What is the best jetting for my 2003 cr 125 with pro circuit pipe,evry thing else is stock,i live in ohio

whatever a professional manual or pro sez-jets are a delicate thing cause you cud end up rebuildin again--the web n fixya has books or getta hold of a motocrosser

your factory jets should be ok with stock carb and exhaust,unless it came from way down south or someone changed them.factory jets are main jet #380,pilot jet #30

The stock main jet is #350. As to fuel, it is best to run premium gas.

Stock Jetting for a 1994YZ250 is:

350 main
45 pilot
J8-6EJ33-61 needle

Adjust your jetting down one size for each 1000' of elevation above sea level. Needle clip position on the needle fine tunes the pilot circuit.

Scott

Clean the carburetor. Move the clip on the jet needle downward one or two notches if your model has a stepped jet needle.

best thing go to a dealership in your area, they will advise you and the cost of a jet is very small.

Standard main jet for a 1999 CR125RX is #162.

i changed my pipe on 07 dt125lc and did not rejet, seemed fine until i holed a piston. best to rejet me thinks, about £20 instead of £150 for piston rebore bearings and jets plus the time to re build.

Scatter gun approach > thoughts to check out > > >
Clean and dry the air filter? Go back to a stock main jet, at least to check things out? Are "O" ring seals on the main jet going bad?Partial internal collapse in exhaust (remove exhaust pipe & do a quick run down the alley), Install a new stock NGK spark plug, Be sure battery connections are clean and tight, Be sure intake manifold and carb are good and tight.

Hope this helps!

Yes. The application of a proper pipe on a two-stroke motor is critical. The 250 pipe has way to much volume and not enough back pressure to allow the 125 motor to breath properly.Also check the ignition timing carb jetting and power valve for proper working order per Honda specifications.,