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Another jetting (?) problem.

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Last sunday my bike started acting strangely. Few days earlier it felt a little powerless at full throttle. No starting,idling or low/mid-rev problems, just the feeling that not all power was available. And it didn't pull wheelies like it used to. As I was riding it sunday morning, it didn't tolerate full throttle. After a couple hard accelerations it started jumping, blasting flames from silencer and hardly pulled at all. After a while things got back to pretty much normal until I tried to accelerate hard again. Eventually it started jumping and blasting at any throttle and then stopped and wouldn't start again.

I removed spark plug which was wet and sooty. I didn't have a spare plug, I was using it already. So I cleaned it and bike started well again. As I was getting fed up with the bike I left it in my garage and haven't tried it since.

Any ideas why it has started to wear out plugs? I called to local Yamaha importer's maintenance and the suggested that I might have a loose main jet. Would'n that be just a little too convenient? Has anyone ever experienced something like that? I'm planning to remove the carb and check it out next weekend and would appreciate hints and tips.

My bike is 99 WR with yz timing, SPES yz-silencer, stock header, 48 pj, European model needle OBDXM at #4, 170 mj.

It's getting cold here in Finland, last sunday was about +2C (~35F). Wonder if I should cover radiators partly?


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The wet and sooty plug indicates you flooded it from being rich. The cool weather would prompt one to richen the jetting and DXM#4 with a #170 looks like it could be lean for the mid-top end. General observation: from 0-1/4 throttle sounds rich and 1/4-full throttle sounds lean. Conflicting symptoms.

Start by cleaning the carb entirely, accel pump rod and diaphram, check the float bowl for moisture/dirt, fuel passages. Maybe even fuel filter or tank venting.

(How many turns out on the pilot screw?)


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Andre- make sure there are no obstructions in the vent line as well. That can cause weird symptoms that just pop up unexplained. Also check for air leaks around the carb and airbox fittings.

But I tend to think James is right on- do a through cleaning (check the carb slide for cracks!) and give her another try.

Also, having flames blast from the silencer could be a condition caused by misadjusted /stuck exhaust valves. The ignition occurs as the exhaust valve is still open (*maybe some carbon buildup) thus blasting the flames through the pipe that is normally held within the cylinder. This would explain power loss as well. Just my $.02. good luck

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