Yamaha YFZ450 hard to start when hot: valve clearance
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Applies to: Yamaha YFZ450 and similar high-revving 4-stroke sport quads
Symptoms
- Starts cold but won't restart hot
- Needs lots of kicking or cranking
- Loss of top-end power
Why it happens
Valves, usually the intake side, wear into their seats and the clearance closes up. The valve doesn't seal fully, so a hot engine loses compression.
How to fix it
- With the engine cold, remove the valve cover and set TDC on compression.
- Measure all valve clearances with feeler gauges.
- Replace shims to bring clearances back to spec.
- If clearances keep closing, the valves and seats need work.
Tools and parts
- Feeler gauges
- Valve shims
- Valve cover gasket
What the factory service manual covers
Engine: valve clearance spec, shim selection, cam timing. The full procedure, specs and wiring diagrams are in the factory service manual for your machine.