Erratic Spark - 1973 Johnson 85 HP

[RDJuhl]

Hi,
I purchased a boat which had a 1973 Johnson 85 HP 4 cyl engine. We have been having problems with cold starts (I have reviewed and follow the cold start procedures identified on this forum).

I have also performed the basic tests including:
1 - Compression - 3 cyl are very consistent at about 113, 1 cyl a bit low in the 95 range
2 - Spark - I conducted the "spark test" and confirmed that I can get 7/16 inch spark when using the spark tester.
3 - Fuel - we appear to be getting more than enough fuel...have not yet rebuilt the carbs...trying to work my way through other problems first. When attempting to start, we appear to be flooding the engine. Once I get the motor running, it blows a lot of smoke for a few minutes, then appears to run pretty good, although we do not think it is topping out like it should at WOT. The tachometer is not yet working...one of my other problems I need to solve.

Regarding the spark test...if I continue cranking while conducting the spark test, it appears that the spark is not consistent, meaning it will be there for a few engine cycles, then it disappears. If I let off the key, then try again, it may show the spark again.

My question: What would cause an erratic spark?
1 - Bad coils (I have replaced one coil and spark plug wire) - but still appear to have erratic spark on the cylinder with the new coil.
2 - Poor spark plug wires (if I touch the spark plug wires while the motor is running, I get a slight jolt out of the one new spark plug wire I have replaced - but not the older wires)
3 - Other ideas?

Thanks for your help...this forum has been a great learning tool for me as a first time boat owner.

 

[ezeke]

Re: Erratic Spark - 1973 Johnson 85 HP

Grounding is a major problem on the series. Make sure that all connections are very clean, especially the ignition coils and the grounding straps on the rack that holds them.

The power packs have screw terminals that should be sealed to keep moisture away.

You will get better information using an inductive timing light so that you can "read" the spark across a range of RPM on the running engine.

The low compression reading should be a concern; you may have a blown cylinder head gasket.

You can only use solid core spark plug wires on these engines, not automobile wires.

 

[RDJuhl]

Re: Erratic Spark - 1973 Johnson 85 HP

Thanks ezeke!
I will clean all connections and replace the spark plug wires with solid core and see what happens.

I appreciate your advice!

 

[RDJuhl]

Re: Erratic Spark - 1973 Johnson 85 HP

I thought I would provide an update...and validate your comments regarding grounding. I cleaned every electrical connection I could find which was related to ignition...key switch, coils, power pack, spark plug wires, grounding straps, etc. The spark plug connections inside the boots had significant corrosion on a couple of the wires. The engine now fires up very quickly. I am still waiting for new spark plug wires, and have one cylinder (#4) with a weak spark, so have ordered a new coil as well.

What I did find interesting, while cleaning all the connections, was that some appeared to have liquid tape "between" the connections, rather than just over the top of them. I will be applying the liquid tape over the connections after I am sure the cold start problem is solved.

After letting the engine warm up, I also repeated the compression test, and now have very consistent reading between 103 and 109.

Many thanks for your "expert advice"!