YZ400/426 Jetting help needed

[stnkbg1]

I've got a 99 YZ 400 that I just rebuilt into a 426, using all factory parts. As far as I know the only other mod is a big gun full exhaust. It always used to start on the first or second kick, and it still will, but only with the choke off. If I try with the choke on I cant kick hard enough to get past the compression stroke. Once running the bike is strong and fast, but if I'm running a constant throttle setting (like when I'm on the freeway) it kinda hickups a little, and there's a TON of popping on decel. I tend to think that it's running a bit lean because of all the popping, and because raising the needle seems to help some.

I was planning on goin to a bigger main jet, but I just pulled mine and it's a 180, which is way higher than I've seen in peoples posts on here.

Have any suggestions for me? I've rebuilt a few carbs in the past, but they were all either CV carbs on sport bikes, or on cars, so I'm not exactly sure how these ones work.

Thanks

Dallas

[SRT426]

http://motoman393.thumpertalk.com/tech/files/426jetting.xls

[stnkbg1]

I've seen that sheet, but I'm not really sure how I would use it since it appears to only tell you adjustment factors for changes in temp and altitude, not displacement and air flow. Plus it looks to me like you need a good base line to adjust from, which I do not have.

[rdefonce]

I've got a 99 YZ 400 that I just rebuilt into a 426, using all factory parts. As far as I know the only other mod is a big gun full exhaust. It always used to start on the first or second kick, and it still will, but only with the choke off. If I try with the choke on I cant kick hard enough to get past the compression stroke. Once running the bike is strong and fast, but if I'm running a constant throttle setting (like when I'm on the freeway) it kinda hickups a little, and there's a TON of popping on decel. I tend to think that it's running a bit lean because of all the popping, and because raising the needle seems to help some.

I was planning on goin to a bigger main jet, but I just pulled mine and it's a 180, which is way higher than I've seen in peoples posts on here.

Have any suggestions for me? I've rebuilt a few carbs in the past, but they were all either CV carbs on sport bikes, or on cars, so I'm not exactly sure how these ones work.

Thanks

Dallas

If I recall, the carbs are the same on '99 400 and the 426, accept for the jetting/fuel screw adjustments.

Since you basically have a stock 426 accept for pipe (not sure how that will affect jetting), setting it to stock 426 specs should get you closer to ballpark.

Also FWIW, my 400 backfired on decel far less after I installed a richer pilot jet and opened up fuel screw some.

[stnkbg1]

all of the research I've done shows a 162main and 42 pilot for the 426, and 175main 45 pilot for a 400, which leads me to believe the carbs are indeed different. I tried bumping up the main today and it's way too much, so I went back to where it was. I'm thinking of goin to a 48 pilot (from 45) as all my issues seem to be with the throttle shut. Overall the bike runs great, I'd just like to not wake the neighbors more than I already am comin down the hill leading to my house.

[Speedracer747-400]

all of the research I've done shows a 162main and 42 pilot for the 426, and 175main 45 pilot for a 400, which leads me to believe the carbs are indeed different.

Your correct the YZ400F came with a 175MJ, 45PJ, and OBDVR needle. The YZ426F came with a 162MJ, 42PJ, and OBEJP needle. If your popping really bad on decel it sounds like the pilot circuit is lean. Did you adjust the fuel screw correctly? How many turns out are you?

[stnkbg1]

Right now the fuel screw is 2-1/4 turns out, and that's where it seems happiest. If I turn it in or out more it starts having other issues.

[Speedracer747-400]

The fuel screw adjusts the mixture from closed to 1/8 throttle opening. At 2 1/4 turns out your still in the acceptable adjustment range of the fuel screw. Here's a good article on adjusting the fuel screw and determining the proper pilot jet size.

https://www.thumpertalk.com/forum/showthread.php?t=327405

You might want to give your carb a really good cleaning especially the pilot circuit. Also check to make sure your fuel screw has the spring, washer, and o-ring in place.

[rdefonce]

I'm thinking of goin to a 48 pilot (from 45) as all my issues seem to be with the throttle shut. Overall the bike runs great, I'd just like to not wake the neighbors more than I already am comin down the hill leading to my house.

You're on the right track; a richer pilot should reduce the popping.

My 400F sounded like a popcorn machine on decel, until I installed richer pilot jet! (along with about 2.25 turns out on fuel screw)

[Speedracer747-400]

If the 48 pilot jet doesn't fix the excessive popping on decel with a closed throttle you might have an exhaust leak. Check the joint between the head pipe and cylinder head. Install a new Yamaha crush gasket and maybe some red hi-temp RTV and torque to spec.