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2005 Yamaha YFZ 450 intake blows out, exaust sucks in

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put new piston, jug, crank, and bearings in 05 yfz 450, according to service manual. when i try to start it carb blows out, exhaust sucks in. could i have put somthing together wrong? :aargh::mad::confused:

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to me it sounds like the timing is out approx 180 degrees. Is it a zero tolerance engine? does it backfire when you try to start it? The first thing I would do is rotate the engine over without the plug(s) in and put your finger over the hole to find the compression stroke. once you feel it triing to blow your finger off watch your timing marks. your mrks should be in sight of each other. if they are, slowly rotate it over until the marks line up. then remove the cover for the valves. they should be closed completely. if not the timing is out. I hope this helps somewhat.

your frielndly neighbourhood mechanic.

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Thanks Gearhead01, I will give that a try and let you know how it works out. I appreciate you taking time to help me out.

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Did the same thing this guy did new piston new jug and everything turned 180 timed correctly but I back fire the carb off any ideason what It is 

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