Grounding Straps on later 18-20-25’s?

[johnyrude200]

I am in the process of installing electric start on one of these motors, and noticed a difference in the later 18-20-25’s compared to older models.

There is a grounding strap going from the bottom of the magneto to the port motor bracket, then another one from the bottom of that bracket to the lower cowel/pan.

This is present on both electric and non-electric start motors. However, for older ones (say 1967), this strap is not present. It is on both motors with and without the low-tension magneto.

I was wondering what is the purpose of this grounding, to prevent stray currents/electrolysis on the motor if there is a leaking/damaged spark plug lead?

[fleetwin]

Well, I am asssssuming your are seeing the mag plate ground strap on engines equipped with low tension ignition. It is required on the low tension systems because the external coils/ground are part of the ignition primary circuit, so a good mag plate ground is needed to complete the circuit.
The primary circuit is self contained on the mag plate on universal mag motors, so a great ground isn’t required even though the secondary circuit from the plug wire must return to the mag plate through that bronze bushing. Not the best ground, some guys add ground straps to the universal mag engines as well.
A mag plate ground is not required/needed for CDII systems, except for the UFI systems where the powerpack is under the flywheel.

[johnyrude200]

I actually was looking at the motors with and without the low-tension magneto. I observed this strap configuration on both ignition varieties.

These ground straps aside, I’m guessing they have no influence on the starter system, so long as the the negative ground off the starter (attached a the lower port bracket screw), runs back to the battery.

[fleetwin]

http://www.marineengine.com/parts/johns … +Crankcase
Are you referring to #28?
If that ground strap goes from the engine bracket to the lower motor pan, than yes, it is to ground the motor pan which is rubber mounted. This is probably an attempt to fight corrosion on the engine pan.
Can you post pictures of the ground straps you are referring to?

[johnyrude200]

Yes, #28 is correct. There is another one from the bottom of the magneto to screw #36 as well.

I guess these straps were an update/improvement; I looked at 7-8 different motors today (almost none with electric start) that were a mix of the 18-20-25 variety, and across different years spanning from 58-76, and it seems the straps were added in the 1970 range.

Maybe this is something I should add to my 1967 20hp I had at the meet a couple of weeks back. I’d like to see that motor last as long as possible, it’s already in very good shape!

[fleetwin]

Grounding the mag plate to the block will ensure a nice ground path for the secondary voltage to return to the coil on universal mag engines.

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