03 YZ450F Won't Start

Here are the details. I am new to the 4 stroke scene. Had lots of 2 strokes. Went riding yesterday with my son and couldn't get the bike to fire at all. Changed the plug and it seemed somewhat gassy. We ended up pull starting it and it ran for about 3 laps. But it seemed to act as if was choking. It died and hasn't started yet. The carb was cleaned this spring and the bike has been riden at least twice amonth since April. The last couple times out it had been getting harder and harder to start. I have had the bike about a year and have done no engine work to it besides oil changes and carb cleaning. A couple of guys at the track said the top end probably needs redone. What are some thoughts? Carb problems or top end? It has good spark. What kind of compression should I seen on a gauge? Any help would be great.. Thanks

Check the idle. Last time I ran into that quirkiness was when it was too low. Just a quick easy idea. :)

I have moved it from 1 extreme to the other. It didn't help

Have you checked the valve clearances?

No, Would that be al ogical place to start

I would prolly go thru the carb again first. What readings are you getting on the compression guage? Remember there is a compression release on these bikes, so getting a reading might take a special guage. Checking valve clearance is not a bad thing to look at also. When did you clean your air filter last? Make sure the accelerator pump in the carb isnt tore.

I havent done a compression check yet. Just trying to gather info on were to go and what to check. All good info thanks everyone

Typically thare are only 3 reasons it will not start after changing a plug

1 - carb is dirty jets clogged or bad gas, or air filter is too moist

2 - CDI - electrical issue

3 - needs a valve adjustment

most likely it is 1 or 3

when was the last time the valves were checked?

How many hrs are on the bike?

I don't know about the hours I have put around 20 hrs on it maybe. The spark looks good.

Is the fuel screw still in the carby? They can unwind and fall out, and the open hole allows too much air in the fuel mix. If it's still there, make sure it's got an o-ring on it too(and a spring to hold the screw in place)

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