KX125 Bog

I have a 2007 KX125 Canadian model, which would be the same as the 2005 US model. I bought it used with low hours and a full Pro Circuit exhaust and VForce reed cage. The reeds are in good condition and I just did a fresh top end. Current jetting is 400 main, 35 pilot and needle clip on the 2nd groove. Using 91 octane, with synthetic oil mixed at 40:1.

The bike has a terrible bog around 1/4 - 1/2 throttle. I am totally out of ideas.

Any help?

try moving the clip up to the top position. Also, if you can, I suggest a new pipe, get the FMF fatty. The PC is a topend pipe and really sucks down low. I had a PC works on my sons 125 and he wanted the FMF, WOW what a difference.

Also, I have his jetting at 370 main, 35 pilot, clip on top and AS out 1-3/4 turns.


At 5700' I run a leaner straight section and a leaner cutaway, can't remember if my pilot is 32.5 or 30. Also, I have run PC works and platinum pipes with no problem down low.

At one time I tried the Vforce2 where the petal tension was adjustable for high OR low rpm response, I threw it away.

Was going to say I have a 410 main, with a 37.5 pilot, needle in stock clip. i'm almost near sea level, so it will not be even close to what you need.

Forgot to mention I am at sea level. 72 degrees and she fouls plugs bad.

If it developed the bog then pressure test the engine. Sounds like a chipped reed to me.

If you're at sea level then my setup should work. If you're not always on the pipe it may foul plugs, but I have no spooge and it runs perfect with a 410, and I actually have the 40 pilot in there right now because i'm to lazy to order the 37.5 because its just a practice bike and i'm hardly on the pilot circuit with it on the track. The needle is in the stock position, and it runs perfect. Little bit of bog off idle due to the 40 pilot, but not a huge deal. This is with a stock exhaust. I'm tempted to say you almost seem a bit lean with a PC exhaust at sea level. Thought you normally had to richen it up with an exhaust. It could be a lean bog there between the pilot being so lean with a 35, and the main being a 400. Try the 410, 37.5 pilot if you want really good response off idle, or a 40 if its all you have laying around, with the stock needle in the stock clip.

EDIT: just saw jeekinz post, deffinitely check your reed petals, did not even think of that.

Put the needle on third clip.

400 main sounds too lean for sea level.

My set up for my KX125 was 410 main, 37.5 pilot, stock needle on 2nd from top clip, around 1.5-2 turns out on the screw. I am about 500 ft above sea level... this was with the gas mixed 28:1.

I'm with gixxer, 400 is to lean. I know the stock 420 is Fat, but 400 seems lean like I mentioned previously.

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