25 and the hybrid

TANKAR's Tech Tips for small plywood boats.
Making the most of your mini-max or doodlebug type hull.
25 and the hybrid:
Warning: Do not try this at home unless you have good insurance. I take no responsibility for anyone doing what I have done by putting more horsepower on a boat than the hull is rated for and will in no way be held accountable for anyone else's foolishness in this regard.

After looking around I decided that a 1950's era 25 horse Evinrude Big Twin at a local dealers establishment was priced right and got it really cheap. This gentleman was a specialist in old outboards and outboard parts and I had done business with him since I was a kid racing hydros. I knew that the motor I bought from him had low hours because I had seen it in his shop since about 1967. It had a 25 horse cowl on it and a smaller carb, and after replacing the head gasket I confirmed that it had a smaller displacement than my larger bored 35 horse cylinders. It was also an electric start model and I gave it the same weight reduction course that my earlier 35 horse received short of the lower unit mod.

Lighten up the port side.
Lighten up the starboard side.
Lighten up the lower half and another view of the lower unit mod that ended up on the hybrid.

It was basically just a super stripped stock 25 horse outboard with no performance mods when I first hung it on the transom. I took it out and although it had plenty of pep, it didn't seem to really get up and move out or have quite the top speed of the old 35 horse. I couldn't afford another bigger motor so I checked things out a little closer. Upon further examination I realized that I could simply remove the smaller manifold, carb, and reeds and replace them with the ones off of my old 35 horse because the manifold was the same shape as both the 25 and the 35 horse block. In other words, the two blocks that I had could interchange intake manifolds with a simple bolt off/bolt on process. The only difference in the two intakes was the carb inlet diameter and the reed size. I bolted on the bigger equipment and then thought about the exhaust. I wondered.....Would the old modified lower unit bolt on as well? IT DID!

I actually had to replace some of the timing linkage that worked the advance with the throttle as well if I remember correctly which was also a simple swap.
After I got done messing with my experimental hybrid I put it in the test tank and it freakin' started right up on the first pull! Just to make sure that I wasn't hallucinating I shut it off and pulled it again. It started again on the first pull. I made sure that the water pump was pumping and then I shut it off and collected my thoughts. This motor seemed to rev like crazy and was a little "kicky" at idle. Throttle response was scary. It was immediate. I couldn't really rev it out in the test tank though and top RPMs weren't going to be had until the motor was on the hull and up on plane where I could adjust the high speed jets with full advance on the timing. I put the motor on the hull and got up to Nettle Lake in the extreme N.W. portion of Ohio for it's first test runs. After putting the boat in on the beach I pulled the starter rope and the motor fired up. I headed for deeper water and turned around to adjust the idle jets screw.

After leaning the screw until the motor coughed then giving it 1/4 turn back rich it was time to push the throttle open all the way for the first time. I slowly brought the hull on plane then pushed it until it seemed to be "lugging". That told me that I needed to lean it down some so I turned around and started leaning out the mixture. After about a half a turn the motor really started cranking out some revs and it really took off. I pulled the dead mans throttle wide open and re-adjusted the mixture a little more. By this time the boat must have been going 40 MPH or more and I was running out of lake already. I headed back varying my throttle position on the way to see if my final adjustment was good over the entire RPM range (which seemed to be the highest RPMs I have ever seen an Evinrude turn.)

I pulled up in front of the campground where I had launched from and a crowd was watching. I decided to show off a little. I came to a dead stop and then I just punched the throttle. This was when I found out that I really needed to hold on tight to this boat at times. The boat jumped about 10 or 15 feet right out of the water reaching a height of about 3 feet in the air. After a few more high speed runs up and down the lake it was quiet time there with a no wake limit enforced by a water patrol sheriff.

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