I have a 1973 85 HP Johnson motor pushing a 1981 16 ft ranger boat. We have had the boat & motor only a short time, so do not have good reference points on how this engine performs.
The engine appears to be consuming a great quantity of fuel...perhaps 1 gallon per mile...
Question..when we trim out the motor to get the boat to plane, we have the motor tipped down quite a bit (cylinders are lower than the carburetor)...would this cause fuel to pass right through the carburetor float into the cylinders...causing this large consumption of fuel?
The motor is a bit difficult to start, but runs fine once started. The spark plugs are "wet" and have some carbon on them.
I have checked the compression, and three cylinders are about 110, and #4 is 92...a bit low on this cylinder...
as a rule of thumb, outboards woll burn 10% of the HP of the engine per hour at WOT. or in your case 8.5 gallons per hour. you can decrease your fuel consumption by backing off on the throttle after you get on plane. are you running at WOT when you are measuring this?
I agree with ssmyth. I have a 70hp 1996 Evinrude on a 18ft boat. I have loaded the boat with a lot of junk (my inflatable dinghy, lots of stuff in the cabin, etc..) and it consumes between 1ltr/mile to about 0.6ltr/mile if I drive carefully.
Rapid change in the throttle and RPM increases your fuel consumption as fuel will be sprayed into the cylinder without actually burning. Also, in my case as I use this engine on tidal thames (in London-UK) it very much depends on the tide direction as you imagine; going with the tide=lower consumption and vv.
I was asking a friend of mine who owns a Mariner 90hp on a Humber 24ft Rib. He also complains about the high fuel consumption. 2 stroke engines all have this problem!
What you can manage - as the others have said- is not what your engine burns at wide open (regardless of load or conditions, that won't change), but where your most efficient sweet spot is, somewhere between 70-90% of WOT, probably. You'll have to figure or guess where you're getting the maximum distance per fuel burned (MPG) and that may indeed change depending on load and conditions. For example, it might be 70% for a lightly loaded boat or favorable wind or tide because it only takes 70% to get it onto an efficient plane. Other conditions might require more oomph to get it up to a good planing speed, so maybe 80 or 90% or even closer to WOT might be the sweet spot that day. It won't be as good as the day that you can use 70%, but it will be the best you can get under the current conditions. Hope that makes sense.
My boat tends to get the best MPG at 80 +/- % I can plane it lower, but the speed improves quite a bit with just a little more throttle, thus the MPG improves.