1971 Mercury 7.5 Phase Maker Learnings

[Minnboater]

I thought it would be useful to post a few of my learnings on the 1971 Mercury 7.5 Thunderbolt ignition system --known as the phase maker. I've learned a ton from this site over the years and am hopeful some info I gathered might be useful to others.

Background: A few of years ago I bought a nice looking Mercury 1971 7.5 that had been sitting under a tarp in a guy's back yard for at least 20 years. I hauled it home and quickly observed that my new outboard had no spark at all. Both coils were showing open on my ohm meter when I put the probes between the spark plug wire and the pigtail ground wire. I picked up a pair of used coils and voila, a bright blue spark emerges and this little motor runs like a champ for 40 minutes on the water and several runs in a tank or on earmuffs. After the last run on earmuffs this motor is dead, dead, dead, No spark at all. It goes into the back of my shed and I forget about it.

Flash forward a couple of years, and I begin to think rationally about what might be wrong with this nifty looking motor. The stator and the maker points design and terminology confuse me so I get educated on the architecture:

• It's called a phase maker for a reason. The points aren't the traditional breaker style where the spark jumps the point gap on it's way the spark plug. These points "make" the ground connection to the external ignition coil. The stator spins around generating voltage (400V +), and the ground from the points trigger the spark for each coil.
• The ground side of the circuit passes through plastic insulator blocks on their way to the ignition coils; these blocks are a critical component to the system. A cracked block means a dead system. My blocks were nice and white with no cracks.
• The condition of maker points isn't as critical as breaker points. They complete the circuit by closing, and as long as they aren't terribly pitted they should work fine.
• I examined the wiring between the maker points and the coils, and quickly saw that it was bad. Mercury used untinned copper wire in those days. I replace all of it with fresh 10 gauge stranded wire rated as good for 600V. My rewiring effort makes no difference at all.
• CDI makes replacement stators for this motor and they have an abundance of electrical troubleshooting tips on their website. Step one: bypass the points. Remember they complete the ground in the electrical circuit, so just disconnect the points connection to the coil and tap a jumper wire to the negative terminal on the coil to see if there's a spark. My motor had no spark. Interestingly my volt meter shows 150V when directly applied to the stator wiring and ground, when I think it should be 400V. Hmmmmm. I don't know if that's because the stator energizes a capacitor for discharge into the coil and I'm reading continuous voltage. I'm not an engineer and I don't know what to make of that.
• I put my voltmeter on those new/used coils I bought 3 years prior. One probe on the spark plug wire and the other on the grounding pigtail. One is close to the 600 ohm spec and the other reads open. Now in my head I figure I have two potential root causes; it's either a defective stator or two bad coils. I don't know if the resistance test is conclusive or not.
• I read somewhere that the diodes and capacitors in these stators (Mercury part # 174-3996) sometimes will pop out of the epoxy when they go bad. I examine mine closely and see no evidence of a big pop, but under light and magnification I see a small circle of black in the slightly opaque green epoxy
• I head over to my local outboard boneyard and they test my two coils, and tell me that both of the coils test out fine. After the coil test I put my ohmeter on both and both are close to the 600 ohm spec.
• I conclude the stator is bad. I buy a used one from the boneyard. Back in business. I've got a big blue spark that easily jumps a 1/4 inch gap on both coils now. I also put the voltmeter on the stator directly and voltage leaps up to 500V when the starter is pulled.

Key learning: the stator on the 7.5 phase maker goes directly to ground through the stop button. Push the stop button and the ground side of the stator circuit open stopping the motor. Pushing the stop button when running the motor on earmuffs may result in the motor chugging for a second or two. Chugging will generate 400 volts that will strive to find a ground. The resultant electrical backlash no doubt cooked my stator. The boneyard guy told me that the Mercury 4 HP phase maker didn't have a stop switch and it's stator reliability was significantly better than the 7,5 and 9.5. Enough said, and I move my stator ground off the kill switch. I will shut the motor off with a combination of low throttle and full choke.

Other learnings:

• The resistance test on Mercury ignition coils isn't a perfect test of goodness.
• The 174-336 stator can be tested directly with a voltmeter If you see at least 400V it's probably good.

 

[alldodge]

Pretty good write up, thanks for sharing