early 80s omc 70 vs 75 hp

[JoshKeller]

hey guys, i have a 1984 johnson 75 hp 3 cylinder powerhead on an outboard jet. it runs well, but since I fish several days a week, I picked up (for free) a 1983 70 block and tore it down, put new bearings, honed it, new pistons and rings. compression is 125 per cylinder. One thing I didnt realize was it was actually a 70 and not a 75. Will I be able to use my 75 carbs, head, and electronics on this motor and have a good running engine with the 70 in the event of a failure on the 75? or are there major internal differences in the ports? thanks guys.

[EMDSAPMGR]

These early 3-cyl blocks were all 49 cubic inch blocks. Not much cubes for making 70/75 hp. Both blocks are identical, except the 75 block is ported higher and makes it's hp at a higher rpm than the 70. Similar parts on both. The 75 has different (performance) heads and may have slightly larger throated carbs. Here is the deal. If you were going to put the 75 on a 15' light boat, the engine would work perfectly. If you were going to put the 75 on a 17' fishing boat with lots of gear and extra folks on board, the 75 engine is the wrong application. Will have a poor holeshot. The 70 is considered a low-port, grunt engine. Better hole shot and load-carrying capacity. You will find little real top end speed difference between the two engines,. So, put your 75 stuff on the 70. OMC fixed the anemic hole shot problem when they bored all 3 cyl blocks out to 56 cubic inches. This happened some time in the late 80's. I have run a 76 75 hp short shaft 3 cyl and also my current 92 70 hp. The 92 is a great torque engine-hands down a better engine than the old 76 75 hp.

[JoshKeller]

Sounds like the 75 stuff will work om the 70 block. Thanks! And im actually happy with the performance on my light 16 ft tracker grizzly. It does low 30s with the jet, which is plenty fast in the shallow river i run.

[sharpeye Mike]

The carbs are the same except for the bottom jet. The 70 HP has a 52D orifice plug and the 75 HP has a 56D orifice plug, everything else is the same except the porting.

[JoshKeller]

So even though the heads have a different part number, they can be used interchangeably with no difference?

[EMDSAPMGR]

Yes, both engines are the 49 cubic inch model. You might want to consider using the 75 carbs, too. If you put a high compression head on a 70, the 70 carbs may not feed any extra fuel to keep the powerhead cool-what with the extra/higher compression heads. The 75 carbs would put through slightly more fuel and better keep the combustion chambers cooler. When you get into changing performance parts on any engine, it's not unusual to have to make more than just one change.

[JoshKeller]

Alright, put the 70 block on my motor, as i had a free day and was bored. Looking at both heads (70 and 75), i can tell of no difference, just a different part number. The inside measurements on the cylinder appear to be dead the same. Maybe the 75 is slightly shaved to give a little more compression. Anywho, The 70 mounted on my boat has compression of 135, 135, 135. good hot blue spark that jumps 1/2" on all 3 cylinders. Timing at idle was -2, and at WOT (using joe reeves method) was 15, which puts it at 19. Thats spec on my 75, i assume same on the 70?

Now for the testing. Motor started easily, but idled a little fast - stuck butterfly on one carb fixed that, and it now idles at 900 like my 75 (keep in mind this is a jet, no lower unit gears). When giving it throttle, it has a slight hesistation around 3000 rpm, then took off to around 5200 rpm the first time i gased it, slightly stumbled to 4900. Let off the throttle, rechecked timing, shut it off. Restarted motor, and same thing. hesitation at 3000 (give or take), and top out around 4900. Did this 10 or so more times, all with the same result. checked alll 3 cylinders spark with my timing light, then Pulled boat out of the water, rechecked compression (still dead on at 135 on all 3).

Would this be the 56d jets, or likely something else I should check before ordering new orifices? i noticed the carbs and reed cages had different part numbers for the 75 and 70, would that effect anything?

[EMDSAPMGR]

Is the sync and link set to factory specs?

[JoshKeller]

Yessir, i set everything from the timer pointer adjustment to the soark advance and throttle plates according to my service manual

[JoshKeller]

Got it running up to 5400 rpm. Removed the airbox and it was noticeably peppier and turned 5600. Does this confirm its running rich? These plugs are from about 20 minutes of continuous running.

[Mark75H]

The only rich plug is the center one. The other 2 are perfect