Why doed this happen

My wr400 is more responsive when i have the cold starter knob out and when i push it back in the responce is not so good, why is this.

Thanks

Do you mean the "choke" or the "hot start"?? Choke out means a richer mix going into the engine (more gas), Hotstart out is a leaner mixture (more air), either one could be a tell about the jetting set up on your bike. If you run with both out, Then you have to give the bike back to yamaha and cease riding for the rest of your life lolol :)

Race

A good carb cleaning may get it back to normal. I would completly dissasemble the carb and clean in a dip tank (not that spray-can stuff). It could also be an air leak causing lean fuel-air mixture so when removing the carb, inspect boots for cracks, make sure they were tightened properly, look for any loose screws or anything unusual.

I had a similiar symptom with my son's 2-stroker. It turned out to be a plugged up carb, a good cleaning was all it needed.

there are some seals on the throttle shaft that would be devored by carb dip.

I would probably avoid using it on an FCR.

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