Reply To: 1970 4 HP, one cyl American Made Mercury

[David Stanton]

JACQUES wrote:

stuff on thunderblot…phae maker

(21) 1971 Mercury 7.5 Phase Maker Learnings | Boating Forum – iboats Boating Forums

– Current condition is No spark.
–
Yes – I had previously found that diagram that you sent me showing the capacitor. That appears to be an old-school standard magneto – notice how the spark plug wire comes from a coil under the flywheel, and no charge coils, etc. I think they changed to the Thunderbolt CDI sometime during that range of serial numbers, so put both types of ignition in the documentation for that serial number range because there is nothing like the parts in that diagram in the motor I’m working on, but the diagram at the link below is what I have, and a testing table in the 1966-1974 shop manual verified that the capacitor is potted inside the stator . My 1971 7.5 HP “parts motor” that I pulled a green coil from also has no replaceable capacitor, and the shop manual references show that in 1972, the capacitor was put in as a separate, replaceable part. Maybe they moved the capacitor outside the potted stator circuit because it was failing often?
–
I tried the green coil that ohm-tested OK from the 1971 7.5 HP parts motor in the 4 HP yesterday and learned the green coil has the ground wire coming out of the opposite side than the reddish-brown one in the 4 HP. No problem – I sanded some paint off the metal that side mounts to and verified with the ohmmeter from the spark plug wire to ground that the coil ground was in good contact with the engine block. Still no spark. The two resistance checks in the shop manual on the stator wires tests OK, but analyzing the circuit potted inside the stator, that test only verifies that diodes are good, and does nothing to test the capacitor, which could have failed open and that test would still look good. (Note I was an electronic circuit designed for 35 years, so can understand the circuits well).
–
I just looked at your link to the iboats forum and found it interesting that a coil can have open resistance in the secondary and still be good, but I’m thinking a small break in a wire in the secondary somewhere inside the coil may create an open that the spark can still jump across; much like you can pull a spark plug wire part way off an engine and the spark will jump the gap you created and the spark plug gap, but I wouldn’t trust a coil like that too far! In that link it gave the method of shorting the points to test the stator, and yes – that makes sense – a meter should read the charge voltage to the capacitor when tested that way, as long as the capacitor hasn’t failed as a short, and I’m thinking turning the flywheel with a drill is a good way to get that meter reading – Thank you! I will try that, and if that looks OK, in the next couple days, I’m going to haul my oscilloscope to the garage and look at the signal to the coil primary, as that will verify the stator’s condition, and compare it to the scope diagram shown in the CDI Maker-point ignition writeup you sent me in your first post.
–
This motor has taken way too much of my time, but I’ve learned a lot, and now it’s turned into a personal challenge to either get it running, or verify beyond a doubt what is wrong. If the stator is bad, it’s not worth fixing and I don’t have a one cylinder stator in a parts motor. I hesitate to search for a used stator, as it’s difficult to know I’m buying a good one. New ones cost more than the motor is worth.
–
Here is a diagram of what I have: https://www.marineengine.com/parts/mercury-outboard-parts/40-1-cyl/2498136-thru-3296136-usa/flywheelstator-and-ignition-coil-40
Thank you!
Dave