$6K YZ paperweight

If you live at altitude, think long and hard before buying a YZ426. You might draw the short straw. After six trips to the dealer, my 2001 still won't run more than 10 or 20 minutes without fouling the plug. My first hare scrambles on the bike consisted of ride 1 lap, foul plug, pull tank, change plug, ride one lap, foul plug, etc. (My 1999 ran like a top, always started, carberated perfectly, wish I had never sold it). Colorado Powersports has tried a 40, 42, 45 pilot, all needle clip positions, 152, 158, and 162 main, and various modifications to the accelerator pump. All with the same results. The colder the weather, the quicker the plug fouls. Ignition, float level, and TPS check out fine. Diagnosis with an Exhaust Gas Analyzer says everything is OK. Its not a starting problem. It fires first kick with a fresh plug. Although most 426s run fine in Colorado, I know of two other bikes like mine. My friend had a 2000 that after six months of this, with various dealers trying to fix it, made his dealer give him a 250 instead. I am out of luck, I bought mail order. Yamaha is sending out a different needle to try. They seem clueless. If the needle doesn't work, I am going down to the KTM dealer. Maybe I should wait for the new Honda. Anybody at sea level want to buy a used 426?

Sounds like the float needle could be sticking?

Try a 1999 YZ400 needle DVR in clip #4. It will be leaner EVERYWHERE from 1/8-full throttle - P/N 5BE-14916-DR

The service dept. may not have wanted to put it in a much leaner clip due to hesitation on the stand from idle. The DVR will be less prone to this because it has less taper yet will surely be more lean at fully closed throttle throttle because it's bigger there. If it's too lean you can gradually work up from there.

James

Check the plug cap and make sure it is not defective.

Remove the check valve from the vent line coming out of the top of the gas cap. The fuel tank presserization problem is REAL with these bikes and carbs.

Follow James Deans suggestion using the DVR needle.

Good luck

Clark

Originally posted by Clark Mason:

Check the plug cap and make sure it is not defective.

Remove the check valve from the vent line coming out of the top of the gas cap. The fuel tank presserization problem is REAL with these bikes and carbs.

Follow James Deans suggestion using the DVR needle.

Good luck

Clark

Thanks guys for the suggestions but no dice. The second dealer I took it to did all the same stuff (including your suggestions) but no improvement. Then he got real mad and took a carb off a 426 on the floor and installed it. Presto, instant cure for the $6K paperweight blues. However, a new carb is $1200. I think Yamaha is going to take care of me though. I think they want my carb just so they can go through it to satisfy their curiousity. I will keep you posted on whether YZ 426 owners can trust Yamaha to take care of such problems.

Interested in hearing the outcome. Wonder if the needle was being held down properly or if the nozzle was resting too low. DVR in an upper clip is REALLY lean. Let us know if you ever do find out.

Originally posted by James Dean:

Interested in hearing the outcome. Wonder if the needle was being held down properly or if the nozzle was resting too low. DVR in an upper clip is REALLY lean. Let us know if you ever do find out.

The dealer is saying that Yamaha suspects a faulty assembly of the upper carb body to the middle carb body. I guess that has happened before. Looks like Yamaha is going to cover my costs on the carb. I'll be happy, as long as the bike runs well.

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