I have Yamaha Viper that only wants to run on two cylinders... will on three if I use half choke, so I am assuming a carb is dirty and a cylinder is running lean, but this one backfires thru the pipe when choke off and only running on two? Why the backfire or detonation? I heard that the later models came with some type of detonation control sensor and was wondering if my problem might be part of a bigger problem that Yamaha found out about after my model year. Thanks to all.

nice post like it thanks for sharing it keep it up

try cleaning the exhaust valves. they were completely stuck on my sled,
and the symptoms were low power, NO performance in deep snow,
detonation in the pipes when shutting down.
to clean them, you need an allen wrench(think it was 4mm),
pozidrive 2 screwdriver for the cover on the lifter motor,
some lamp oil or paraffin, some cloth or non fibrous paper,
and an appropriate size (4-7mm * 100-200mm) rigid stick to operate
the cloth in the valve-slides(you can use the screwdriver also), old toothbrush to easily clean the valves
and a container to clean the valves in.

regards,
Frode.

Always try new plugs first. It would back fire because the bottomend is loading up with fuel. If the '02 is a two stroke.