Difference between two motors?

[Numlaar]

Hey all,

As some of you may have been following, I have two engines...

Engine #1: 1977, Evinrude 70HP 3Cyl. Model #: 70773S
Engine #2: 1976, Johnson 75HP "Stinger" 3Cyl. Model #: 75ELR76D


Two weeks ago, I blew the Johnson up (burned up #2 Piston).
This week I blew the Evinrude (blew head gasket at #2 Cylinder).

My questions are the following:

1. Is it normal for these 3 Cyl. motors to have so many problems with the #2 Cyl??? (I had a 79 100HP V-4 and it never, ever, gave me any problems)

I have done a pretty thorough check on the BRP site, and as far as I can tell nearly all of the parts are identical, so....

2. What is the main difference between these two motors?
(The only thing I found different was the stinger uses a 55d High speed jet, and the rude uses a 54d jet on the carbs)
3. How does the Stinger get the extra 5HP? (lower unit gearing? The miniscule bump in jet size? what?)
4. Can I interchange the parts (pistons, heads, etc.) to put the 2 together to make 1 running motor?

The reason I ask, is, I got 6MPH more and 600 RPM more out of the stinger at WOT, using the same prop on the two (traded it between the motors)... so if I can interchange everything (pistons, rings, etc.), I want to use the Johnson, because it seems to perform a lot better overall...

I just dont want to say, use the 'rude' carbs with the 54d jet and burn up my rebuild.

Just wondering what made such a "big" difference between the two...

 

[Faztbullet]

Re: Difference between two motors?

1) No... they do have some carb issues in different years with jetting
2) Decals,carb size,cylinder head as the 75 has slightly larger carb and jet as you found and tighter cylinder head(compression)
3) See above
4) Yes, but use the 75hp carbs and head
If the head gasket is just blown on the 70hp just replace it and install the 75hp carbs after a good cleaning and new kits. If you rebuild the engine(boring and oversize pistons) rejets the carb .002 larger.

 

[Numlaar]

Re: Difference between two motors?

Thanks Faztbullet!

Since the rude block/cyl/pistons seem ok, The fastest and easiest solution for me would be to just replace the head gasket on the rude, since it seems the head is what generates the extra power, would it be also ok to stick the Johnson head (along with the carbs) onto the rude to beef it up a bit?


/edit - Nevermind, you already answered, I just missed it.

Thanks!