Weak spark on 1973 20 Hp Johnson

[fleetwin]

Yes, the bowls are the same, except for the high speed jet, but the bodies are different…
Did you check to see if the choke is staying fully closed while cranking? Are you advancing the throttle to its stop in neutral? It is not going to start easily at dead idle when cold..

[reivertom]

fleetwin wrote:

Yes, I remember you posting about the new coils. But, Mumbles post is a good reminder for the rest of us when checking coil secondary resistance. Those poor/rusted connections often show up as an infinite resistance reading on the high ohms scale…A good coil could be discarded only because of a poor/rusted plug boot/terminal…

If you don’t mind, could you tell me what years this type ignition was in use? I am looking for a similar size OMC motor, and would rather avoid them.

[fleetwin]

This low tension system was used on some engines from 1973 thru 1976, and a few more years later on some commercial models…. It is best to be able to recognize the system when you see it though only because model years are often misrepresented/unknown on used engines.

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reivertom

[Evinrude 25]

Hello,
Rusted or poor terminal connection rules out in this case since the spare plug cable comes together with the coil as you could see in the picture. It is Mallory Marine Ignition Coil 9-23106. I have checked the new coil resistance vs the old coil. I don’t see the different. I suspect the old coil is still good. I am going to put the old coil back to see if I could get the spark!!! It is just an experiment.

[GLEN DEUTSCH]

Evinrude 25

Can’t over emphasize the importance of that thin round plastic bearing under the mag plate. I fiddled for two months thinking that a little crack in the plastic and a little wobble would not hurt. Could not get it running. Frustrated. Finally, replaced the ring and it runs great.

[Evinrude 25]

Other point I read in other post about the point setup gap. I setup 0.020 on both points (New OEM OMC) I found out one side was Ok to set the gap more than 0.02″ but other point I barely got less than 0.020″. Then I used the Ohm meter method to find the correct timing. That point was just made it at the very maximum opening adjustment. The adjustment screw turned at max. Why??? Manufacturing inconsistencies??? Or the wear out of the plate bearing?

• This reply was modified 3 years, 8 months ago by Evinrude 25.

[fleetwin]

GLEN DEUTSCH wrote:

Evinrude 25

Can’t over emphasize the importance of that thin round plastic bearing under the mag plate. I fiddled for two months thinking that a little crack in the plastic and a little wobble would not hurt. Could not get it running. Frustrated. Finally, replaced the ring and it runs great.

The 20hp did not use the plastic bushing, it still had the conventional brass ring and aluminum retainer set up…
But, you make an excellent point. Those plastic bushings should be considered throw away parts and replaced during every servicing, just like gaskets…

[fleetwin]

Evinrude 25 wrote:

Hello,
Rusted or poor terminal connection rules out in this case since the spare plug cable comes together with the coil as you could see in the picture. It is Mallory Marine Ignition Coil 9-23106. I have checked the new coil resistance vs the old coil. I don’t see the different. I suspect the old coil is still good. I am going to put the old coil back to see if I could get the spark!!! It is just an experiment.

I don’t think you have an ignition problem, the engine sparks well while cranking, correct? Have you checked the choke linkage?

[Evinrude 25]

Yes, it has spark. Now I need to focus on the carb.

[RANDY SCHNEIDER]

I may be all wet, but I thought the carbs on those had a plastic low speed needle jet seat. Im pretty sure I bought one from Ed for a 20 I had to work on a while back. Seems they can get smashed or crack. I had to use a straightened fish hook to snag it out. Its a distant memory, but I remember fising that bushing and gettting the plate flat again. Might be something,
Randy

[Author]

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