misfiring

[COSTA FORTUNE]

I have 2 40HP 3 cylinder Yamaha outboard motors. The problem I have is that both motors misfire from cold starts. It appears that the spark plugs are oiled up. Within +/- 30 seconds both motors clear and run perfectly. The recommended spark plugs for these motors are B7HS’s. I have now experimented to attempt to get rid of this problem and changed the spark plugs to B6HS’s. This plug is a hotter plug. The problem of misfiring from cold starts has gone. I have run the motors for +/- 10 hours. I then removed the plugs to inspect the condition on top of the pistons to ensure there was no burning taking place; they still have carbon on them. It has now also come to my attention that both thermostats have been removed. Should I replace both thermostats and return to the original plugs or should I continue running with the B6HS’s. Please help!

 

[mike pulcella]

Re: misfiring

Hi Costa, Welcome to the club. I hope someone out there can help you.

 

[vinney]

Re: misfiring

Reinstall new Yamaha thermo's and install new NGK BR7HS-10.

 

[jim dozier]

Re: misfiring

Also it may be that your "misfire" when cold is just that the engine is running lean (2strokes hiccup when too lean) when cold (that's why we choke them to start). Perhaps in addition to Vinney's good advice you could try enrichening the idle circuit a tad. Reinstalling the thermostats will speed up the warmup and keep the engine at proper operating temp.