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2002 YZ125 strange bogging

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First off would like to introduce my recent purchase, a 2002 yz125 ImageUploadedByThumper Talk1402198823.825438.jpg

This is my third bike (previously owned a cr125 and a rm125). By far enjoy riding this beast over my two other bikes. But I've been having some bogging issues..

So from here on I will start a log on what I've done to the bike so far and hopefully i can narrow down the cause and help others learn as well.

I gave this bike a FULL bottom and top end rebuild; new crank, seals, bearings, piston and cylinder.



- My compression is at 120psi constant (cold and hot). This thing runs like stink and the front wheel will never want to stay down, even in 4th gear!

- running a 50t rear with a 13t front sprocket.

- stock exhaust with fmf shorty silencer

- carbon fiber reeds with stock cage (not sure brand of reeds)

- 420 main jet with a 30 pilot jet

- needle position is on 3rd

Everything else is stock.


But after 30 minutes of riding, the bike will start to lose power and eventually bog out. Strange though I can keep rolling on quarter throttle but once i hit half/full it'll sputter and slowly bog its way down until it stalls. Note that this all starts happening once the engine has been running for a decent amount of time (30-40mins).

So this is where the troubleshooting began.. Pulled the plug and it seemed to be running rich top end. Adjusted the needle to every position to see if I got any improvements.. Still bogs. So i swapped the main jet to a 410 and it was WAY too lean.. Back to the 420 it was. My bottom end was sputtering quite a bit so i downsized to a 25 pilot with 1 1/2 air screw out which improved my bottom to mid range power quite a bit. Finally I was getting somewhere, but still bogging out after I hit that 30 minute mark of riding. Started to think maybe I needed to change my throttle needle? Running a #79 throttle needle and heard the #77 was my best option for the 02 yz's. So thats on order.

Meanwhile I managed to score a 2001 yz125 carb for 100$. I read online that the main jet on these carbs were a 460 with a 25 pilot. Throttle needle is a #75 so it's bigger than what's in my 02. I said heck with it why not see what happens if i throw it in my bike. Went out for a rip and it ran AMAZING. I have yet todo a chop test on it along with a full 30mins + ride (it was getting dark). So more updates hopefully tomorrow.

If anyone knows what could be the cause to this bogging, commenting would be greatly appreciated!

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Sounds like over heating (water pump is damaged) or a dying stator


There is no reason to think it is  the carb, as the carb requirements will not 'change' after 3 min of use.


30 minutes in would be most likely related to heat damage (stator) or heat build up (non functioning cooling)

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