25hp, 2s, Tohatsu, astonishing symptoms:

Tohatsu M25C3, serial 51176 The following symptoms follow a complete and thorough carburetor cleaning along with removal of the flywheel for visual inspection of the exciter coil. It's a low time motor from which any sort of corrosion is essentially absent.

Motor barely starts, won't stay running with plug wires connected normally.

Disconnect bottom wire from plug, connect it to a third plug that is grounded and the top cylinder runs perfectly. The engine will run all day with very diminished power ('cuz one cylinder is off) but it will start easily, idle quite nicely in neutral, throttle up and down normally, etc. and etc. Sounds and behaves like a normal single cylinder enigne would sound.

Reconnect the bottom wire to the bottom plug and the engine will soon quit, after a stout backfire or two.

In one scenario, the bottom wire is connected to the bottom plug and in the second the bottom wire is connected to a ground. That's the only change.

With the top plug wire connected to the top plug normally and the bottom wire grounded, one can swap the plug wires (ground the top, connect the bottom to the top plug) and there will be no change, i.e., it will run on the top cylinder quite nicely.

However, grounding the top wire and connecting the bottom wire normally, the engine will not run at all.

Because it idles beautifully on the top cylinder (with the other cylinder disabled) I have eliminated carburetion. Lastly, it will not run on the bottom cylinder with the top disabled.

Compression on both cylinders is 130psi on the dot.

Ideas?

PS: I'd like to hear only from those who have not yet attained the rank of Chief. Just like when I was in the Navy, the lifers coudln't find their butts with a flashlight and my experience on this forum is similar.

Re: 25hp, 2s, Tohatsu, astonishing symptoms:

Either plug wire will time in correctly for either cylinder; The wire also fires on exhaust. So you can put either plug wire on either plug, and it should run the same.

There is a possibility that you have a slipped crankshaft. If the previous owner severely over-revved the motor, the press-fit, multi-part crankshaft may have come out of time. I would verify that TDC for the lower cylinder occurs precisely 180 degrees from the upper cylinder TDC.