Increased gas consumption

[kazankota]

Here's a question that I can't seem to figure out. My motor is a 1987 Mercury 70 hp outboard. I recently had the carbs and fuel pump rebuilt so the motor has been running pretty well although I feel like I burn a lot of gas. I was out the other day (Bay and open ocean) and used about 5 gallons in a time span of 45 minutes. During this time I spent probably 10 minutes at most at WOT and the rest no wake and cruising. I realize that this is a vague question but what accounts for increased gas consumption that I may be able to look into, and how much difference do adjustments make to the low power lean/rich carb adjustment screws.

 

[Silvertip]

Re: Increased gas consumption

Unless you have a fuel flow monitor on your boat and had previously recorded fuel consumption data you have nothing to compare it to other than what you said. That my friend is not a good measure of fuel economy. Mixture adjustments affect idle and high speed jets are fixed so there is no way either of those would significantly affect fuel economy. Now then -- a 70 HP engine at wide open throttle will burn about 7 gallons of fuel per hour. I know this for a fact because I have a fuel flow monitor and a 75 HP Merc. It is simple physics (actually not so simple) but an outboard burns fuel at 10% of its rated HP. 150 HP + 15 gph, etc. So, you ran 10 minutes at WOT (let's say 15 minutes) so thats 1/4 of an hour. 7 GPH equates to 1.75 gallons in those 15 minutes. At cruise that engine will burn about 4.5 GPH. So in 30 minutes you burned 2.25 gallons. By golly 1.75 + 2.25 = 4 gph which is pretty close to 5 gallons. Since all of your numbers were very arbitrary, I'd say your fuel use is perfectly normal.

 

[edjunior]

Re: Increased gas consumption

Dang, that's pretty good info. I though mine burned a lot too, but seeing this, it's just about right. Sure am glad I went out and got another tank! Now I can stay out a bit longer. Woo Hoo!!!