01 Jetting fmf

First Thanks for all the info and goodwill extended in this forum. This is my first post as I have been able to find everything by searching; however, I do have a jetting question.

I live at 5000 and ride between 4000 and 6000. I have used stock pipe and jetting and have been happy. I just switched to a fmf pwr core 4 sq and it ran horrible. Please advise on jetting specs. Current - 01 426 with 45, 175, 4th position, 2 1/8 out.


175 main seems awful big for your altitude. Stock is 162, and you should be smaller than that by ~2 sizes. A good rule of thumb for jet sizes is one step for every 2000 feet in elevation.

Both the pilot and the main seem awfully rich. I’d suggest going back to the stock setting, which I think is a 42 PJ and I do think the MJ is a 162. I’ve got a 01 426 with a DSP pipe and I’m using a 45 PJ with 1 1/4 turns out on the screw and a 168 MJ. This works for me up to 2,000 feet. I’m not an expert here, but I think the higher flow rate of the FMF pipe might compensate for the higher elevation.

I think you are running too rich, even though you have changed the pipe. I ride MX/HS at 5000 ASL and currently am running a 155 main, stock needle, clip #3, 40 pilot, FS 1.5 turns out on my stock 01 YZ426. My bike runs great. When the temp is in the 90s I run a 152 main and go 1 turn out on the FS. I have a Alt/temp jetting chart that works well for the YZ426, email me at mikeolichney@hotmail.com for one. (I am going to Crested Butte for a week so it may take a while to get it to you).

If you call FMF they are very helpful on jetting changes with their pipes.

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