Increasing power of a Yamaha 5hp 2-Stroke?

[Tim Good]

I have recently made the conversion from a Mercury 3.5hp 4 stroke to a Yamaha 5hp 2 stroke (5CMH Model).

I want to try and make it as efficiently powerful / effective as possible for use on a small AVON airdeck 2.6m (28kg). Could anyone list some ways to do this from easiest to most difficult. Ideally ways which don't compromise the life of the engine much. At the moment I'm thinking:

- ideal prop for intended use?
- planing fins?
- higher octane fuel?

 

[TSB240]

My experience of improving performance of two stroke engines is major gains are made by improving fuel and air flow, increasing RPM and tuning the exhaust scavenging.
All of these are impractical on water cooled outboard.
I have used octane boosters. The small gain in horse power will usually cause premature failure of your alloy piston.

I am surprised you have a problem with 5hp on a small inflateable. I have used both a 2.3 honda 4 stroke and a 4HP evinrude on a similar size dinghy. It will plane one up if you get your weight well forward. I dont expect to plane two up or with a heavy load of clobber.

 

[LONG_KEELER]

I have recently made the conversion from a Mercury 3.5hp 4 stroke to a Yamaha 5hp 2 stroke (5CMH Model).

I want to try and make it as efficiently powerful / effective as possible for use on a small AVON airdeck 2.6m (28kg). Could anyone list some ways to do this from easiest to most difficult. Ideally ways which don't compromise the life of the engine much. At the moment I'm thinking:

- ideal prop for intended use?
- planing fins?
- higher octane fuel?

Definitely worth checking the propeller.

Perhaps you could let us know what the prop has stamped on it.

 

[PabloPicasso]

Get it serviced and check it is performing as it should before considering mods . Cheaper, sensible, and should give you a great little outboard tender combo

 

[pagoda]

Yamaha used to have a habit of marketing the same hardware with different power ratings (and prices!) . In some cases the only real difference was the size of the carburettor jetting setup . These outboards generally don't have air filters on the carb input side so there is no real air restriction. Get some more fuel through the main jet/needle size combination and you can have some more power. Worth looking at anyway.

 

[jiris]

In the old days we used to play with increasing power of 2 stroke motorcycle engines with a reasonable degree of success.
The mods included:
1) Lowering the engine head by a fraction of mm.
2) Filling the holes in crankshaft flywheel by cork.
3) Polishing the air passages.
4) Larger fuel nozzle.
By combination of these it was possible to get (our optimistic estimate) about 30% power increase.
Higher octane fuel is a myth. It doesn't produce any more heat than the lower octane. The significance of higher octane is, it can be used in higher compression machines without suffering self-ignitions (pinging). You do may have to switch on one after "shaving" the engine head, otherwise you are just wasting money.
A properly tuned prop is definitely a way to go.

 

[lw395]

Power restriction is often by the carb not giving full throttle or being choked down.
Sometimes restriction in the exhaust.
The prop is the first place to look though. If the engine can't reach its rated RPM it won't give its rated power.
A lot of Yamaha 2T engines are reed valve. This is a potential route to improvement.

First thing is to check engines which you think may share components and cross-reference part numbers. Also look at max power RPM, to see if the lower power motor is being restricted by a rev limiter or ignition timing.

But basically, cc's don't weigh very much, if you want an upgrade, buy a bigger motor.
You don't want a highly tuned peaky response on an outboard, you need a wide power band.

 

[Honda Z50M]

My experience of improving performance of two stroke engines is major gains are made by improving fuel and air flow, increasing RPM and tuning the exhaust scavenging.
All of these are impractical on water cooled outboard.
I have used octane boosters. The small gain in horse power will usually cause premature failure of your alloy piston.

I am surprised you have a problem with 5hp on a small inflateable. I have used both a 2.3 honda 4 stroke and a 4HP evinrude on a similar size dinghy. It will plane one up if you get your weight well forward. I dont expect to plane two up or with a heavy load of clobber.

Higher octane will most likely actually reduce power as it retards the ignition timing slightly (higher octane fuel burns a fraction slower than normal fuel) this is why it's ONLY suitable for engines with high compression pistons to reduce pre-ignition caused by having a high compression piston.

Tom

 

[ibbo]

Raising the outboard on the transom may reduce drag?

 

[geem]

Installing a large planing plate will help. We used to use one on a 6hp Tohatsu four stroke. It would plane with two adults no problem on an airdeck inflatable. We routinely use large plates on our 10hp and 15hp two stroke engines