Converting 1972 4hp OMC to CD ignition

[johnyrude200]

I have a service order for a ’93 4hp deluxe. The meat of this order is a failed manual starter, and the plastic pulley was cracked/warped. Digging through my parts bins, I found a 1972 4hp powerhead with starter still attached.

Now granted, I need to inspect it more closely, but at my first casual glance, the main difference I saw was the pulley was still made of metal, not "CPS." The powerheads, less the cylinder head to accomodate the external coils, are identical (minor changes to if a boss is drilled out or not — inconsequential in the big picture).

This sparked more interest to see what the difference in these motors are, and how hard would it really be to change a 1972 4hp ignition over to CD. It looks like the same powerhead, same hardware for the throttle (obviously the older version was a simple lever, but the newer cable throttle looks to tie into the same ‘arm.’).

I’m thinking switching the ignition over would be pretty easy to have a stop lanyard (nothing earth shattering). Obviously the rest of the motor is different hardware (i.e. exhaust housing, stern bracket, gearcase). Haven’t looked at a driveshaft for an old 4hp motor in awhile, but I suspect the crank splines are the same through all production years?

I have to say I’ve always been impressed on how well the 8hp and below motors run, and run, and run, and idle to boot. Must be because things got left alone from the 60’s designs right up to when OMC was bought out.

So I surmise…

1. Swap flywheels and mag plate.
2. Swap cylinder head
3. Reconfigure wiring/coils, and ‘yankee engineer’ a position for a stop button/lanyard

I would say the cylinder head changes compression numbers from the typical ‘ol "anything over 65 PSI is good" to "lets look for at least 100PSI cold/dry" on the newer version. And of course, J4J’s are swapped out for longer stemmed, L77JC4 plugs. Is this where the compression numbers are brought to higher guage number? Or were the rods/porting changed to get higher numbers?

Anyone want to weigh in on how a 4hp Deluxe gearcase vs an original ‘high thrust’ (not weedless) gearcase perform? The deluxe gearcase definitely use a larger diameter prop, can’t comment on pitch differences though.

[dan-in-tn]

1977 is the 4hp model (with CD ignition), but like your 1972 motor. You can compare parts you would need there. I believe you will run into several differences in the later ’93 model engine. It is a fully jeweled powerhead with different pistons, rods, etc. Not sure if the parts will all switch over. Never tried it, but they are not of the same family or generation of engines?

Dan in TN

[PugetSoundBoater]

I will compare the LU on my 1977 2 hp Johnson and a 1979 4 hp Evinrude CD version. I believe they have the same driveshaft. I have parts manuals for both motors if you want a copy ,let me know. Jim PSB

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[billw]

Just my two cents (before tax) but I think the ’72 4 is a better overall motor than the ’93 4 The ’93 is just an eight horse with a four-type power head. That makes them heavier and less portable. The ’72 has a nice, compact mag; whereas the ’93 has wires all over the place and a roll-of-the-dice UFI OMC power pack. Other than just a good, mental exercise, I’m not sure why you would want to put CDI on a ’72……?

Long live American manufacturing!

[johnyrude200]

Well, more just a curiosity to see if it could be done. The newer 4 has full gearshift, tiller throttle, and the stop button kill. The original does not.

Weight wise, they arent that much heavier but I see your point about the overall dimensions. Just curious in terms of debating new vs old school.

T2 – great point about the late 70’s model.

Parts booklets show different part #’s for cranks, pistons, rods. Maybe if I ever have free time Ill pull apart a powerhead from each motor (have a few dead ones here) for entertainment purposes.

I wonder if the flywheel key is in the same place, degree-wise?

[Randy in Tampa]

I agree, why? I converted a 1976 25HP to the older style ignition with a 1957 5.5 HP Mag plate because I couldn’t get it to run right, just traded some parts on the mag plate and she ran great, I would think twice if I were you, but that’s just my opinion… 🙁

quote BillW:

Just my two cents (before tax) but I think the ’72 4 is a better overall motor than the ’93 4 The ’93 is just an eight horse with a four-type power head. That makes them heavier and less portable. The ’72 has a nice, compact mag; whereas the ’93 has wires all over the place and a roll-of-the-dice UFI OMC power pack. Other than just a good, mental exercise, I’m not sure why you would want to put CDI on a ’72……?

[Randy in Tampa]

I agree, why? I converted a 1976 25HP to the older style ignition with a 1957 5.5 HP Mag plate because I couldn’t get it to run right, just traded some parts on the mag plate and she ran great, I would think twice if I were you, but that’s just my opinion, I think the older ignitions are simpler and more reliable… 😉

quote BillW:

Just my two cents (before tax) but I think the ’72 4 is a better overall motor than the ’93 4 The ’93 is just an eight horse with a four-type power head. That makes them heavier and less portable. The ’72 has a nice, compact mag; whereas the ’93 has wires all over the place and a roll-of-the-dice UFI OMC power pack. Other than just a good, mental exercise, I’m not sure why you would want to put CDI on a ’72……?

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