I have 2004 yz 125 fmf phatty pipe with fmf shorty silencer the head is shaved 80 thousnths and bored 20 over .bike is burnin hott what size jetting should be used for main and pilot. stock is 420 mai

I take a small drill and go up to the nearest 0.1 say if you have jet with 104 bore, go to 1.1 drill, means 110 bore, next is 1.2 and so on,
When you find the right size, then buy a jet in that size.

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.

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

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.

you are running to rich by the sounds of it ..dave

I have an 04 125 with a FMF fatty Factroy expansion chamber and a Titanium 2 FMF silencer (shorty) the carb settings are Pilot a size leaner than stock main IS stock jet needle is a setting toward lean and air screw is two turns out. My gas:oil mix is around 32:1 - 40:1.. My spark plug buget is killing me, and I havemessed with it til I was blue in the face can anyone help me?,don't worry I didn't take it wrong it is a good question, and let me put it this way.. I run it like a two stroke lol I have had more than my share of two strokes so I don't lolly gag it and usually for the trails I ride you really can't get out of 1st most of the timeI spend most my time around 3/4 throttle. When I 1st got the bike I had a friend messing with the idle screw on it cause it wouldn't idle at all and he messed with the mix screw by accident so I think it was screwed out more than two turns but I set it according to FMF recommendations with the pipe and chamber. The guy who had it before me raced it in a AMA class I don't know which so don't ask lol, so who knows what has been done to it other than what I can see,,,

I have an 04 125 with a FMF fatty Factroy expansion chamber and a Titanium 2 FMF silencer (shorty) the carb settings are Pilot a size leaner than stock main IS stock jet needle is a setting toward lean and air screw is two turns out. My gas:oil mix is around 32:1 - 40:1.. My spark plug buget is killing me, and I havemessed with it til I was blue in the face can anyone help me?,don't worry I didn't take it wrong it is a good question, and let me put it this way.. I run it like a two stroke lol I have had more than my share of two strokes so I don't lolly gag it and usually for the trails I ride you really can't get out of 1st most of the timeI spend most my time around 3/4 throttle. When I 1st got the bike I had a friend messing with the idle screw on it cause it wouldn't idle at all and he messed with the mix screw by accident so I think it was screwed out more than two turns but I set it according to FMF recommendations with the pipe and chamber. The guy who had it before me raced it in a AMA class I don't know which so don't ask lol, so who knows what has been done to it other than what I can see,,,

I have an 04 125 with a FMF fatty Factroy expansion chamber and a Titanium 2 FMF silencer (shorty) the carb settings are Pilot a size leaner than stock main IS stock jet needle is a setting toward lean and air screw is two turns out. My gas:oil mix is around 32:1 - 40:1.. My spark plug buget is killing me, and I havemessed with it til I was blue in the face can anyone help me?,don't worry I didn't take it wrong it is a good question, and let me put it this way.. I run it like a two stroke lol I have had more than my share of two strokes so I don't lolly gag it and usually for the trails I ride you really can't get out of 1st most of the timeI spend most my time around 3/4 throttle. When I 1st got the bike I had a friend messing with the idle screw on it cause it wouldn't idle at all and he messed with the mix screw by accident so I think it was screwed out more than two turns but I set it according to FMF recommendations with the pipe and chamber. The guy who had it before me raced it in a AMA class I don't know which so don't ask lol, so who knows what has been done to it other than what I can see,,,

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. ,,,