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Layton111

07 yz 450f 478cc big bore +3 mm stroker crank

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I have a 07yz 450f just rebuilt it. I put a cylinder works 478cc big bore kit, +3 mm stroker crank, stage two hot cams and kibble white valves and valve springs. Sense I got it back together I've had a few problems with it starting it and keeping it running. It doesn't want to really start but when it does it sounds good and when u try to take off it dies out or when u go to stop but want it to stay running it dies then don't want to start back up

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Did you change your jetting to go with your engine changes? If not you might want to search around on here for some ideas on where to go with it.

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Whether or not any of the jetting actually needs to be changed is debatable, and remains for the engine to decide.  Either way, the main jet is not affecting your starting problem. However, your problem is most likely to be centered on your pilot jet, assuming you haven't done anything silly such as mis-time the cams.

 

If the carb sat for any length of time during the work you did, there's a better than even chance that the jet will have dried a film of varnish into the metering orifice, and that will not dislodge simply by spraying carb cleaners and blowing air through it.   A film only .001 thick will take a #45 pilot down to the effective metering size of a #39, and that, my friend, is way too small to work. 

 

This is a very common problem. Read this post and check the jet: http://www.thumpertalk.com/topic/681801-bike-wont-start-dont-know-what-to-do/#entry6879695

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