yahama 2 stroke from 4 to 5 hp

hello my name is alex and i am the owner of an 4 hp 2 stroke yamaha (1991-2000)and was wondering if a simple carburetor change can boost it to 5 hp?
alone with no gear i can plane with my boat an reach something like 22 km/h but with fishing gear in that its imposible... that extra hp is the thing that does the trick. i have tested a 5 hp (same generation 2 stroke) and iti does the job well.
can it be done? is it just a jet change or full carburetor ? has enyone done this tune-up ?
thank you

 

Look at the parts listings in a Yamaha outboard parts books to identify the differences between the 4 and 5 hp engines. http://yamahamc.partsandwarranty.com/SyndicateNav.cfm?NodeID=0

Based on a quick look it appears the carburetors are different but I did not look for other differences.

 

It is NEVER just a jet change.

 

But it can be a simple as a different carburetor, or a restriction in the intake system.

 

Frequently with outboards the differences may be as simple as a carburetor. It's basically the same engine, just tweaked to get more HP. My 4 HP mercury comes in 4 HP, 5 Hp and 6 HP , and it's really a Tohatsu. They all weigh the same, have the same cubic inch displacement, gear ratios, and prop. So as David said check the specs, and see what they say. By now the warranty has run out so any changes you make won't void the warranty.

 

Checked the specs the jets seem to be the only different pieces that could affect the power (screw's dont count). i dont know what else to look for

 

The jets are chosen after everything else is set in place to achieve the designed hp.

The jets keep the fuel to air ratio in the correct range, so assuming the correct jet is in the carb now, a bigger jet does not increase the HP, it will typically make it run worse.

If other changes are made to the motor, then a jetting change may be required to keep the fuel to air ratio correct.

People tend to notice a jet may be different, then assume it is the jet change that increased the hp, but in reality the different jet is required because some other component(s) was changed that required the different jet.

 

ondarvr is correct. Different jets imply something else is different which affects airflow, probably the carburetor body. The parts book presumably does not list the carburetor body separately so a difference in carbuertor body would not be apparent from comparing parts book lists of carburetor parts.

 

this is the parts book that i found . they are all listed to witch engine they fit or work... main nozzle and jet pilot is what i cand see different
http://www.marmom-marine.com/parts_catalogs/yamaha/pc_4acmh_04.pdf

 

In the Cylinder and crankcase list there's also a difference : bolt nr. 28 and 29. Maybe one of them is longer and partly restricting the exhaust ? Hans

 

The crankcase and crankshaft/rod/piston are also different. Probably the displacement is different.

As ondarvr said a difference in the nozzle and jet alone will not increase power. There also needs to be a difference in the airflow which the differences in crankcase and crankshaft/rod/piston would provide.