67 Evinrude 18 modified to a 25 ?

[Ironhorse2022]

Morning all,
Im considering buying a nice 67 evinrude 18 hp with the intent of modifying it to a 25 by changing carb, intake and exhaust manifolds. I’d appreciate your thoughts on a few things
1. Are there more differences than just carb and intakes ? Ive heard yes and no.
2. On a front steer, 180 lbs boat with 350lbs of cargo/people, what max speed can I expect from an 18 vs 25 ? Not sure it’s worth the effort of upgrading.
3. I don’t want to be running at wot all the time traversing lakes. I’m thinking that might be the case with an 18 ?
4. Any special reason to avoid a 67 evinrude 18 over a later model 69 or 70’s of 18-25 hp ?

thanks

 

[Pappy]

With that engine you can literally bolt on a 25hp complete powerhead and go.

 

[Ironhorse2022]

That would probably be the way to go rather than changing out individual components. I’ll probably run it as an 18 for a season then decide.

 

[moedelawn]

Love those old motors. I have 2 1969 25hp Johnsons. Just rebuilt one. Had to get the pistons from England. Repainted and need to paint the cover. One of them my dad bought new in 1969 for $400.00. The gears were replaced in the late 80's with the spline shaft and Atomic electronic ignition replaced the points. The gears costed as much as he paid for the motor. I run it occasionally for nostalgia. When i was 14 dad would drop us off at the launch and tell us what time to be back. No cell phones! Pushes the 15' southfork 25mph with just me. It's really like an 18 hp by current standards. The new 25 yamaha pushes the same boat 32mph.

 

[airshot]

Being a two stroke, running at wot constantly is not a problem like it would be on a four stroke. Difference between an 18 and a 25 would be a couple/ few mph at best!! If you looking for more consider jumping up to a 30-35 hp

 

[VinTin]

I believe there are internal engine differences between the 18 and 25 hp OMC's.

 

[Pappy]

I believe there are internal engine differences between the 18 and 25 hp OMC's.

You would be correct. Plus you are effectively adding somewhere around 38% more horsepower and no extra weight. It is a really good and inexpensive modification.

 

[Ironhorse2022]

Pappy- would you recommend looking for a good 25hp power head vs building the 18 into a 25 ? I’m guessing the former is probably less expensive.

 

[cyclops2]

Used OLD OLD LIED ABOUT Parts ??? A really bad risk of spending money.
Buy a RUNNING MOTOR tested on the back of your boat. ONLY SURE way to get a GOOD running engine of the correct hp. What good is a pile of crappy 30 hp parts or WEAK engine ? NO returns on used parts in most cases

Complete running engine. ONLY Are you a boater or a frustrated mechanic ?

 

[Ironhorse2022]

Understand your point about the risk of buying used. As to your question, I suppose I’m both a boater and not-so- frustrated mechanic. Half the fun for me is refurbishing things back to good condition. Using it is the other half.

 

[cyclops2]

Excellent !!!!!!
NEVER lie to yourself.

 

[RaisedByWolves]

a pile of crappy 30 hp parts

Hey now…. I have many piles of crappy 30hp parts.

I can generally sus out wether a motor is viable right on the tailgate. It may need some random parts from the crappy parts pile, but it’s not rocket surgery.

Usually a $100 parts motor can be turned into a runner for about that much again in parts, carb kit, plugs and water pump.

Best I ever did was have a parts motor pay me $50 before I ever even looked at it.

 

[Pappy]

Pappy- would you recommend looking for a good 25hp power head vs building the 18 into a 25 ? I’m guessing the former is probably less expensive.

Sure! they are plentiful. Get the later model gearcase and use it as well.