Outboard motor modifications

[Fun_Bucket]

Hey guy's,

I'm wondering if there are any members of the forum that have experience in outboard motor's, and would be able to offer me some advice.

My main reason for posting here rather than a boating forum, is that i've found by reading through a few posts in some other forums, that most boating enthusuast's who join forums, are too sensibly minded to go about answering my question properly.

So, i have a 2.85 m Zodiac. I'm currently using a 15 hp motor. (hull rated to 8 hp) Before i go any further, no lectures about overpowering ect, i've heard it all before, and i am well aware of how it affect's the handling of the hull.

I want to gain a bit more power without going up to a larger motor. 15 hp is the highest output using the 262cc 16cu in block shared between the 6-15hp range. Going up to a 20-25 hp block, brings a weight increase of 14kg's.

Does anyone have some advice on what i could do to my motor to gain some extra hp?
I've heard about-
Hotter plugs
Intake/exhaust porting
Advanced timing
Bigger carbs ( allthough this whe going up a hp class in the same block, so there are no off the shelf carbs available to my knowledge that i could use)

I have accses to 100/130 aviation fuel for less than what you would pay for PULP as i work with a commercial pilot. But i've heard that the compression of the motor is not high enough and therefore would not be suitable.

Would it be worth increasing the compression of the motor? I've read about cylinder boring and head shaving. Do you think it would possible to go far enough to take full advantage of higher octane fuel and still retain some sort of engine reliability?

Many more questions to come, but i'll leave it at that. If someone on here has some useful knowledge , then possible i could talk one on one with them...

[gopher]

why not get rid of the whole lot and get a real boat?

[Fun_Bucket]

Thanks mate, that really help's a lot.

Being a 2.85 m SIB, it deflate's into a bag that fit's into the back of my focus. The outboard then lay's flat on the floor, and the ply insert's sit in sideways. This eliminate's the need for a trailer, along with registration.

Plus, i've always been into small RIB's SIB's ect. It suit's my needs well, allow's me to handle the boat and motor unassisted, and gives me a fair bit of freedom. You'd be suprised at just how far you can go, and the places you can acsess, with a setup this size.

We buy what we can afford. It would be like me telling you to piss your car off being an S pac, and get yourself a V8 instead.

[Tree cutter]

One of my uncles owns a boat shop and has had a lot of experience building and tuning two stroke motors for racing. He told me when i was looking for ways to get more power out of my racing go cart That i should not raise the compression by shaving the head because it will only give you more power down low and will stop the motor from revving out. You need to raise the height of the ports for more revs so you get more power. With motorbikes you can put 2 barrel gaskets on to raise the port height but i dont think you can do this on an outboard because the crankcase and barrels are one piece. It is best to warm up the cylinder with an oxy and press out the steel cylinder liners so you can open up the ports with a die grinder and then press the liner back in. Another way to get more revs is to machine a bit off from the bottom of the piston skirt if it is a piston port or reed valve motor. I was told to take 1mm off and give it a run then take another mm off and try again until you lose power and then buy new pistons and machine them down to the shortest lenght before you lost power. Reducing the crankcase volume will give you more power down low in the rev range. Most engines have crank pins that are hollow but some dont have plugs on the ends. If you plug the ends of the crank pin you will reduce the crankcase volume and gain low down power . Make the plugs out of aluminium. You can put a tapered edge on the bottom of the transfer ports to help a bit but dont open up the transfer ports too much or it will be dead down low and then go ballistic when it finally hits the power band. I would try to fit carbys off a 25HP engine and open up the inlet ports to match.

[Tree cutter]

My uncle once put nitros oxide on a customers racing boat with a 200HP evinrude but it kept blowing the gears in the leg and it had a flat spot when you shut the nitros off which he couldnt get rid of.

[wana s' charge stock vt]

would bigger carbi's and higher pitch prop work? bigger carbi's just to cope with the longer range prop to get it up n moving.

thats just what i were told to do with mine, i got a 40hp on a 13ft runnabout, but i can get the 50hp carbi's(off shelf) and bigger prob and i'll be flying, and sh!ting myself

[Fun_Bucket]

would bigger carbi's and higher pitch prop work? bigger carbi's just to cope with the longer range prop to get it up n moving.

thats just what i were told to do with mine, i got a 40hp on a 13ft runnabout, but i can get the 50hp carbi's(off shelf) and bigger prob and i'll be flying, and sh!ting myself

The off the shelf carbs you talk about are the 50 hp carbs used in the block by the manafactuer's to reach the maxuimim hp. My carb is allready the largest off the shelf item available. I'm not sure a larger carb will fit, but i will try and find out.

[wana s' charge stock vt]

oh ok, gottcha. mayb jjust get bigger jets put in your carbi,