I have a weird problem,Briggs engine 12.5hp model #289707-0121-01 the exhaust valve gets stuck open, and then the mower won't start. and if I remove the cylinder head and reset the exhaust valve the mower starts and runs very well i may add. What causes this and what do I need to do to fix this.
Check that the exhaust valve spring is not broken. Probably best to replace it regardless - it's obviously weak. Valves can get stuck open after long periods (years) of non-use, but it's usually the inlet due to gum from stale fuel.
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