02 YZ-426, First 4-stroke, Runs great for 10 mins and then.....

[westtexashacker]

Guys,

I'm in a real pickle. I bought a 02 YZ426 a few weeks back to run in a local GP series. I drove a considerable distance to pick it up and when arrived. It didn't start. Bike looked okay otherwise - I believed the seller when he said it ran yesterday. Brought back home. Changed spark plug. Started and ran for a few minutes and then it started to miss. Would not - start again. Pulled brand new plug out to find black, sooty plug. At this point, any reasonable former 2 stroker would say, I have a jetting issue. Figuring my Yoshi pipe and header and at 2800' elevation, I figured I needed around a 165. Pulled main and it was a 158 instead of a 162 standard. (The needle was in the 3rd groove and the pilot jet was standard.) This flies in the face of logic to me. Anyway pulled carb, found a little contaminant (dirt) cleaned carb, Replaced all jets, cleaned breather, took out and it ran great yesterday for around 15 minutes again. Problem is most noticeable in the mid - full throttle. But thats not the biggest issue. I have 1 week to fix it before the next race. Any ideas? ?

Thanks,

Hacker

[Harry Paratestes]

I'm at 1800 ft. and I run a 155 main,needle in stock position and 38 pilot. It's still a little rich so you are probably a lot richer. Might also check the float level,it might be too high. Consider the GB mod for the accelerator pump, it'll help clean it up too.

[WR426madness]

I would start by making sure the float level is corrrect and that the float seat is working properly. Then set the fuel screw to 1 1/2 turns out and work from there. If that doesen't work take every thing in the carb back to stock and start from scratch. ?

[Fastest1]

Have you checked your vent on your gas cap? Sounds like a malfunctioning one.

[ca412686]

I had the exact same problem and tried the same stuff you did and nothing worked. The problem was the fuel picot. It was semi clogged. Try cleaning that first!

[skthom2320]

Anyway pulled carb, found a little contaminant (dirt) cleaned carb, Replaced all jets, cleaned breather, took out and it ran great yesterday for around 15 minutes again. Problem is most noticeable in the mid - full throttle. But thats not the biggest issue. I have 1 week to fix it before the next race. Any ideas? ?

Hacker - I don't quite understand your current problem. Does it run but after it warms up it starts sputtering at mid-throttle?

Are you running pump gas?

If it's sputtering on pump gas at mid throttle, try lowering the needle one clip (raise the clip to the 2nd from the top) to lean it a bit. But don't discount the vent tube thingie either.

[westtexashacker]

It runs well for about 10 - 15 minutes. Then the spark plug is fouled. I am using pump premium. Since last entry I've gone way down in jets to lean it up. The more I went down the longer I was able to ride. (before fouling the plugs) What I'm interpreting from the popping is the fouling of the spark plug, and the bike miss firing. The float appears a little off so, I'm going to tweak it and give it another go.

Thanks,

Hacker

[Fastest1]

Check the AP squirt time also.

[skthom2320]

OK, so it runs great for 10-15 minutes then starts running bad, then your plug is fouled?

Since last entry I've gone way down in jets to lean it up. The more I went down the longer I was able to ride.

It would be helpful if you included the details about "way down in jets" as well.

A 426 at your elevation with a pipe ought to run reasonably well with one to two steps leaner in the MJ, needle clip, and pilot.

The things I would suspect are:

1. Gas - make sure you really clean out the carb good, get rid of all the old gas in the AP too. Also there were also a number of old threads pointing to alcohol-laden gas like this one => https://www.thumpertalk.com/forum/showflat.php?Cat=0&Board=UBB6&Number=81619&Forum=UBB6&Words=plug%20fouling&Match=Entire%20Phrase&Searchpage=2&Limit=25&Old=allposts&Main=81603&Search=true#Post81619

2. Coil

3. CDI

Anyone you know with a 426 that you can swap CDI and/or coil?

[Fastest1]

I have a CDI and Coil for a 2000 if you need it. Let me know.