1967 Mercury 350 (35hp twin) fire problem at cylinder#2 (bottom)

[flugger]

At the end of last summer, I discovered that my motor was running on 1 cylinder due to a "totally blown" coil and point. So I changed both, made the ajustements but now the engine is running bad with both cylinder. I mean it's running fine with just top cylinder but if both are connected, it misfired at different RPM and sometime stalls. I switched condensers: same result. It seems that I have spark while running => I check it with a spark plug connected on motor case. The only thing I can see is that the new coil is a bit different then the other: the longer connector (to the point) is at left instead of right . Can it be a clue? Is there a "polarity". Or something else I missed...

Thanks in advance.

 

[hotrod53]

Re: 1967 Mercury 350 (35hp twin) fire problem at cylinder#2 (bottom)

There are different coils for Mercs. For my '77 500 I saw that there were 2 types, a cheaper one and a more expensive one .. guess which one mine took..LOL. They discontinued and changed numbers on coils, first off see if you have the right one. Check your plugs and wires too.

I would start at the end and work backwards, check the plugs, measure the wires from the cap top ground and compare the two cylinders, I would also do a compression test just to be sure. If they resistance readings are different, pull the wires and measure the wires and then the coils seperately. Be sure your coils are grounded well. Be aware, if you have a broken plug wire, in short order the coil will grenade itself, that voltage has to go somewhere and it will blow thru the coil case to ground if it has to.

I assume you have one carb, right? Check your wiring too, a 1967 harness can be pretty bad. I say that because my '77 harness is getting crispy. You didn't mix up the trigger wires on the two coils did you?

 

[flugger]

Re: 1967 Mercury 350 (35hp twin) fire problem at cylinder#2 (bottom)

Thanks for quick reply. I'll check it. Yes it's a carb on and compression is OK on both (110psi).

 

[flugger]

Re: 1967 Mercury 350 (35hp twin) fire problem at cylinder#2 (bottom)

Finally got time to check. Bad news is my new coil is defective or incompatible. I swapped coils and now the problem follows the new one. I check resistance between connectors: old one 6.5Kohms, new one 9.6kohms. According to a lot of sites, it must be between 3 and 8kohms. The good news is that everything else seems to be ok (as far as I know). Now the mission is to found one coil comptaible before end of summer Thanks again hotrod53