Hello, I just bought a 2002 YZ426F, and I'm having a little jetting trouble. I usually ride MX at 5000ft. elevation, I put on an FMF PCIV spark arrestor, and I was wondering if I should go leaner on the main jet. I currently have a #158 main and it backfires and pops alot. I've also fouled two plugs in three rides! Believe me I'm full throttle alot on the track. I have had two tanks of gas total through the bike. I have not re-checked the valves yet. I run 93 octane pump gas which may or may not contain ethenol, I don't recall looking at the pump. Can anyone offer advice? Thanks.

I'm at sea level so I can't help with the jetting (I assume you'll need to go leaner?), but I can tell you that my '01 is a bit finiky with gas. I was fouling plugs when I was getting gas at a Mobil station near my house, but after switching to chevron or sunoco I have had no more fouling problems. It may be worth trying a different gas, but if you're running rich then that could foul the plugs too.

Oddly enough popping on decel is "usually" associated with a lean condition, but I always thought that the higher elevation guys had to jet leaner. I'm sure someone here can help you with the jetting.

Originally posted by YZFORBILLY:

Hello, I just bought a 2002 YZ426F, and I'm having a little jetting trouble.

The main jet is not your problem, although a 158 may be pretty close for your altitude. I don’t think you will need to go much smaller, if any, than that. The backfiring is a little confusing, that usually indicates a lean spot (or exhaust leak), but the plug fouling is likely due to you being too rich on the pilot. IMO a small amount of popping on decel is common with most aftermarket pipes.

I ride at 4k to 5.5k ft. of elevation, although it is probably a bit hotter down here, but here is what I have used in my ’00 and ’01 YZ 426:

40 pilot

~85 Pilot Air Jet

EKQ needle, clip # 3

160 main

The Pilot Air Jet stock size is 100, a WR stock size is a 75. If you go smaller on the pilot jet, which you should do (stock is 42) then the popping you are experiencing will likely get worse unless you go smaller on the Pilot Air Jet. You can get an adjustable one from Sudco (800-998-3529, two PNs: 021-230, 021-235, about $14) or try ordering the WR 75 PAJ (PN: 7A9-14943-17-00).

The stock needle is an EJP. The EKQ is one size leaner just off the bottom and a half clip leaner everywhere (so I am 1.5 clips leaner than you). The EKQ is an optional needle for the ’00 (PN: 5JG-14916-E1).

If you don’t want to buy into all this crap I’m layin’ on you about needles and air jets I would encourage you to at least try one or two clip positions leaner on the stock needle and one or two sizes leaner on the pilot (38 or a 40). This should take care of the plug fouling, and the bike will run much better for you.

Hope this helps.

BTW if the bike is new I wouldn’t even bother checking the valves. They should have a good 40 hours of use before you need to change shims.

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