Wont stay running/just rebuilt,,67mm all new

Have the spark plugs been replaced? Have the plug wires been tested?

usually a 2 stroke will chuff/backfire out of the exhaust if it low on compression, did you put the rings in the right grooves and up the right way and is the piston in the right way around as there is a front and back ,the ring pegs are supposed to face the inlet side of the barrel.

Is the idle properly adjusted? The ringing is normal for a 2-stroke.

Might try changing the spark plug. Good luck.

Check reeds, for cracks, splits,correct gap,and reed stops. This should correct backfiring. Check thermasrats for correct opening temp. Is water tale tail pumping out water?

I would first make sure the power valve is decarboned and moves smoothly. Jetting is trial and error as so many things can affect the mix. Temp. humidity, gas quality, oil ratio etc. Tht being said you can make small changes and keeping a log with results. First write down baseline settings. Include main jet and pilot jet numbers found stamped on the jets and needle number. You don't say where it is you are trying to get power. 125s have poor bottom mid power, it is all on top for racing. Don't change and race or you may keep going when you should stop and melt something. Try to test at the altitude where you will race and read the plug often, cleaning or changing the plug between tests. Be alert for the surging feel of lean mix with racing idle do not let this continue. Remember 0 to 1/4 throttle, pilot and needle base width. 1/4 to 1/2 throttle controlled by needle mid taper. 1/2 to 3/4 throttle needle tip and main. 3/4 to full is the main jet. Find out the stock settings and make sure to start near there. I don't know about a spes pipe but pro circuit has charts for jetting and maybe a support line. I really feel you will get more from a clean powervalve, repack the silencer, clean airfilter and maybe v-force reed. Gearing can offer some options. I just read about a James Dean jet kit for cr250 that puts more power in the low mid. I might try that for my bike. Good luck, just remeber " too lean, too long- too bad"

if the bike was running fine before you replaced pipe,you may need larger jets,or richen fuel mix,also check the reed valve it may be on the weak side.try a new spark plug also if you have not already replaced it.

Lack of power is usually bad valves. Clean the air cleaner and do a compression check. Could you take a moment and rate my answer? Thanks!

carb sync problem.. check online for some really easy to make carb sync tools from home..

the tubing with liquid fill worked well for me

cheers

make love to the bike