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YZ400 carb in YZ426 bike

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I bought a used YZ426 last fall and just found out that it has a YZ400 carb. It was running fine all winter and summer. Just this fall it started to sputter 1/8-1/2 throttle. I took the carb apart and cleaned everything and I thought I put everything back the way it was. Now it is worse in the 1/8-1/2 throttle range. I tried moving the needle up and down 1-2 positions with no change. Has anyone else put a YZ400 carb in a YZ426 bike? I am at sea level with 195 main, 55 pilot, 82 starter, clip at 3, and 1.5 turns out.

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I forgot to mention it is a 2000 YZ426 with all stock engine (I believe). It has an FMF powerbomb header with FMF silencer, no mods to airbox but it has a Uni filter. I ride between 0-1000feet, 40-90 degrees F.

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I took the carb apart and cleaned everything and I thought I put everything back the way it was. Now it is worse in the 1/8-1/2 throttle range.

Did you get the slide plate oriented with the square end down??

If the plate is installed upside-down you will have jetting problems from 0 - 1/2 throttle.

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The slide was assemblied correctly. It won't even fit upside down. I have the carb apart again. The main air jet is a 200, the pilot air jet is 75 (stock is 100), the needle is marked OBDTM, the float height is .45inches (stock is .35 for 400 and .31 for 426). When I first assemblied it after cleaning, I did notice that the front carb boot was not tight to the engine. Could air have been entering here requiring larger jets? Does the jetting seem at all right? Why would it be so much higher when both carbs are 39mm? Stock jetting for a 39mm 426 carb is 162, 42, and 72. I’m at 195, 55, and 82. I am thoroughly confused.

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If the pilot air jet is a #75 and the needle is an OBDTM, the carburetor is from a WR400- 1998/1999.

The jetting you would NORMALLY use is-

#45 pilot jet

#75 pilot air

--no WR400 air cut valve--

OBDTM needle #3 or #4 clip position

#175 main jet

The YZ426 used a OBExx needle and needed a MUCH smaller main jet. The YZ400 used a OBDxx needle with #175MJ, which is very common. A #195 main jet is excessively big (too big in most cases). The #55 pilot jet is also too big with the slide plate correctly oriented (but not if upside down) or might be OK with an air leak.

The 82 is just a choke jet and has no bearing with the choke off.

If you had rich low speed jetting sputter, it would be from the pilot jet or needle clip position. The '99 YZ400 used a #45PJ with #100 PAJ and the '99 WR400 used a #75PJ with #75 PAJ and air cut valve(adds to the pilot air jet).

The same carb on a '01/'02 KTM 400-520-

#48PJ

#100 PAJ

OBDTM #2-#4

#175MJ

Am I helping or confusing things?? 😢

James

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Are you recommending a 45PJ in my setup? Another thing I noticed when I took the carb apart the second time was the pilot screw was 1 turn out. I'm sure before I messed with the carb, I recorded 1.5. So I must have set it wrong during assembly. That's one thing to try. But that would richen up the mix. I had tried the needle up and down 1-2 positions with no change. I'll be able to try some things this weekend.

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A #45 or #48 pilot jet 'should' be a better choice with a DTM needle for a steady idle and good response in this version of FCR carburetor. Note that you will need a '98/'99 Yamaha FCR pilot jet and not the newer version.

James

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JD,

Would it be best if I got your jet kit? What main jets are included? I know I will still need to get a pilot jet. Should I also get a #100 pilot air jet (instead of the #75)? If I get the #100, is a 42 or 45 pilot OK? I really want to get the carb acting like it should for a 2000. I ride trails until I find a coal hill to climb. Thanks,

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The jetting kit would help. Keep the #75 pilot air jet and try a #42 pilot jet. I've run the same setting before on the same carb with excellent results in low speed response and starting.

James

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I bought and installed the jet kit; blue needle at 4, 170 main jet, pilot at 1.5 turns out, 42 pilot jet. When I first tried it, the idle would hang up after revving. I adjusted the pilot and it kept getting better until I was at 3 turns. Then I put in a 45 pilot jet. Started at 2 turns but the idle would still hang up a little. I am now at 3 turns out and the idle still hangs a bit. I think I need to go to a 48 pilot. The air temp was around 45 degrees at sea level. This should be the coldest I'll ride in. Since I have to order a 48 pilot, should I consider getting a pilot air jet? I'm at 75 now.

The throttle response was awesome. A big difference over what I had. I can't wait to get it out on the trails and hills. Thanks!

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Make sure you don't have any air leaks. A #75pilot air with a #48PJ seems richer than needed in most cases.

James

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The more I look into this, the more problems I find. I now noticed that the engine is really a 400cc (by the big 399cm3 on the cylinder). And after going through the previous repair bills, I believe that it has 426 cams (both intake and exhaust). Will the cams mess up my jetting?

I think there could be air getting in at the throttle shaft on the cable side. There was dirt up in around the top of the throttle slide the first time I took it apart. Where might I get the seal for the throttle shaft? I don't see it on any parts finder.

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The motor is a 400? The jetting seems to be a WR400... The cams are a 426...

This is getting stranger all the time.

Judging by your last post, use the pilot jet that allows the bike to idle down OK and maybe try the blue needle in the 5th from top clip position.

James

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