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this table will help you. I used to own one.. beautiful bikes easy to work on too!
Valve clearance: Inlet 0.13 0.19 mm
Exhaust 0.18 0.24 mm
Valve head thickness: Inlet 0.5mm 0.25 mm
Exhaust 0.8mm 0.5mm
Valve stem bend 0.05 mm TIR
Valve stem diameter: Inlet 4.975 4.990 mm 4.96 mm
Exhaust 4.955 4.970 mm 4.94 mm
Valve guide inside diameter 5.000 5.01 2 mm 5.08 mm
Valve/valve guide clearance
(wobble method): Inlet 0.02 0.07 mm 0.18 mm
Exhaust 0.06 0.11 mm 0.21 mm
Valve seating surface:
Outside diameter: Inlet 30.8 31.0 mm
Exhaust 26.3 26.5 mm
Width 0.5 1.0 mm
Valve spring free length: Inner 35.5 mm 33.6 mm
Outer 40.5 mm 38.6 mm
Valve seat cutting angle 32 degrees, 45 degrees, 60 degrees
Here is some rule of thumb specs
intake should be between .003" to .005"
exhaust should be between .006" to .008"
When it's set right, it should feel like pushing a butter knife through butter right out of the fridge, also be aware that sometimes the lock nut can either tighten the clearance or loosen it.. This is a job to really take your time on
it can be caused by a number of things including.
lean mixture--air leak intake or exhaust,
leak in exhaust gasket
low ignition voltage- bad plug,lead\resister cap
tight valve clearance
and a number of less likely problems like faulty kill switch, ignition switch\cable connecter,igniter.
If it is not the standard exhaust, the jetting may need a slight change
So first give it a tune and service, check valve clearances,new aircleaner, drain carbs, tighten exhaust bolts and clamps, intake clamps and bolts,it may need exhaust gaskets
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
"Valve Clearance" refers to the small gap between the valve lifter and the valve stem that allows the valve to completely close. Remove the Valve Cover and set the crankshaft to the timing marks (only found in your service manual), for each particular valve to be completely closed. Loosen lock nut and set with a feeler guage then retighten the locking nut on the adjustment shaft. Four valves = four adjustments. Does this help?
with the cylinder set at top dead center compression. using a feeler gauge between the tappets and the valves there should be .002-.003in clearance on the intake valves, and .006-.007in. clearance on the exhaust valves. thank you.