OK, calling in the reinforcements. My KX125 carb issues?

OK, so some may remember that I was having jetting issues with this 125. I narrowed it down to the tab on the float had a wear dimple in it and wasn't allowing the needle seat to move properly. That's fixed now. Float height is set at 9.5mm according to manual. The bike runs great and has no sputter. Current jetting is 400/37.5/2nd pos/1-3/4 turns.

The problem I'm dealing with now is after running WOT, the carb is running dry and it's causing it to lean out. The idle will hang until the carb can refill. I cleaned the petcock, cleaned the whole carb, it has good flow, etc.

I'm even still getting some over flow if the bike it tilted slightly to left. I mean it comes pouring out the overflow tube. Can't figure that one out either.

This bike is absolutely P-ing me off. I need to get this thing right. My daughter is ready to fit on it and the 85 is getting sold.

Help me out here fellas, I'm at a loss. any idea is worth trying right now. Besides a block of C4.

How do you know the carb is running dry? The hanging idle to be a bunch of things from cooling to air leaks.

At first I thought it was a bad crank seal, so I rebuilt the engine with all new seals. I did have a bad r/s seal that was sucking in trans fluid.

After getting it running and that problem was solved, I began dealing with the float issue of either not getting fuel or getting too much. So a guy at work and I started going through everything and he had seen the dimple in the float tab. Very small but was more than enough to make needle seat stick. We polished it out and reset float height. Now it runs great but has this issue now. If you just normally rev it it doesn't hang like it would if there was an air leak or other problem. If I hold it for a few seconds above 3/4 throttle or ride it WOT and pull in the clutch, it'll hang.

After 2 weekends of trying different things we narrowed it down to just not getting fuel fast enough. I even put on a larger fuel line just to try (10mm fuel injection line) and clamped it so it wouldn't leak. I even took off the gas cap thinking maybe the breather was clogged. No luck.

The use of explosives is starting to sound pretty good right now.

EDIT: also I did a plug test for S&G's on Sat. I put in a new plug and just rode it around easily with out getting too far up in the revs. Checked it, it was nice and dark. Put in a new plug and commenced to ripping around WOT for about 3-4 min. Plug was white.

Edited by SAPPERS

I can run mine WFO and I'm running a 360 main. I'd pull the ignition cover and pressure test the engine. Just revving the engine on a stand isn't the same as if it were under load.

Good thinking on the gas cap vent, forgot about that one.

You can double check your float settings using the clear tube test.

I want to do the clear tube thing but I need the actual tool to do it. The one that screws in the bottom of the float bowl. These don't have a drain.

Also, if I just cruise around and tap into the powerband for a second and pull in the clutch, it doesn't hang. If I'm on it hard, I can start to hear it ping and run lean. Slow down to a cruise and runs great, hit is instant.

I gotta get this thing right cause I don't want to sell it and want to use the money from 85 to buy another 125. This one is my sons but he's giving it up for her. He has his 250 but still likes to race the 125 at times. Plus, I won't have anything to ride with only 2 bikes.

Have you tried a bigger main? Maybe the lower throttle range is fat enough to compensate for a lean main during the short blips, but only for a short time. Seems unlikely the bowl is drying up. Also are the vent passages clear? Some basic suggestions here i know, but i know how i get when really frustrated with something so i tend to over look the simple things.

Could it be possible that the main is lean enough, so that when you run WOT it is leaning out the air/gas ratio in the bottem end enough that it allows the idle to hang for a bit before the pilot circuit can catch up with enough gas to cancel out the lean condition :excuseme: lol I'm grasping for ideas here :smirk: I was going to say a bad gas cap causing a vapor lock but you got that covered.

I might try the main. It ran so good before having a 370 in it. Now I have 400. Maybe I'll go to 410 or 420 and see if that works. Good thinking Moose. Thnks.

I might try the main. It ran so good before having a 370 in it. Now I have 400. Maybe I'll go to 410 or 420 and see if that works. Good thinking Moose. Thnks.

If it ran good before, then there's obviously something wrong and you're trying to compensate a lean condition with jetting. A pressure test is the only way to confirm the engine is sound. Then look into tuning issues.

I still say air leak...for the record.

hard to get a good plug color read with a new plug, have to look at the ground strap, if you do a wfo pull and thinks it's fuel staved, can you coast to stop and open the bowl and verify there is no fuel? you may just have a fuel needle and seat sticking closed from wear and having the fuel level too high (your comment about overflow at small lean angle).

Edited by highmarker

replaced float valve? If this is a Mikuni TMX (jet numbers listed suggests this) you can try a richer nozzle (main jet screws into that). That helped my TMX equipped 02 CR250 - it was always lean no matter what I did until that. Ultimate fix is the Keihin PWK however.

I have a leakdown tester I use for my car engines, I'm gonna rig it up this weekend. But if this weather stays like it is, I won't be doing any test rides. Ughhh, it's freekin cold. Burnin up the propane in the garage!

Just a bit more info.

You might have read I was having some bogging issues with mine while I'm turning. Figured the float level was too high...and it sure as f*** was! I didn't take a measurement, but it was about 11-12mm, basically as high as it could go. lol

The carb is a bish trying to set the float. I made a template/guage out of cardstock, basically a bridge to go from one side of the gasket surface to the other with a 9.5mm cutaway. You can't see in there so I mounted the carb in a vice at 45* using those little tits on the side where the slide is and carefully set them. So tiny in there, took like 15 minutes. I used a pick to pry/jam the float to where it just touches the tab, then go-no-go with the gauge.

Seemed the longer I was trying to look at the pin on the needle, the smaller it got! :lol:

Edited by Jeekinz

I had a 82 RM250 that the petcock did not flow enough to keep the carb full at WOT and I ended up putting it in the drill press and bored it out as much as I could get away with. Fixed the starving problem but it would not fully shut off after.


Put needle on third clip position.

Jeekinz, that's what I made mine from too.

Ed, I'm gonna check the petcock again today. Maybe I missed something.

Jim, might try some jetting again today but its 18* right now.

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