i have a 1972 50 hp johnson.  it seems like i get less then 1 hour run time on 5 gallons of gas.  any idea if thats the norm?  is there anything i can do to improve on this?  if not i will have to get a bigger tank for my boat to run a long day of fishing on a larger lake then i normally fish.  thanks in advance for any help

How big/heavy is your boat? How full? Is that hour at full throttle? There's no question that those old motors burn a LOT of gas, but 5 gals. an hour is an awful lot of gas, unless the boat is way underpowered. Another factor can be waterlogging. If it's an old fiberglass boat with foam below the sole, the foam could be waterlogged...at about 8 lbs. per gallon, water weight is significant, and the old foams were like sponges.  Is it well-tuned? Are the carbs clean, fuel lines tight, gaskets good, etc. ....

That's a rather old motor, no where near as fuel efficient as later models.  Then you have to figure the rings, pistons and cylinders are gonna have enough wear the loss of power caused by blow by is going to be rather significant.  Then the carbon build up in the exhaust system, etc.  All this adds up to loss of power and efficiency.

With all that said, I would love it if I could run wide open for 20 minutes on five gallons.

yea ben is right carbon is  probably a big factor on that old of a motor....the carbon will absorb fuel

it is all fibor glass no bottom foam on the hull.  That would be a mix of 3/4 throttle to idle. can't run full throttle or it gets air gaps in the proppeller area.  its a 14 foot boot 4 feet are inclosed for the bow rear 2 feet are for gas tank and battery so i don't think its weight but i'm sure it hasn't had its carb cleaned.  spark plugs look newish. thanks again

If I were you I would get the motor serviced if you haven't had it serviced lately, you won't be able to a whole lot better gas mileage than you already have with a motor that old. But the tune up should help a tad. I've got a 98 yamaha 15 and I only carry a 5gal tank with me no matter which lake i'm going to. I run all day with that and usually only use about half of the tank if that. On the other hand, my 85 mercury will guzzle a 14gal tank in around 6 hours and its a two-stroke.

FWIW the rule of thumb on a two cycle outboard with a carburetor is.

It will burn 1/10 of its HP in fuel in an hour at full throttle.

Paul