Horsepower is horsepower it is a standard for doing work.
This motor started as 4 HP and went to a 5 HP and then to a 6 HP with out a change in volume or compression.
Let us see if we can find out where the horse power went!
29.92 inches of Hg at 59 F is what is used as a standard day to measure from for aircraft performance purposes and dynometers correct reading for these standards and more.
Less pressure equals less HP.
More temperature equals less HP.
More alcohol mixed in the gas equals less HP. (If not jetted for it)
Premium fuel when regular is called for equals less HP. ( You are not doing you engine a favor)
Adding an accessory after the HP has been established (charging system) equals less HP to the prop.
Let's deduct 5% for manufacturing variables such as a minor difference in compression ratio, lump in the intake track, cam timing variance, piston height, deck height, valve clearance, gasket alignment just a bunch of differences between the original prototype and the production engine.
Now the big hit, 10%! The engine was designed and tested to meet EPA air quality laws on pump gas without alcohol. It was jetted very lean then. Now with the addition of up to 15% alcohol before they have to call it GASAHOL(?) it is running so lean that it cannot equal the HP of the test engine by a long shot. Add to that the fact that most marinas only carry 91 octane and you could loose even more.
How about 3% for the alternator, it takes HP to make electricity.
2% for the weather.
So there you have it, 20% loss or 1.2 HP. Now your 6 HP motor is only putting out 4.8 HP.
What about the power of my 4 HP 2 stroke I hear. Well, two stroke engines are delivered jetted rich because they run leaner during the break-in period because the extra oil leans out the fuel mix. Two strokes are still jetted rich to protect them from over heating and sizing or galling a piston. The new gas is leaning them further and bring them into the sweet spot for max power but any more and they will start to self destruct. I will guess your 4 HP is putting out closer to 4.4 HP (10% increase and no loss from an alternator)
Who to blame?
Blame the manufacture 5%, you for 3%(charging system), the government 10% (epa and fuel) and God 2%.
On second thought don't blame God (cuz he will give you some great sailing days to make up for it).
Let's try to drop your losses to 7% or less.
The easy stuff first.
I am sure you have fresh gas of the 87 octane the book calls for.
Remove anything that obstructs your view of the intake side of the carburetor. Now turn the handle all the way open and check the throttle plate is 90^ to the air flow and has not over shot the mark and started to close. If it has look for a stop screw or linkage and adjust.
Next take a plastic straw and slide down the carburetor throat, holding tight to the side and see if hangs up on any lips that may protrude at the junction of the engine and manifold or carburetor and manifold. I hope not because the carburetor will need to come off.
Is there an air filter or flame arrester for the carburetor? Make sure it offers no restriction. Put every thing back together.
Did you find anything? Get out the instruments and run a base line pull test. Log and add the barometer and temperature readings.
Disconnect the battery, any change?
So now you have broken the dock lines and shot out into the bay looking much like a torpedo running straight and hot! You can stop testing now. Nope?
Take off the prop and let's check the rev limiter, the brochure says it has one. I guessing the motor should top out at over 6,000 rpm. It should rev this high without the prop if not then the problem may be electrical.
I say that before we get into jetting we throw off the lines and go for a sail. This is a good time to see if any small changes have gotten us over the hump. Nope? then on to jetting.
Jetting, or re jetting as it can be called is not hard if you use the rule of two.
THE NUMBER ONE RULE IS TO ALWAYS SET THE FLOAT LEVEL PER FACTORY RECOMMENDATIONS BEFORE ANYTHING ELSE!!!!!!!!!!!!
The Rule Of Two
Always go up two sizes.
Improvement? Go up two more sizes.
Keep going up by two until you start loosing power then go back one.
Go back one more if necessary.
Now there is a slight chance that the carb is running to rich and you will have to lean it out. The rule of two still works.
I will bet you won't find a large array of jets for this carb so you will have to drill them. The factory will probably have a leaner high altitude jet but none larger than stock. Take your stock jet or a replacement to a good hobby store and check for availability of letter sized drill bits(I think they are the smallest). The trick will be finding a size that will fit in your jet and then use the rule of two and buy three larger ones. If you don't have a ---PIN VICE---get one there too. ( holds tinny bits so you can drill with your hands or put into a regular drill.---Use your hands)
You can solder the hole closed and then re drill it smaller if you are good and don't get any solder on the threads.