Tuning a -1986 Yamaha 15hp

[vStrandell]

Hi, I'm new at this very nice forum so I don't really know if this is the right place to post this. And sorry for my bad english

http://www.boatracingfacts.com/forum...ad.php?t=13813

So, I got one a boat called TG Special, made in finland by Tage Gustafsson back in the 70s, it's V-type boat that is 4 meters long, about 1,8 wide, weights about 43kg/94.8pounds (without engine and driver) and got quite big wings and a Mariner 9.9/15hp eninge made by yamaha in 1986. I got it in the middle of the last summer and some tuning was already done on it.
The carb is a 20,5mm modded stock carb (std 19,5mm) with a 1,43mm nozzle. I noticed it had stock metal reeds still on, will fix it in the spring. It has standard crankshaft and pistons. The inlet&oulet ports have been risen by about 1,3mm. the engine head is modded to, the pressure is now about 13bars or 27.1 p/sf (std 8,5bar?) And is has a standard exhaust pipe. The gear case has the standard shape also. I had a about 9" - 9.5" X 15" cleaver on, it hadn't really enough blade surface or enough pitch, and the engine rev 6650 (digatron tachometer) and it gave it a modest 36,7 knots (just 42.6 mph). I found a 10" x 17" Radice Tipo hydro mibral prop, would like to have about 6300 - 6400rpm, i will use it from now on.

Wich is the best reeds for this engine? I have thought about boyesen power reeds or then malossi reeds. Are the 13bar pressure in the cylinder too much? And are the intlet&outlet ports in the engine too high? Could i get more power with modding the standard pipe? Is there any point with making a nosecone on the gear case? I already have external cooling water inlet. Is there anything else i can do? I will drive on pump fuel, wich is pretty good i Finland (98 or 99okt)

[Fast Fred]

one Bar, think thats about 12psi, so thinkin about 150-155 psi, boyson reeds should work for you. you can always add more timin, it hard to take it back once you have gone to far. i'd pull 5drg off the stock timin, start there.

[vStrandell]

one Bar, think thats about 12psi, so thinkin about 150-155 psi, boyson reeds should work for you. you can always add more timin, it hard to take it back once you have gone to far. i'd pull 5drg off the stock timin, start there.

Thank you for your advices!
The converter said it would be 188psi, do you think the Boyesen reeds would work with that?

[Fast Fred]

thats what i'm runnin in mine, boysons be just fine, runnin LL100 fuel, think your fuel may be a bit weak at that comp'

[microbream]

a 20mm carb sounds a little on the small side, i run a 24 on my 164cc motor and it loves it.
i too have the 190 psi comp and run pulp fuel over here which it 95 octane.
i would suggest some port work to increase the port time are values up to 6000-6500 rpm, and a bigger carb to start with, oh and use at least 30:1 fuelil mix

some guys dry stack those but i would not recomend it unless you like to melt you bushes

[Powerabout]

one bar is atmoshperic pressure so 14.7 lbs

[j edwards]

Where did you get the carb? I have the same engine a i wanna put a bigger carb on on maybe a 25 hp carb?? What size do you have and what type of engine did it come off of i gao axcess to 25 and 30hp carbs!

[tsrt]

30hp yamaha carby make it scream