4 when the power valve opens fully it TAKES OFF. Three times apart - at a complete loss. There are no tears left.
Post by johnnyb300 on Mar 29, 2017 18:55:18 GMT -5
Looking at your first post, you said it revved fine on the stand when in neutral, but then had a miss on the stand when you put it in gear. Does the rear wheel turn freely when in neutral on the stand (sticking rear brake or.....)? Try adjusting the external air screw in and out until revs peak while at idle. Then set the idle if needed with the idle screw. Check the float level if not already done. You can also try different clip positions on the jet needle. Note: Make sure the needle has no up/down play in it. Is it stock gearing? It may be bogging if it's geared too high. That's about all I've got for now.
Rear does turn freely. But also turns slightly when in neutral and running. Can't adjust that hydr clutch. Rear brakes ok. Replaced all jets with stock and adjusted again. No play in needle. Did adjust the float a bit. I've tried all clip positions. I don't know about the gearing. I do know that when you open it up full it takes off like a rocket. But between idle and full, it feels like I have a 400lb dumpy fish wife on the back. Also, runs the same with choke on or off. Think I'll ride it around for about an hour tonight (pouting) and see if it gets any better or worse.
99% of the time, if it runs the same or better with the choke on, it's a fouled pilot jet. Pull the pilot jet (again) and clear it out. Soaking it in Pine Sol and blowing it out with compressed air helps. Holding it up to light, you need to be able to see daylight thru it.
Post by gregtowers on Apr 10, 2017 10:09:33 GMT -5
Let me rephrase the 'choke' bit. The bad part (1/4 - 3/4 throttle) runs just as bad with the choke on. Not really worse. The low and high end of the throttle run MUCH worse with the choke on. Rode around the lease an hour and a half (fantastic after the rain by the way) and nothing changed in the middle. 3/4 - full is great. Just can't do that in the tight trees. Didn't roll it off into the ditch and leave it there as planned. Giving it one more shot. Removing pilot AGAIN, even though it's new.
Post by johnnyb300 on Apr 10, 2017 11:45:57 GMT -5
Greg, I pulled the shop manual I have on CD for my 2012 to get some troubleshooting ideas. Guess what? This manual also covers the 2005 125sx. You're welcome to borrow it. If so, I'll bring it to the club meeting on 4-11.
Greg, been watching this conversation for awhile and I had a similar issue with the vintage cr125 I was restoring. Ran great on the stand started and idled fine but as soon as I kicked it into gear it would load up till I could get through the flat spot and get into the upper RPM range. Played around with jets and float heights on the original carb to no avail. I ended up borrowing a Mikuni in good condition from a a guy in Fernandinho to run some tests with and even being the wrong size and jet specs it woke up the engine with good power off idle. Called Sudco and bought a right sized Mikuni with an assortment of jetting and that solved the issue. So I'll go out on a limb and tell you to find one of the club members to see if they will loan you a good operating carb and bolt that mother on and see what happens. Then again it might be a crank seal. Try the carb, you just might be surprised.
Post by gregtowers on May 18, 2017 10:25:26 GMT -5
Finally got around to doing this the right way. Put it off long enough - Threw away the manual. (actually just closed it) Purchased two sets of 10ea pilots and mains. Got the gold needle (DGH) that ships with Keihin 39mm (although not for KTM). Started in the middle...Pilot: ended up dropping four notches from stock. Air screw is 1 1/2 turns out. Needle: 2nd groove. Main: Ran four plug chops and ended up dropping two notches from stock for a tan insulator. Leaner than KTM or FMF says, but runs good. Middle has calmed completely down now. Plug chopped at 1/2 throttle, which I have never done and needle is a bit lean, but it sputters when raised a notch so I'll let that go. I am the king of slow anyway so I'll never run so hot that it seizes. Hopefully. Still think my YZ is superior in every way, but as far as ease to work on, nothing has come close to this KTM.
No I'm not going to say what jet numbers I ended up with. I don't need all the screaming "you idiot, too lean, piston is going to melt..." blah blah blah. I'm telling you, the bike runs fine, puts out a little spooge, and plug is tan, not white. It's fine.
Last Edit: May 18, 2017 10:27:43 GMT -5 by gregtowers