omc 25hp

[looseunit225]

Hey guys just pulled my 25 hp evinrude 2 cylinder single carb engine down to give it a new set of rings and so on

also looking for a new lower unit for it

im looking at doing a little bit of mod work to the engine and i was thinking about opening up the exhaust and the inlet ports a fraction and also to have the exhaust come out of the top of the mid section

im just a little worried about the carby being able to keep up with the small mods

if anyone could help that would be great cheers

[Phil McDaniel]

Which 25 are you talking about? 22C.I. or 31.8??

[looseunit225]

how do i know weather it is one or the other?

[Glenno]

Hi, The difference between the two is easy to tell. The ones pictured here are the larger bore 521cc. The smaller bore 360.5cc looks very similar to the one with the over prop exhaust but the leg has very much rounder edges and is thinner. As you can see I modified the gear case on the over prop exhaust motor and added a 35hp carb (bolts directly on) cleaver prop (now on 15hp)and the motor went better. The gear ratio in the through hub exhaust model is 2.1538461 to 1 wich is not to good for speed with small hp in my experience. In saying that you will be able to add heaps of speed to the standard motor. There where a few early through hub 25s but I dont have pics and I believe there gear ratios where better for speed. I race 25hp class in Australia and have found the tohatsu nails all other brands by a considerable speed. Have fun with it, I am currently working a 15hp omc here are some pics of the gear case with low pickups.

[looseunit225]

yeah my engine is the late model engine

its a 98 model 25 evinrude with the thru hub exhaust

i was thinking about taking the bridge that is in the middle of the exhaust and the inlet ports and clean up the block down near the crank to try better the flo?

[Glenno]

See if any of the GT pro guys have any input before going to wild on porting. I believe they are reaching speeds of around 78kph. I rebuilt the late model 25 in the previose pics and noticed that they have a masive amount of airspace on the intake side of the block especially behind the reed block. I was wondering wether a wedge shape reed set up could take up some of this space to create more crank case pressure. Bolting on the 35 carb is a must, whatever you end up doing. Also be carefull with the exhaust tube (megaphone) Some racing mates played with theres and lost speed. Out of interest the tohatsu's are getting around 90kph on a light (around 60kg) well set up wing boat, the best I have got is 88.4 on my 101kg wing boat hull. There is a pic of my mates boat with Johnson- evinrude 25hp and it did around 76kph. Also if you can get your hands on to a racing gear case and adapt it to you leg you will really fly. Ps: it is a short shaft is'nt it?

[looseunit225]

yeah its a 20 inch mid section and the engine is going to be bolted to a small tiny

i was thinking about doing a bit of port work giving it a bit more fuel than releasing the exhaust out of the mid section for a better sound

[Glenno]

So its a long shaft. You need to get it up out of the water for more speed so a short shaft is easier as it means a smaller jacking block. But in saying that a taller block should do the trick. A nose cone to bring water up to the standard water pick up and about a 17 to 19 pitch prop. Ron Hill has these, I think your motor is a 14 spline but just check. Have fun

[arkansasbowhunter]

I have a 1999 OMC also. I am interested in doing some mods for the motor. According to the difference in micro fish pictures only the carbs on the 25 and 30 hp are different. I purchased a 30 hp carb and put it on but didn't notice any increase in top end. Will changing out to a 35 hp carb increase my top end?

[Glenno]

Hi, The 35hp carb is off the older models around 80 and older I think. I have never put the 35 carb on the later 25s but they are the same motor so i dont see why not. The only difference will be your carb has a small choke nipple which is fed by a choke pump. The older carbs have a choke butterfly. If you deside to get one, I would just bolt it on go for a run, start using the choke butterfly. If it works well Custom drill the choke hole and install a nipple. Have fun!

[Jfarrelly]

Where did you get, how'd you make the nose cone. Looks good

[MWhite]

Where did you get, how'd you make the nose cone. Looks good

The 20,25,30 35 or all 31.8 cubic in motors, the carb get bigger as u go up. We use to river race down here where we live with those motors, the guys that run them tryed lots of things to make them run better and ruined a few blocks because of porting on them,, if u ported on them it would kill it, a stock motor run better, so what they did was they cleaned up the reed block off a 35 and cleaned up the casting on the blocks, and put on a set of stacks, made by bob snider called hustler pipes, made just for that motor, and it made a big different with lots of gain. just saying.

[champ20B]

Some of you guys are talking about porting and mods and only getting 78+KPH? That is only about 48 or more MPH.....

I was able to achieve this with my 1997 25hp Johnson simply by using the bigger (35) carb, and a 17" pitch standard design aluminum prop. ( I bought the aluminum 15" pitch from the dealer then had it stripped, polished and pitched to 17" without any cup.) The boat I ran it on was a flat bottom mini cracker-box style runabout I thrown together that was 11' length and 48" wide at the bottom and weighed about 140+ pounds. I was 140lbs back then as well. The rig would just hit 51mph on a GPS with no trouble, and get there quick too.