Excessive fuel usage ?

[goodrich]

I have a 1990, 90 Evinrude with a freshly rebuilt power head and a new power pack. I'm seeing about 3 MPG or should I be looking at GPH? Either way, I'm sucking the fuel. I'm running a 13 x 17 prop, deep V, 1500 LB hull weight, at 4000 RPM, trimmed out I'm running about 25 to 27 MPH. Tops at 5300. Is this wrong or normal? Thanks for any input.

[Joe Reeves]

As far as your fuel consumption... that's not my line of expertise, however, 5300 rpm at WOT indicates a good match (Prop/Engine/Boat) as the full throttle rating of that engine is 5500 rpm and you're closer to that than most boaters.

I'd suggest that you double check the synchronization (timing/carburetor) to make sure that the throttle butterflies open/shut at the same time... and at the proper time. You don't want one shut at idle and one slightly cracked open.

Also, with the above... the butterflies should just start to open when the scribe mark on the cam is aligned dead center with the center of the roller.

Check the idle timing which should be set to specifications as per your service manual.

And at that idle timing mark is when the scribe mark on the cam should be dead center with the throttle roller.

The throttle roller is no doubt one of those cheaper (idiot engineer designed) two piece designs that has a clear outer shield over a smaller black roller instead of the black one piece roller that has proven to be superior for decades... the outer diameter of the roller should be approximately 3/8" If it is smaller (about 1/8"), replace it as that smaller diameter screws up everything and it is impossible to get the synchronization as it should be.

[goodrich]

The motor idles well and runs well at higher speed. Could what you're describing possibly be causing my lack of fuel economy?
sounds logical. A few months ago I spun a prop 10 miles from the ramp and had to hold it @ 1500 RPM for the trip home. About two hours. When I checked my fuel, I noticed I'd used as much as I would have at cruising speed for the normal 30 minute trip. Thank you so much for your time and expertise.

[Joe Reeves]

Originally posted by goodrich View Post

The motor idles well and runs well at higher speed. Could what you're describing possibly be causing my lack of fuel economy?
sounds logical.

Not an absolute cause.... however, if everything isn't exactly as it should be, there will be of course a flaw in the engine's performance.

Originally posted by goodrich View Post

A few months ago I spun a prop 10 miles from the ramp and had to hold it @ 1500 RPM for the trip home. About two hours. When I checked my fuel, I noticed I'd used as much as I would have at cruising speed for the normal 30 minute trip.

This fuel usage/waste is due to the fact that the engine has been force to "lug", that is to fight to keep the boat moving through the water. It could be compared with having overdrive or third gear go out on your vehicle... then driving it home (30 or 40 miles) in first or second gear. That'll make the ole fuel gauge drop in a hurry.