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HELP! Bike Won't Start

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I've never had a problem starting until yesterday.

Run-down: '03 250F Bike is jetted close to perfect. Plug should be fine because it started up two weeks ago. Gas is still ok.

It did drop in temperture about 10 degrees. Last time I rode it was about 65 and yesterday it was 55. I turned the air/fuel screw out a eighth of a turn.

So the only symptom that I am hoping will help someone diagnose is the bike is basically backfiring. Its never done that before. It doesn't even try to turn over, but every 15 kicks or so it backfires (loud pop out of the pipe) I've tried with the choke in, and out. Began with no gas, then primed it with a quick little wick of gas one time, which did nothing.

What exactly is causing a backfire, and what could be my problem????

Bike is well maintained, but it might be due for a valve shim.

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ya, i have the same problem currently. not sure what it is , but hopefully someone will know

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Someone answer this please. I have a 05 yz450f and it will start but, will backfire first.

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10 degree temp change is nothing. not a factor at all.

spark, fuel, air (and to some extent compression).

prove you have each. one at a time.

i'm betting bad spark plug or plug cap came loose/off.

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If you've exhausted the above options, check your valve lash. Temp changes can impact ability to start if your valves are off. Just takes an hour and some basic tools to eliminate this as a cause.

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I had a 02 426 with the same problem, it ws basically flooded, the plug was fouled. I faught it before I found out the problem.

If you pull the hot start (if yours has it) and pull the compresion release in and kick it 10-15 times it should help clear it out and if you are lucky the bike will start. :worthy:

This worked for me, not every time, but sometimes. But the only time my bike ever got flooded was when my friends tried to start it and didn't know how.:D

This technique saves a lot of work, compared to pulling the tank and seat just to get the spark plug out.

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Well if it pops, you have spark. Also with the symptoms you describe, the hotstart is stuck in the carb.

You are going to have to get into the carb. Clean the pilot jet first, then remove the hotstart plunger, clean and apply a light coating of grease. After that it should start up with a few kicks. If not, install a new spark plug.

Now next time you store it for more than a week, use the drain at the bottom of the float bowl and store the carb dry.

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