Volvo Penta AQ 171c petrol marine engine. 1600 overhead cam. Twin carburettor fitted into a 22ft windy day cruiser. It is quite an old engine. Problem is when started it runs fine from cold start. When engine has been running for 20mins or more at 20000 to 25000 revs engine starts missing or stuttering. Then have a problem starting engine when it is warm. The engine is not overheating. Have had leg serviced oil has been changed. Engine is usually very reliable. Everyone who has looked at it says it is only something small. Problem is trying to find exactly what the problem is. Any help would be appreciated

 

when was the last time the engine had a tune - new plugs/wires/cap/rotor etc? Are the plugs the correct version for that engine? - I'd pop them out to see what they look like) You may also want to consider testing/replacing the coil as part of a "stage 0" tune up just to make sure the spark is correct. Next is fuel - Have the carbs ever been rebuilt? (any signs of leaking/gas smell to suggest that there's some internal leaking making the engine run rich? (cold engines like a richer mixture). What weight oil was used during the last service? Older worn engines may need slightly thicker oil to help build/maintain compression. Using the wrong engine oil weight can expose this issue.