2002 YZ426F - Cold weather issues

Hey there, as the title says I have the old 426! Anyway, its bored and stroked, sadly I don't know the specifications because I got it from a middle man, it leaves a 2002 crf450 behind like a 125 2 stroke would to a 125 four stroke, if that helps explain the power.

As it became colder out, I noticed it would like to backfire more and doesn't like idling :thinking: (on average about 40 degree's out)

I'm new to owning yamaha's, any suggestions would be appreciated! cheers! :cheers:

Go up one size on the pilot jet, you will need to adjust the fuel screw slightly but it will fix it. You may want to go one up on the main jet too.

I would just try the fuel screw first,back it off some to see if it runs better.

Basically you are running to lean. As the temperature drops, the air becomes more dense, so more air actually is going through the engine than before. You need to increase the fuel to keep the same air:fuel mixture. Not idleing and popping on decel when you close the throttle are both signs that you are lean on the pilot jet, fuel screw or both.

Try richening the low end with the fuel screw by turning it OUT, maybe 1/4 turn at a time until the popping stops. I can run my WR426 in any temperatures from 30degF to 100degF, just with adjusting the fuel screw. Sure it may be a little off at the extremes (I jetted it while it was about 60-70degF outside) but usually just a bit of fuel screw adjustment and any popping or sluggishness goes away. I only ride trails so I don't really care that I am not achieving "maximum performance".

If you can't get the popping to go away with the fuel screw, go up one size on the pilot as already suggested.

Edited by Pooley

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