Excessive fuel consumption by Merc 500?

[badllama]

I finally got my recently purchased 1965 16' Starcraft onto the lake. It's powered by a mid-70's Mercury 500 outboard (2 stroke, 4 cylinder, 50 horse). It starts easily and idles well, smoothly advances to WOT and runs well at speed (about 20 MPH with two full 6 gallon fuel tanks, fishing gear and two adults) though it seems to use quite a bit of fuel.

Since this is my first outboard I don't really have anything to compare against. I've owned 2 stroke dirt bikes in the past that would run a few hours on 2.5 gallons of pre-mix. This Mercury will drink down a six gallon tank in about 45 minutes or so at WOT. Is this typical of a motor this size pushing a boat this size? My last boat was a 20' Bayliner with a 4 cylinder Volvo four stroke and I could cruise for hours on 10 or 15 gallons.

Is this Merc's fuel appetite typical or should I be looking for better fuel efficiency? As it is I'll probably have to get at least one more tank for a day on the water, maybe more if I'll be pulling a tube or skier.

Any opinions?

 

[trendsetter240]

Re: Excessive fuel consumption by Merc 500?

I finally got my recently purchased 1965 16' Starcraft onto the lake. It's powered by a mid-70's Mercury 500 outboard (2 stroke, 4 cylinder, 50 horse). It starts easily and idles well, smoothly advances to WOT and runs well at speed (about 20 MPH with two full 6 gallon fuel tanks, fishing gear and two adults) though it seems to use quite a bit of fuel.

Since this is my first outboard I don't really have anything to compare against. I've owned 2 stroke dirt bikes in the past that would run a few hours on 2.5 gallons of pre-mix. This Mercury will drink down a six gallon tank in about 45 minutes or so at WOT. Is this typical of a motor this size pushing a boat this size? My last boat was a 20' Bayliner with a 4 cylinder Volvo four stroke and I could cruise for hours on 10 or 15 gallons.

Is this Merc's fuel appetite typical or should I be looking for better fuel efficiency? As it is I'll probably have to get at least one more tank for a day on the water, maybe more if I'll be pulling a tube or skier.

Any opinions?

Typical fuel consumption for a 50HP 2-stroke is 5 gallons per hour at WOT. You will see some variance in fuel economy based upon the engine design but not much. Most of the fuel economy improvements have been at cruising speeds and below.

Your 50hp is a crossflow scavenged 2-stroke which are typically the worst for fuel consumption. I had a 4-cyl 50hp merc and it was a pig on gas.

What RPM do you get at WOT? It could be that you have the wrong prop for your setup.

Also 20mph seems a little slow, I would expect at least 25 out of that setup.

Cheers

 

[badllama]

Re: Excessive fuel consumption by Merc 500?

I thank you for your response. It's very helpful to have access to the knowledge of experienced boaters!

Getting the tach working is the next on my list of stuff to do, so I'm afraid I can't tell you right now what RPM is at WOT. Though I have a sense that the engine isn't quite pushing like it could.

Perhaps I need to adjust the throttle cable to get a bit more travel out of the carb linkage? Engine doesn't accellerate 'till I'm well past the point when it engages forward gear.

I'm sure having the tach working will help pinpoint things better. Any idea what WOT should be spinning at and what the redline is on this old Merc?

I appreciate and value your information!