How can I increase HP on a 72 Evinrude 25HP?

[Captain Caveman]

I took my '72 Evinrude into the shop a few weeks ago for winter maintenance. I told the guy that it doesn't seem to get the same power as buddy's with similar boats and 25HP outboards. He said that the 25HP Evinrudes from this era were really more like an 18HP motor.<br /><br />Is there a way to mod the carburetor or anything else to squeeze out some more juice? I've rebuilt my truck carb many times so I'm sure I could handle this job if practical. Thanks.

 

[rodbolt]

Re: How can I increase HP on a 72 Evinrude 25HP?

not very practical. it kinda was a juiced 18. the cranks are kinda weak at 25 hp the later thru hub and commercial 25 motors shared the same block bore and stroke as the 30-35 the 72 25 was a tiny bore. but a 3 cyl suzuki or yam will still run off and hide from them.

 

[Dhadley]

Re: How can I increase HP on a 72 Evinrude 25HP?

Absolutely. Torque is torque. The 35 style motor can swing a much larger prop, in diameter and pitch. Plus the newer gearcases are a ton sleeker too.<br /><br />The 25 crank is weighted differently than the 18 and 20 so it will spin up more. You can get more out of the 25 but it requires a complete teardown and really good gas. Really good. In the long run you'd be better off to look into one of the larger bore motors Rodbolt mentioned.

 

[Captain Caveman]

Re: How can I increase HP on a 72 Evinrude 25HP?

I realize that the answer to more power is typically to buy a newer and larger motor. I just wanted to see if changing the jets, reeds, etc. would make any horsepower difference.<br /><br />Thanks

 

[Dhadley]

Re: How can I increase HP on a 72 Evinrude 25HP?

It will to a very small extent. But the torque from the larger cubes and other improvements will win out.

 

[rodbolt]

Re: How can I increase HP on a 72 Evinrude 25HP?

ironranger<br /> the motor will repond to standard hotrod tricks such as insuring the ign system is top notch and the points open at the same point for both cylinders, make sure the stator plate is not sloppy. make sure the timing is correct on both cylinders,may mean you have to make the #2 cyl marks. insure the exhaust chamber is not clogging with carbon,may try running power tune, or Combustion chamber cleaner with every tank. make sure the boat bottom is smooth make sure excess weight is left ashore( talking about extra coolers not wives ) make sure the prop allows for maximum operating range and is mounted correctly on the transom.

 

[herothc2003]

Re: How can I increase HP on a 72 Evinrude 25HP?

talking about props,could you put a bigger or more agresive pitch prop on a 73 18 hp?

 

[Captain Caveman]

Re: How can I increase HP on a 72 Evinrude 25HP?

rodbolt - Are you kidding? The wife gets left on shore to make room for the beer cooler.

 

[G DANE]

Re: How can I increase HP on a 72 Evinrude 25HP?

The 72 25 hp's were rated at the cranck, in opposite to the later than 84, having bigger displacement too. But at WOT, compared to i.exa an 1980 25 HP its horsepower to horsepower. I worked a lot on these. The 72/73's small displacement blocks are much more sensitive to the right prop selection, allowing them to spin up to 5500 rmp to produce their horsepower, and allowing their hole shot torque. Get a tinytach ( search on web, few $)and insure it spins up to 5500 on wot, light load. Give it a ignition tuneup and clean carb. I think you will be surprised. ( Oh I didnt read enough, I guess you will get that at winther service ) When it gets to 5500 you get MUCH more power. Decarb it too, means a lot.

 

[McGR]

Re: How can I increase HP on a 72 Evinrude 25HP?

I agree with GD regarding the prop. I had a '69 25hp for quite a few years. It was a great fishing motor... easy to start, would troll all day and had reasonable get up and go... with the right prop. I once replaced the prop on this with one I ordered through a catalog. Thinking bigger is better, I chose the largest pitch available. I can't tell you how poorly it performed with this prop. So, I decided to make this prop a back up and went to a local OMC dealer. He set me up with a lower pitch prop that made a HUGE difference. Yea the newer style larger 2cyl 25s have a little more grunt... but with decent tuning and the right prop matched with a decent hull that isn't grossly overloaded...the old "hopped up 18" will do OK.

 

[Captain Caveman]

Re: How can I increase HP on a 72 Evinrude 25HP?

Good to know about the prop. I had suspected that the prop may be an issue. It doesn't seem too banged up, but I did ask them to machine the edges since I heard that the outside edges produce the majority of the power. I suppose it's worth looking into a different pitch. At 25HP, I more worried about how long it takes me to get from lake end to lake end (many hours) versus how fast it gets up on plane.

 

[Dhadley]

Re: How can I increase HP on a 72 Evinrude 25HP?

With the different port timing on the 25 compared to the 18/20 and the "balanced" crank, the 25 is capable of making more power closer to 6000 than 5500. Yes, the blocks are the same as far as porting but the wrist pin is in a different place on the 25 piston, effecively raising the ports. And the 25 head is a bit different too. The bottom line is -- dont be afraid to turn it up a bit.

 

[Chinewalker]

Re: How can I increase HP on a 72 Evinrude 25HP?

Iron Ranger,<br /> One thing to keep in mind - pretty much all of the advice here is stuff that will basically make sure your motor is putting out its rated 25hp. If your 30+ year old motor is like most I've seen, they're a bit on the tired side, the throttle linkage has some slop, the rings have some carbon in the grooves, the crankcase seals are less than perfect, etc. That said, all the work you do will, when you're done, give you a 25hp motor. A fairly high-strung one at that. As good as the 25s were, they made their 25 horses from the time-tested 22-cubic inch block. Getting 25hp from the block required a lot of tinkering on the part of the OMC engineers. Getting MORE than 25hp isn't terribly realistic on anything other than a stick-tower, 16:21 gears and a 115 lb. hydroplane (as run in 25SS class APBA racing in the early 1970s). <br /> Follow the good advice to prop the motor to your boats load at 5500 revs, do a good de-carboning and make sure your motor is putting out it's rated power....<br />- Scott

 

[Captain Caveman]

Re: How can I increase HP on a 72 Evinrude 25HP?

Thanks everyone. I sure learned a lot.