OMC 120HP I/O won't keep running..

[OMCboater]

ARGGHHHH!! I am so close. Owned this boat 3 years, did the wood fiberglass work and got the engine/drive back on. 1969 GM 4cyl with rochester 2GC carb. coil, points, condensor, plugs, cap, wires, rotor are all new.

started in the driveway (leg in a pond) and it started and idled incredibly well.
Put it in the lake and the engine was badly flooded due to a sunk float in the carb. fixed that.

back in the water and now it won't keep running. When I start it, it cranks and catches and as long as I am cranking you can hear the engine firing, but it dies when I stop cranking. We have strong hot spark when cranking as evidenced by removing the plugs.

I am suspecting something in the ignition circuit that is providing power to the coil while cranking but dropping it in the key 'on' position. I checked the coil with the key on and it showed .5v at 1 terminal and .35 (or so) at the other. these numbers don't make sense to me. Shouldn't one of them be 12v and the other 0?

 

[whywhyzed]

Re: OMC 120HP I/O won't keep running..

A basic illustration of points ignition systems that might help you
http://www.classictruckshop.com/clubs/earlyburbs/projects/trouble/igndiagram.jpg
The Ballast resistor in the pic might actually be a resistance wire on your boat.
Sounds like yours might have failed open.

The coil is designed to work on 9 to 10 V. During cranking, it gets 12V to make a nice hot spark for initial start.

 

[whywhyzed]

Re: OMC 120HP I/O won't keep running..

another pic i like better:

3 is the path when key in RUN
4 is the path when key in CRANK

 

[OMCboater]

Re: OMC 120HP I/O won't keep running..

Thanks for those diagrams. We spent waaaay too much time trying to isolate to a fuel problem (because that is what we had messed with) but in retrospect we should have spent that time on ignition. I will be diagnosing tonight and see where it goes.

looks like the deciding factor is the voltage at the coil terminal with the key on - if I don't have 9-10v then most likely the resistor has failed.....?

Thanks again.

 

[wire2]

Re: OMC 120HP I/O won't keep running..

With key "on" you should have ~9v at the + coil terminal *IF* the points are closed.
If they're open, it should be a full 12 volts, same as battery + post.

 

[whywhyzed]

Re: OMC 120HP I/O won't keep running..

Referencing Ground (engine block, or neg battery post) with your multimeter's black lead, you should see approx 9V at the coil with the key on, and 12V when cranking, regardless of points open or closed.