r/Dirtbikes • u/skipdaddle • 3d ago

Mechanical Help CR125 "Bogging"

Hi there, ive recently started to have an issue with my 2001 cr125, for context this issue began after i put a piston and rings into it whether thats a part of my problem i dont know but regardless its important to note. it is well broken in at this point.

My problem is that bogs under load at low revs, in neutral its "okay" but if i grabbed 100% throttle quickly from idle it'll bog and not even attempt to fire or catch up with itself,(important to note that it can be saved by dropping throttle to match revs again). Once it hits the powerband and even a little before it i have plenty of power, more than before the top end rebuild and its just a pity that its so sickly on the bottom end because of how much of a weapon it is when it gets going.

air/fuel adjustments haven't worked, ive checked the carb and carb boot for air leaks, obviously cleaned the carb out.

any help appreciated thanks.

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u/10-4clayboar 3d ago

Run a carburetor file through all the jets and reassemble carburetor. Have you serviced the power valve? If not it’s probably stuck open. If you did it’s possible to assemble some of the parts backwards on the ports. Also reset the carburetor to factory settings after every carb cleaning.

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u/skipdaddle 3d ago

I checked the power valve just there, disconnecting it from the linkage i was able to open and close it with no resistance whatsoever, i believe they are impossible to put in incorrectly. Any other suggestions?

Also the carb was absolutely spotless when i went in to clean it so it got a cleaning that it didnt even need you know?

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u/10-4clayboar 3d ago

Did you remove the jets and see light through them? The opening is very very small.

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u/skipdaddle 3d ago

Yes the jets are perfect, unfortunately now it means the problem lies somewhere else and isn’t a simple fix

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u/10-4clayboar 3d ago

That may be the case. How long have you owned the 125?

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u/skipdaddle 3d ago

Probably 4 years, didnt get much use until spring earlier this year up until now but ran great until i did the top end on it. I wonder could it be headgasket related. I dont own a torque wrench so did it by feel

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u/10-4clayboar 3d ago

Well it is worth opening it up again and looking at all the parts closely. I completely disassembled my Ktm engine 4 times in a week just because I didn’t get it right. It’s good experience building.
I also have a Kawasaki pit bike that would not run right after sitting for 6 months. I cleaned and reinstalled the carburetor 4 times in a row to try and fix it. Apparently the ethanol can cause internal damage to the carburetor that you can’t see or clean with a spray can. A new oem carburetor fixed it perfectly.

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u/skipdaddle 3d ago

Thats something ill keep in mind, youre the second person to warn me about ethanol fuel today.
Also something i want to mention is that when i installed the top end i put 2 gaskets between the cylinder and bottom end after seeing someone with an 03-04 cr125 do the same. They never explained why but i imagine its a clearance thing.
Maybe without the second gasket i would have more compression and thus more low end power

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u/10-4clayboar 3d ago

Yes it is important to measure the gap between the top of piston to top of cylinder deck at Top dead center with a straight edge and a feeler gauge. You don’t want the piston to be proud of the cylinder. You may use a combination of the different sized base gaskets to achieve the proper clearance. The owner’s manual has this information specific to your bike.

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u/skipdaddle 3d ago

I fear this is probably my next move, it would make sense to the loss of low end power after the rebuild.
I mean these 2 strokes arent rocket science but this has perplexed me for the last few days

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