Yamaha 2.5 Four Stroke running problem

[MADRIGAL]

I had the engine oil, impeller and spark plug changed last winter, and the engine had been running well until I tried to increase speed to outrun a thunder storm last Sunday. The engine speed increased to about half throttle, but would not speed up beyond that even with the throttle fully open. When I put the choke half on, the engine speed increased. Any explanations would be appreciated.

I don't normally run at more than half speed getting in and out of the sailing club and holding her head-to-wind while getting the mainsail up, so I don't really know how long I've had this problem. I don't usually use marine fuel, but get the highest octane petrol from the Shell service centre down the road.

 

[VicS]

Always worth trying a new spark plug even though yours is new.

Otherwise symptoms suggest the carb needs cleaning.

E5 rather than E10 petrol probably a good idea but for the lower ethanol content rather than the higher octane rating.

 

[penfold]

How old's the fuel?

 

[MADRIGAL]

Fuel was fresh in April, and I thought it was ethanol-free. I did wonder about the carburetor. Are they much different on a four stroke than on a two stroke?

 

[penfold]

I'd agree the carb may have some grot build-up, worth having a go at it with a tin of carb cleaner. There's not a lot of difference.