Re: General Prop Issue
Because of the design of the carbs and because of the relationship of timing to carb butterfly opening, the "sweet" spot is at about 1/4 BUTTERFLY opening. This does not translate into 1/4 throttle control handle movement. You need to find that yourself.
Note that this engine is the same block and carbs as the old Chrysler engines. These old engines had a collar around the control lever hub. There was a clear area and a painted blue line showed through at "CRUISE." This was the economy setting and corresponded to about 1/4 butterfly opening.
Take the carb covers off and observe the throttle butterflies. You will note that at approximately 1/4 opening the timing is almost fully advanced, giving the best economy. Also, with someone else operating the boat, brace yourself and look into the carbs as the driver opens them for a hole shot: You WILL be amazed at the amount of fuel atomizing in the venturis. When that "finger" on the carb advance cam pushes the roller on the bottom carb down and the butterflies open all the way, those babies like to suck gas.
ADDENDUM: DO NOT PLAY WITH THE JETS. Do not lean the low speed needles in an effort to acheive better fuel economy. Either is a fast way to buy another engine.
This, of course, is predicated on the engine running in the recommended RPM range at full throttle. Running too small a pitch prop is much like running an auto in second gear on the highway: Of course you will use more fuel.
BTW: With my Force 125 running 60 MPH at 5900 RPM, I burn through 15 gallons in something like 2 hours. Actually, that is not too shabby! Given that I am not at 60 ALL that time, it still works out to somewhere around 5 miles per gallon.
If your gear ratio is 1.95 to 1 as the three cylinder engines are, then the top theoretical speed with a 19 pitch prop would be: 46 MPH at 5000 RPM. If the ratio is 1.78 to 1, then the speed would be 50.
As stated above, check your tach and check the speedometer against a GPS. If the numbers are correct, then step up to a 21 pitch. Stainless if you got the money and it's burning a hole in your pocket.