Weird Mercury 650 Problem

Heres a weird one

The motor is a 1975 three cylinder two stroke. Mercury 650.
Good compression on all three cylinders, good spark on all three cylinders.

The problem we are having is that after running for about three seconds or so the motor dies out, wont hold idle and dies.

While checking to verify that all three cylinders we took out the top spark plug and cranked the motor over and it idled fin running on the two bottom cylinders.

We all found this very odd, haha.

We noticed that the spark plug on the non-running cylinder was real dry like it wasn't getting fuel.

Next we tried screwing the top plug back it without the lead connected to it. Wouldn't run this way either.

The motor only idles when the top plug is completely removed from the cylinder head.

Our fuel pump is good. Starter motor is good. Spark plugs are new all with big spark. Compression good on all three cylinders. Coil seems to be good. Fuel line is all new. Brand new battery.


I'm pretty stumped. Anybody got any ideas?

Re: Weird Mercury 650 Problem

Wow, that is an interesting problem. I havent had alot of experiance with boat motors, but i have a 2 stroke 25hp 1996. i think my motor has these things called reed valves. Being a 2 stroke it doesnt have regular valves for example as a car does. I recently had to rebuild my carb and when the carb was off i could see them in the hole. Like i said im not to up on this but maybe one could be stuck. Has your motor sat for awhile? Have you recently rebuilt your carb? i had an interesting problem of my engine idling for a bit then quiting, it was the new gasket blocking a port on the engine block. good luck