Vintage Johnson Coil Resistance

[BrayD]

Posting here rather than in the electrical forum as it's related to motors.

I picked up a '57 18hp Johnson a few weeks ago and I'm freshening up the ignition. Looks like the coils have been replaced at one time, and I have one that is reading pretty high across the secondary. I still have spark at the plug, and my understanding is a higher secondary resistance will just produce a stronger spark. Resistance across the primary is just over 1ohm on each coil.

I admittedly only have a superficial understanding of these things though.
In an effort to be frugal, I'd like to run it as-is if it won't hurt anything. Will the higher resistance cause any performance issues or risk early failure of other components?

 

[BrayD]

Update - further inspection shows the coil may be cracked. Playing it safe and just ordered a pair of OEM OMC coils.

 

[mbullen]

For the record, every type coil will have different specs, but for the standard vintage OMC stuff, I've found the following to be true:
Primary = 1-3 ohms (lead to lead)
Secondary = 3k-10k ohms (either lead to plug stud)

Some folks extend the coil life by covering small cracks with liquid tape, but in my world, if you've done the work to remove the flywheel already, there is really no reason not to update the consumables while you're there.

 

[Sea Antsy]

Agreed... once your have paid the price of admission by pullin the flywheel might as well remove.. REMOVE coils and check for any cracks esp where the wires enter/exit.. and those wascally cracks are usually hiding on the bottom side nearer the hot power head...any opportunity to allow moisture inside the "POTTING" case will not go well....moisture and 10's of thousands of volts..nawww

 

[BrayD]

I kept reading 3k-8k or 3k -10k. Rarely saw reports of anything north of 20k, and when I did they decided to replace them for the sake of being thorough and it solved other mystery performance issues they were having. It'll give me the warm and fuzzies to start fresh with OMC coils.

 

[BrayD]

Update - installed new coils and wires. New coils measured around 4.5k across the secondary windings.

Motor ran pretty well last weekend. Idles really smooth and has plenty of power at the top end of the RPM range. It stumbles really bad as I roll on the throttle though. Almost like a cylinder drops out under acceleration. Once both cylinders come to life it pulls hard all the way to WOT.

Pulled the flywheel off again and set the timing with my multimeter last night. I'll revisit my carb linkage-to-ignition plate timing as well. New plugs for good measure and we'll see how it does this coming weekend.

 

[BrayD]

Plugs and timing did the trick. It’ll idle smooth under 700 rpm and rolls on strong all the way to WOT.

I may ask this in a new thread but it’s worth putting it here. Target WOT is 4500. It currently spins up to around 5100. Is there a rule of thumb to get me in the ballpark for a prop?

Edit: Pulled the prop off and it’s a Michigan SMC 43. eBay says that’s a 9x10.

 

[Pappy]

Go up to a 12" pitch prop. don't get hung up on diameter.