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Hi mate have you checked valve clearances they may need adjusting on the intake side your describing a issue of either a leaking intake valve due to clearance to tight or worse case valves are to old and not sealing correctly. hopefully they just need adjusting as is cheap and easy to do... if need replacing then its pretty big job with getting the seats re cut and buying the valves etc. Happy to do the work for you if you anywhere near Perth wa
You need to remove the valve cover and rotate the crank shaft to TDC then with a feeler gauge you check the clearance between the cam lobe and lifter bucket. There are 3 on intake cam and 2 on the exhaust. The clearance will most likely tighten as the valve seats and valve wear in. If the valve clearance gets too tight they will hang open and not seal for proper combustion. Valve clearance for my 2002 YZ426 are intake=0.10-0.15 mm (0.0039-0.0059 in) exhaust=0.20-0.25 mm (0.0079-0.0098 in) the 450 should be the same or very close
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