I searched the archives on this topic, but I couldn't find any info on my particular question. I tore my bike apart last week and fixed some of the dealers mistakes, they had removed the throttle stop completely, so I trimmed it and put it back in, I also put in a bigger pilot jet (#48), after putting the bike back together I ran it, and it felt very rich in first and second gear. When I say it felt rich, it felt like a really rich or cold two stroke. It felt very bubbly, but cleand out in third gear. It felt rich with the throttle wide open. At lower throttle settings it feels fine in first and second gear, my question is am I hitting the Rev-limiter, or am I too rich on the main jet. I don't experience the bubbling sensation in taller gears, but it may be because the enging has enough RPM to overcome the additional fuel. I am running a 172 main, with a White bros E-series, s-bend with twelve disks, airbox removed and grey wire cut.

Thanks for your help,


What year is your bike? 01 426 or older? What made you put in the 48p was there a problem? What is your overall set up? altitude? need more info.



It is a 01 426, and I live at sea level (240 feet), in Lemoore Ca I typically ride in the desert around China lake ca and mesquite NV. I put the bigger pilot in the bike because I was getting some good backfiring on decel, and the new pilot seems to have helped that problem. I am currently 1 1/2 turns out on the fuel screw. I hope that is the info you are looking for, thanks for the help.


Brett, did you jump from a 42p to a 48p or did you try a 45p first? a 48p is too rich. Did you run through the fuel screw from 1 turn out to three turns out at 1/4 turn at a time before changing the pilot? how did you end up at a 172m thats 3 sizes up from stock. I have a 01 WR 426 and when I ride at sea level I run a 162m. If I run any bigger than that I get pretty rich. I don't think that your pipe would make 4 jet size difference. I'm running a stock pipe wide open. lid off wire cut. but I have mine YZ timed. I tried a ton of setups and a 162 is where it runs best. I would try a smaller main first like a 168 then try the fuel screw. If it still feels rich at 1 turn out then try the next size smaller pilot. Keep trying the rewards of good jetting are sweet!!!!!!!!!!!! JW


I did try a 45 pilot from 1-3 turns out and it didn't seem to help my backfiring problem, so I went a bigger pilot.

It has been really hot here as of late, and I think that I might be a size to big on the MJ, so I will going down one on the MJ to try and solve my problem. Any thought to changing the needle jet, I know a lot of guys are running EKP needles. I love the bike, but I am trying to fix some less than optimum set up work from the dealer I bought my bike from. I am still pretty new to the whole 4-stroke thing, but I will never go back, I love this bike.

Thanks for the help,


have you looked for air leaks? that might be your problem. I tried a 45p and a 48p They where way to rich. I think I would look at the air boots and make sure there is no leaks. check where the head pipe and the muffler join aswell if that clamp is loose it will cause it to pop as well. When you say backfire do you mean popping at the pipe or does it actually backfire through the carb?



I have not looked for leaks as of yet. I wouldn't be surprised, when I got the bike from the dealer, I had them install the White Bros exhaust, and the mechanic had installed the disk backwards, and there have been several other things that have been wrong with the initial set-up of the bike.

The back firing i am experiencing is mostly at the tail pipe (popping). However, there is the occasional backfire through the carb.



Nedley, Hey besides the popping at the pipe how did the bike feel when you tried the smaller pilots. I think you should jet the bike by how it feels but if you have a air leak your jetting for nothing because no matter how your jetted Rich, lean or sweet you'll always get the popping. Air leaks first, check the clamp on the pipe, rule out all air leaks. I still think your probably running way to rich. I put a 48p in my bike last night and got it good and warm and took it for a ride on a pipeline by my house and I got some popping on decel. So I think you might get some popping in a too rich senerio as well. I went home and put in the 42p and the bike was perfect. Very crisp. I think I would do some experimenting I would try a way smaller main. remember the 01 426 runs alot smaller jets than the previous years. I'd try a 168 main and a 45p and see what the bike feels like then I'd try a 42p and see how the power feels. Then I'd even try a 165m and run through the pilots again. See if this gets you an idea if your running lean or if your running rich. This might take you an hour but it will give you a feel for rich or lean. Then you can decide were to stay at and fine tune from there.

Try a range of jets and feel how the power hits. Then you'll know if you found the sweet spot!!!!!!!!

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