Need 3000 ft elevation jetting for 2002 yz250 with fmf SST pipe and fmf spark arrestor

Hard to give accurate info with out seeing the bike with jetting but I would suggest take a reading from the fuel air screw if the bike running better with the fuel air screw in carbi at 2.5 turns out or more then definitely increase pilot jet up 1 or 2 sizes if you get the bike to run well between one - two turns out on the fuel air screw that is where your jetting should be
I wouldn't be concerned with main jet at all - it should be fine for wide open throttle unless you feeling power loss when you open it right up
also if you are getting backfire or popping when de accelleration that also hints toward lean pilot so a size or two change in pilot jet would be worth trying

There is no need to change jets. Messing with them will do more harm than good.

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

this should get you close, will still have to fine tune due to exacting conditions.....humidity, temperature, pipe, silencer and reeds....however try this:

Main 360
Pilot 20
Needle H1-70
Needle Top Clip
Air Screw 2 Out

make love to the bike

Hi, when you change your pipe to the FMF you reduced the back pressure and gave it more flow. That causes it to run a little "rich". Look at your spark plug and check the color of it, it should be a nice tan color. If its a dark brown then you need to "lean" out your bike. The easiest way to do that is by taking the slide out of the carb and taking out the main jet, the lock washer is set to the middle position for a stock set-up. Move it down 2 notches and then run it and see what your plug looks like. The black "goo" is the unburned oil from not running lean enough. Another trick to try is run a NGK-EV plug. it burns hotter so it will help with the oil problem (and make it a little more snappy also). And as an added bonus it will be easier to start.

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.

change the jetting...youcan contact FMF for thier setup recommendations for running that pipe with the bike...very common problem and jetting will take care of alot of the issue

I know there were a few different models: X440S only FMF Powercore aluminum silencer/stainless header combo E440/C440 standard header/silencer with midpipe attached (long can) (2 spark arrestors) S440 standard header/shorty silencer with seperate midpipe (1 spark arrestor) X440 standard header/silencer (NO spark arrestors) Just wondering whether anyone has ever done any hp tests between these. From my experience the X440 can would flow the best because of the lack of spark arrestors but I'm wondering how the FMF powercore compares? The FMF has shorter header runners leading to the merged portion so the shorter header should deliver more low end torque. Also do I have the S440 can info right, only 1 spark arrestor in the shorty?,The so-called S440 pipe did not have a separate midpipe. It had a fixed welded inlet, same as the longer can, and was full open. Also, I have a New 03 X440 here with the SAME exhaust, ...but another with a long and full-open version. So, you really cannot tie all exhaust versions to a particular model, since too many owners will get confused. Everybody would need to check their own machine to be sure. But if ANYBODY out there has a separate midpipe?, I'd really like to confirm wazzup -- Up until now, I have thought that the only ones around are from the small batch that I had found over in Holland. And, on just a few bikes that were assembled in Europe just prior to the bankruptcy. ,,,