Bought an 05 RM125 this year and it ran fine. Recently put a top end in it and it would stutter about where power and should be.
Checked power valve, cleaned carb and put stock jets in, no change. Put old piston back in and it ran even better than before. Probably jetting.
Tried another brand of piston and same stutter above 1/2 throttle.
I’ve only been running it up and down the street with new pistons. Is this break-in related?
Upgrade to enjoy this feature!
Vital MX fantasy is free to play, but Premium users receive great benefits. Premium benefits include:
- View and download rider stats
- Pick trends
- Create a private league
- And more!
Only $10 for all 2026 SX, MX, and SMX series.
Are the pistons all flat topped not domed, do they all have the same 29.5mm compression height?
Everything looks the same to me, codes in the middle are the same.
The top of the old piston looks damaged, from debris or detonation, is the big end OK, do you have an air leak?
+1 on potential air leak, it could be you didn't properly seat carb boot when reinstalling.
Forgot to connect the powervalve actuator? 🤐
Can't think of anything much else that could change when swapping between pistons. Break-in won't cause this
The Shop
Luxon 4-Post Bar Mounts
$189.95 - $239.95
Free shipping: VITALMX
Do the pistons measure the same?
Ring gaps the same?
Did you deglaze the cylinder?
Was there an A or B piston offered? Guessing that would show in the part number but just thought I’d throw it out there.
the one on the left is size A piston, the new one is B-size piston in the picture buddy
Oh, I didn’t notice that and didn’t have options when I ordered. I see 53.95, 53.96 and 53.97, which is A B or C?
I’m convinced it’s piston related as I’m sure I didn’t fix air leak when putting old one back in, then cause air leak when trying new again.
53.95mm size A
53.96mm size B
53.97mm size C
53.98mm size D
Post a reply to: Stutter after top end job