25 to 35hp johnson

[pdt]

the triple suzi's and yams will be faster top end due to them being bigger engines in cubic capacity from the very start.

but with a bigger prop you'l go faster top end.
if you have a very fast pull away you could sacrifice a bit of that and go up 2 more inches in pitch for the top end your seeking.
but remember you'l lose the pull away and suffer big time if a friend comes along for the ride.

37mph is bloody fast for what you have it on anyway.
what speed gain did you get with the jackplate ?

post some pics.

[14apache]

Think a 35hp head will lose performance. I put the 35hp head on mine was afraid it would detonate with the more squeeze. I never touched the timing on mine I am a pussy. lol

[pdt]

also remember the more you try to get from it, the shorter time it will live.
better off staying a bit on the safe side and make it last longer.
anything over 160psi is putting a lot of strain on the internals, ok so its quick, but is it worth sacrificing its life for an extra 3mph ???

.

[14apache]

I would almost sacrifice my life for 3mph. lol That's the disease of racing talking.

[Fastjeff57]

The 25 hp motor is 30.5 cubes, where the 35 hp is 34.5 cubes, so you can't do a jetting or reed valve change to make ten hp.

Jeff

[speedfreak]

FastJeff57, We bored my 25hp to .060" over a stock 35hp. Same as 14Apache did.

All good points’ and information guys. I think I’ll leave well enough alone (cheaper too), guess “I want my cake and eat it too", this stuff is addictive, to 14apache point. At this point, the engine is still very reliable, starts and idles well all the time no matter how cold. I have removed the oiling system and premix 40:1 with Amsoil HP. I always burn 93 octane ethanol free fuel. Everyone that rides in it wants to buy it from me. I gained about 2.5 - 3.0 MPH from the addition of the trim and tilt, which that thing has other great advantages other than some rpm/speed gains. I don't want to get to the point where the engine is not reliable. I’m not a “racer”, so I can’t come home after the weekend and rebuild everything. I need reliability and thus far I still have that. Wasn't this OMC's "Mirage" series motor? I hear nothing but good things about the design of these engines once the oiling systems are removed. I just always wondered what it would take for a true 25 to 35 conversion other than finding a 35 horse power head and sitting on top of my 25hp short shaft lower unit. For my 25hp, I got the Cu in's (bored), jets (35hp), etc. always wondered what else internally was different (like the 35hp head, porting, exhaust, etc).

[speedfreak]

FastJeff57, check that .060" over a 35hp above. It was actually .030" over a 35hp bore. Just used standard OMC part number piston.

[speedfreak]

Picks of my little 1997 25hp OMC 3 banger! Hope this works!

[NERSTROM]

I think the Mirage was one of the best engines I did at OMC. The 25 and 35 share the same block and lower unit castings making all port passages and exhaust tuning the same between the two versions. Increasing the bore size of a 25 and readjusting port timing makes it a 35 block. I may have posted some history on the origin of the design in an earlier post. The block is of lost foam construction and due to the small bore size there was a tendency for glue lines to partially block the posts. Unfortunately, this is the case with many of the blocks. Someone willing to take the time to grind out the glue lines, especially in the finger ports is going to be pleasantly surprised at the increased performance. The air box that came with the engine was a patented design and while reducing noise also increased power. I have never found air horns to be of much benefit, although they do look nice. Darn cold here in Chicago!

[speedfreak]

Mr. Nerstrom, Thank you for posting and sharing your information and expertise regarding this Mirage 3-cylinder. I'm not exactly sure of all the engines you designed at OMC, but you knocked it out of the park with this 25-35hp 3 cylinder Mirage. This thing has been bullet proof since the day I bought it. I have been "tinkering" with it off and on trying to get my 25hp closer to a 35hp+ to run with those peskie little 2 stroke 3 cylinder Suzuki’s, Yamaha’s and Tohatsu’s that were manufactured in that similar timeframe. As far as the air horns, I designed/drew those up on the computer one cold, rainy weekend and had a local machine shop turn them out for me. I didn't really see any gains from them, didn’t seem to hurt performance any. I can always put the original air box back on.
So far the modifications to my 25hp (in my quest to get to a 35hp+) have been:
1) Increased bore size to .030" over a 35hp specification
2) Changed the carburetor jets from a 25hp to a 35HP
3) Went with a set of Boyseen reeds
4) I think the dealer may have increased the timing a degree or 2
5) Remove the oiling system, I premix 40:1 just for some added protection.
This was all completed by a authorized Johnson/Evinrude dealer.
Looks like maybe port timing adjustment and cleaning up glue lines in the finger ports may be next on the list.
By the way, Cold in Arkansas also.