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It is killing me not to ride and I can't get my bike to START! Please help

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I have a 2003 Yamaha YZ450f and I can't get it to start even after towing it behind a car! It died on me a month ago when I was out ridding. I was out on a trail and my bike died. Then I got it started but I had trouble keeping it running. When I would open up the throttle the motor would die, then it would only start if i kicked it into gear when I was rolling down a hill. The problem got progressively worse over the next 20min until it wouldn't run at all. After a long hike out of the woods, I started trying to figure out what was wrong. The bike still has spark. It still has compression. It still is getting gas. I tried priming the plug and it still wouldn't start. I have changed the plug and the fuel but don't know what to do next. Does anyone have any idea what is wrong because I am stumped.

Thanks Steve

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Air, Spark, Fuel, and Compression = running engine

The only thing you did not mention was air filter.

Does a shot of ether get it running?

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When have the valves been adjusted? Could also be a CDI issue or hot start is sticking in plunger in the carb

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"Check to make sure your fuel mixture screw is still in the carb."

Thats good advice. They can vibrate out and they don't like to idle and start without them..

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I thought it might be a CDI issue but I thought I wouldn't get spark. I have never adjusted the valves.

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Air, Spark, Fuel, and Compression = running engine...

One edit to this statement: ...Spark at the right time !

Check that the indicated TDC on the flywheel is actually TDC by probing for TDC through the spark plug hole. You could have a sheared woodruff key. And, as long as you're checking valve cleareances, check the valve timing.

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I thought it might be a CDI issue but I thought I wouldn't get spark. I have never adjusted the valves.

how many hrs on the bike please?

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I don't think it has a fuel mixture screw, at least the screw is not mentioned in my sevice manuel

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Check all timing marks to make sure the timing chain didn't jump a tooth or two. Exact same thing happened to my 03 YZ250f. Found bad chain tensioner. Re timed, replaced tensioner, fired up in 1 kick.

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take spark plug out. insert the plug into spark plug cap. ground plug to motor.

kick bike over to see if you have spark at plug. also look at plug to see if its wet, if so you have fuel. with that info you now know what direction to go in.

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I don't think it has a fuel mixture screw, at least the screw is not mentioned in my sevice manuel
Look again. Start on page 3-17, and then look at page 7-5. For a more detailed how-to, use

The Procedure Listed Here

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Yeah or make sure your airfilter is as clean as a whistle for future reference. That's what blew my motor:improper caring

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