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I have the above engine on a 13 footer, the boat is clean and dry however I get terrible fuel mileage. I know this motor is old technology however it seems like it should do better. I'm no stranger to working on outboards but looking for some suggestions or someone to tell me this is the best it gets. I know this is a hard question to answer without any data. I tuned the motor up this summer with points/cond and went thru the carb. Runs like a top, doesnt load up and foul plugs. My thoughts are to check and possibly reduce the main jet, check timing again, increase pitch of prop. I also have some two stroke mod history and thinking of increasing the compression and clean up the porting to create more power utilizing the unburnt fuel. Any thoughts on this issue?
What kind of fuel mileage are you getting? Under what conditions are you operating the boat? Do you have a tach where you can read the full throttle RPM with the prop you have on it now? What altitude are you running at? What position do you have the trim pin set in? Is your hull bottom painted or is it smooth gel coat?
Based on what you have said I think the plugs are telling your mixture is about right and both plugs are firing. Going too lean with the jetting is about the fastest way to burn up a 2 cycle si I wouldn't mess with the jetting unless you are at high altitude.
I would check the timing and make sure it is advacing to the right setting per the manual as you add throttle. As I recall on these engines you set timing at both idle and fully advanced. Improper timing will defintely affect your mileage.
I have the same engine in a '64 and it has a feature that kills the ignition to one cyclinder while you are idling. This does not cut the fuel to that cylinder so my guess is this outboard is pretty inefficient at idle by design. Not much you can do here.
I would set the trim pin in the highest hole and see if it will run without porposing. If it porposes, then lower it one hole at a time until it quits. Most run in the highest or 2nd highest hole. Trimming too low will get you poor economy if you are running the boat at speed a lot.
Lugging a 2cycle will also get you poor fuel economy so make sure your prop will let you get to full RPM at full throttle for the load you are running in your boat. If it is not, then reprop for the load and you may need more than one prop if you run it with significantly different loads. Lots of cool old props for these motors on ebay and craigslist once you know what you are looking for.
Most outboards run most efficiently at speed when running at about 3/4 thottle. With the cowl off you can find this place by advancing the throttle until the spark advance stops. Thats the sweet spot.
I'd do these things before I went to modifying the engine. You can buy a lot of gas for your old motor for the price of a new motor so how bad is the mileage really? My priorities are a motor that starts and runs like it should everytime, then it needs to be setup to run well with the boat its on (right prop, trim, engine height) and after that I'll take whatever it gives me on mileage knowing that if I keep the throttle in the sweet spot when cruising the motor will do its best.
thanks for the response uncledeck. I agree with not modifying the motor and leave it stock, it probably would keep us from paddeling back. The motor runs like a top, I'll check the timing and look for other prop choices. The hull is clean and I have it trimmed out on the last notch. I agree too that I can buy alot of gas for the difference in price of a newer motor.
As a reference my 1966 33hp Evinrude would travel 25 miles on a full tank of fuel in 1977, while running 80 to 95% throttle.
Estimated from fuel usage on two identical trips. Distance estimated today using Google earth.
