81 Evinrude 25 HP convert to 35 HP

[cchildr84]

Ok so I know this question has been asked a 1000 times if it's been asked once. I have an '81 Evinrude 25HP. In my research for the '81 year model everything associated with the power head between a 25hp & 35hp is exactly the same. That is, the crankshaft, pistons, rings, all bearings & seals, block, intake, flywheel, etc., with the one difference being the carburetor.

Where the difference comes in, is starting with the driveshaft then all the gears in the gearcase, and the fact that a 35hp is a through hub exhaust and the 25hp is not. Also, per the manual the timing on the 25hp is to be set at 34 degrees and the 35hp
is to be set at 30 degrees.

So my question is, if I were to ever consider doing so, would buying a 35hp carburetor and retarding the timing down to 30 degrees provide me with a 35hp motor. Is the driveshaft and gears on the 35hp simply beefier or are the gear ratios different such that simply changing the carb & timing wouldn't produce the same results as a true 35hp.

Please keep in mind this is all out of simple curiosity, I don't see that I would ever consider doing this. I've put to much time, effort, and money into re-building my 25hp to ruin it.

 

[boobie]

Re: 81 Evinrude 25 HP convert to 35 HP

Sorry, can't tell you. But I will say the best thing is "to leave a sleeping lie".

 

[jasper60103]

Re: 81 Evinrude 25 HP convert to 35 HP

The only one that has done this with documented success to my knowledge is johny25, but he did it with the later 1980's 25hp that share the same lower as the 20 and 30hp motors. He has a very long thread on this topic.

 

[Bosunsmate]

Re: 81 Evinrude 25 HP convert to 35 HP

If its only the carb and retarding the timing, give it a go with a second hand carb off ebay or somewhere and post back on how you get on.
Dont worry about the gears, just see if you get a much bigger power boost

 

[gbrowne9]

Re: 81 Evinrude 25 HP convert to 35 HP

i read johny25,s thread and modified my 1989 20hp.i did not buy a carb,i just opened up the intake manifold. the original intake hole looked like a large keyhole. i made it as big as the bore of the carb.i think i gained 1-2 mph on gps.i did not play with the TIMING or anything else. the motor still runs very well and on a 1959 14ft aluminum boat i do 28-29 mph.

 

[cchildr84]

Re: 81 Evinrude 25 HP convert to 35 HP

i read johny25,s thread and modified my 1989 20hp.i did not buy a carb,i just opened up the intake manifold. the original intake hole looked like a large keyhole. i made it as big as the bore of the carb.i think i gained 1-2 mph on gps.i did not play with the TIMING or anything else. the motor still runs very well and on a 1959 14ft aluminum boat i do 28-29 mph.

Yeah, I just rebuilt a 25hp, and I was hoping to get 30mph plus out of it. With a 11 pitch prop Im getting 27 on my 14ft Starcraft seaferer. The boat itself only weights 200# but with the flooring and gear and myself we looking at probably 650-700 pounds. I really thought I'd get to 30mph or more. I have a mariner 9.9 that will push me and the same load at 19mph. So I thought that by more that doubling the horse power I would have gotten more than 8 additional mph. Don't get me wrong I love this evenrude and will so for years to come. Perhaps jumping up to a 12" pitch prop will get me to 30MPH.

 

[cchildr84]

Re: 81 Evinrude 25 HP convert to 35 HP

Well I have a 35hp carburator. When it comes due for the motor to change out the impellar, I may go ahead and switch out the carbs. Another concern I've determined is a possibility is this; with the 35hp carb on there and the extra fuel being pushed through and burnt, will this cause additional exhaust, and can the 25 hp exhaust housing take this additional load since it's a non-through exhaust set up?

 

[emdsapmgr]

Re: 81 Evinrude 25 HP convert to 35 HP

I had a 79 35 hp engine. It was rated to turn a few more rpm's than the 25. The 25 ran up to 5500 rpm's, the 35 up to 5800. Back in those days, the way OMC made more hp, was to install carbs that had larger throats (more air) and larger main jets (more fuel.) Not so sure this is the same between these two carbs. Both use the same .059d high speed main jet, so not sure just where the extra hp comes from. If you come across a 35 carb, check the throat size VS the 25 carb... The blocks are the same part number. Possible some of the higher hp comes via the different lower unit. The 25 hp engine had an early lower unit-without the thru-hub exhaust. The 35 had the thru-hub exhaust. It's possible that the 25 may not breathe quite as well as would a 35 at high speed/rpm's. I always thought the thru hub lower unit was more efficient through the water than the non-thru hub lower unit. Also they had different gear ratios. The 25 was a 12:21 ratio, the 35 was 14:22.

 

[Willyclay]

Re: 81 Evinrude 25 HP convert to 35 HP

Lots of good info in this old thread below. Good luck!

http://forums.iboats.com/johnson-evinrude-outboards/converting-20-hp-johnson-35-hp-274566.html

 

[cchildr84]

Re: 81 Evinrude 25 HP convert to 35 HP

I had a 79 35 hp engine. It was rated to turn a few more rpm's than the 25. The 25 ran up to 5500 rpm's, the 35 up to 5800. Back in those days, the way OMC made more hp, was to install carbs that had larger throats (more air) and larger main jets (more fuel.) Not so sure this is the same between these two carbs. Both use the same .059d high speed main jet, so not sure just where the extra hp comes from. If you come across a 35 carb, check the throat size VS the 25 carb... The blocks are the same part number. Possible some of the higher hp comes via the different lower unit. The 25 hp engine had an early lower unit-without the thru-hub exhaust. The 35 had the thru-hub exhaust. It's possible that the 25 may not breathe quite as well as would a 35 at high speed/rpm's. I always thought the thru hub lower unit was more efficient through the water than the non-thru hub lower unit. Also they had different gear ratios. The 25 was a 12:21 ratio, the 35 was 14:22.

I think I may have stated this above, but per the prop/rpm increase that would likely occur with the 35hp carb there arent any props available above a 12" pitch prop for the 25hp non-through hub motors that I can find. So with out plenty of extra weight in the boat, my concern would be no way of keeping the rpm's down with a lighter load. This is touched on in the link that "Willyclay" provides.

 

[emdsapmgr]

Re: 81 Evinrude 25 HP convert to 35 HP

Bombardier did make a 13 pitch for that 25 engine. 395232. It's been discontinued, but tthere are a ton of them (still brand new) at a few dealers around the US.

 

[Willyclay]

Re: 81 Evinrude 25 HP convert to 35 HP

Could a good prop shop re-pitch a 12inch prop to 13?

EDIT: There is some discussion of this lower unit/gearcase issue in the current thread below.

http://forums.iboats.com/johnson-evinrude-outboards/need-help-1982-25hp-evinrude-carb-637310.html

 

[emdsapmgr]

Re: 81 Evinrude 25 HP convert to 35 HP

Usually a good prop shop can add a little cup to the trailing edge of the blades, effectively raising the pitch by one. That's more common in the larger 3 cyl thru V6 props. Not sure if anyone would do that work on an old-style 25 hp 3 blade prop.... Sure you want to invest that expense in a 12 pitch prop? Maybe cheaper to just purchase the 13 and wind up with a spare (two props.)