modifications,where to find info

[Skoontz]

Assuming you know how to install the nitrous, I would recommend you start stocking pistons, rings, engine blocks and rods. Nitrous is a great bolt on tool to make alot of power with the push of a button, but, the engine has to be completely remade to handle the extra HP the nitrous makes. Ever watch the movie Madison? As cheesy as they made the pushing of that nitrous button, there is alot of truth to what they over dramatized when they blew off the sponson. Having been at that race with my dad in 1971, I was dissapointed in how they made the movie, but it was still fun to see all those vintage boats.

Mark 75 said what I would have. Until you do something below the water line (gearbox) you are not going to see a great gain from the powerhead. Something else. Unltil you do a great deal of work up top, that motor is never going to swing an 18" wheel and get any kind of respectable RPM. We had a 25 on a 10' GW Invader years ago and Michigan Wheel recommended a 17 as a box stock package. We could not ever see beyond 4800 RPM when Red line was 5500. It was so doggy out of the hole it took nearly an 1/8 mile to plane off and lugged very badly on the turns. There is a hole thread here on
OMC 25 HP that you would benefit reading from. Check it out.

[smittythewelder]

Dirtbag, don't feel out of place here with your Minimost. A lot of the older guys had boats like that to start with.

If you MUST hop up that nice fishing motor, see if you can get a little more torque, so you can swing a prop with a lot of pitch. First do web-search on the subject of squish (also try it under quench, squishband, quench area, and compression). Check the various 2-stroke sites (look for Kevin Cameron, Dr. Dirt, MacDizzy). What you'll find is a way to increase compression without causing detonation. This involves milling the deck surface of the block, or head, or both, so that the piston has about .035" clearance from the head at the tightest point. You might have to use the high-octane pump gas. If you disassemble the engine, you could lightly bevel the sharp edges around the ports, but do not knife-edge them and don't open them up. If you disassemble the carb, you could bore the venturi on a lathe, opening it up maybe 1/16", but then you have to re-jet unless you have adjustable needle valves on that engine. You could experiment with spark timing. You should go at least one heat range colder than the standard spark plug.

Have fun.

[dirtbag]

right on this is what I am looking for ,so I have a old lark 40 the carb looks like it will bolt up, would that be foolish,and what could I do to my bottom end ,or should I just attack it with a hole saw ,ha ha,on rc boats they run a tuned exhaust and put the end right below the water,or run a baffle,I am guessing I would have a hard time finding a race lower unit or would I ?

[Mark75H]

Racing lower units are easy to find, but expensive

[tthibodaux]

Racing lower units are easy to find, but expensive

Wouldn't he need to find the whole down housing? What type of gearcase is there for a stock midsection? I have a B1 complete but I dont think it would work with a tiller handle.

[Mark75H]

Correct, that makes it even more expensive; unless you can do it yourself. Like some people say ... speed=$ squared

[Skoontz]

For some things, it's the tinkering and not the purchase that is the fun. But, I agree David L6 on this one.

For the carb question, the 18-20-25 manifold studs are about 3/8" too narrow, and, the inlet hole in the manifold would need to be opened or it would act as a restrictor plate. Second, size. 22 cu. in. with the same RPM as you would have with a 36 cu. in. You would have rejet the low end, and increase RPM on your 18 by 40% in order to efficiently burn all the juice that the 40 HP carb would throw into the engine. To do that, the crank and rods would need replacement, forged units put into their place, preferably H beam rods. Pistons also might need and probably would need forged units.

Dollar and cents of the deal, plan on $1,500 for a crank, $500.00 per rod, and, whatever you can work with pistons. Then you are going to adapt a hot dog gearbox. Cost of the hot dog, at least a grand, then $500.00 a prop, and, and morped midsection, or custom made section. Plan on at least a grand. Then, you have a motor that at best, will need alot of fine tuning and dedication to dial in, when you cqan just find a very decent motor to do what you need to do for the combined cost, that will need very little to do what you want to achieve.

I'm not telling you not to do what you are inquiring about, matter of fact I'm intrigued by your interest. Just trying to lay out the carpet you will be walking across. I've been playing with engines since I was about 3, I leearn every time I read or touch something, and I could see this project taking me a considerable amount of time, so, if you have limited mechanical abilities, or are unsure of how to proceed, I would probably start you playing with a smaller project.

[smittythewelder]

I haven't looked at a motor like his in a long time. Is it like the older OMCs, where the water pickup is a screened-off slot in front of the exhaust stub?

Dirtbag, you can see that nobody here is very optimistic about your chances of waking up that engine very much, and generally expect you to wreck it as a useable fishing motor, and maybe render it permanently non-functional. But so what? It's your motor, and so far as I know it has no collector value, so go ahead and carve on it! You'll have some fun, maybe learn something. But don't expect us to spend much time helping you on the project, or to give you specific directions. YOU have to do the work, study the manual, and read the books and magazine articles and website stickies on engine hop-ups. "Pay your dues," in other words. And come to some races! Get a better idea of what we're about. And good luck! You can have a lot of fun with a Minimost.

[dirtbag]

this is what I was looking for just opinions,thanks for the info ,it sounds like I am best to build another boat and find a racing engine,that has been the lingering thought for awhile

[jeff55vDSH]

What are the differences between the 25mod OMC and the 18hp powerhead? I would think the two powerheads are similar in design. Therefore, what's good for the 25 should be good for the 18. I think the little 25hp powerhead is popular in the 25mod class. There should be a wealth of info to be found about the 25mod OMC. Can anyone here help direct this man to an engine builder for the 25mod class?
If you stick with the stock towerhousing and transom height, Do not cut the exhaust snout off. You will lose power. The snout runs below the waterline and the passing water creates a vaccuum. Thus, "sucking" the exhaust from the motor. The power gain from this action outweighs the drag caused by the snout. Take it from me, I learned this lesson already on a stock KG-7 Merc. Unless of coarse you are able to jack the motor up on the transom. Then the lesser drag from the lower unit will FAR outweigh any powergain from that vaccuum exhaust action. I know it sounds strange, but it's true.
Best of luck to ya with your little boat! Have fun!