How much outboard motors have improved.

[airshot]

I recently purchased a 1992 Mercury 40 hp 2 stroke motor and boat. Just recently downsized from my 22' as it was getting a bit much for this 73 yr old to get on and off the trailer. With allbthe kids grown and living elsewhere, I find myself going it alone much of the time. My last OB was a 1979 Johnson 70 hp three cylinder, and it was an excellent motor for me. That motor used a 6 gallon tank of fuel in about 45-50 minutes when running hard. I was expecting about an hour and a half of run time on this new to me 40. Finally had a chance to monitor fuel comsumption on this Merc. I was pleasantly surprised to find that it only used 3 gallons of 89 octane fuel in over 4 hours of run time, most of that 4 plus hrs was hard running at 4200-4800 rpm. While not new, it is a big improvement over the 70/80's era of OB's. This is still a carb version 4 cylinder model, and to me this fuel economy is excellent. Anyone considering one of these, go for it, definetly easy on gas !!

 

[flyingscott]

You do realize how old that 4 cylinder design is don't you? And hard to believe it only used 3 gallons of gas in 4hrs of hard running. If your top Rpm at full throttle is only 4700-4800 rpm you might need a different prop.

 

[jimmbo]

I believe the WOT Range for that engine would be 5000 -5500, so if you are getting 4800, too much Pitch
Just a FYI, that 44 in block goes back to the late 50s

 

[flyingscott]

Seriously nobody is going to comment on the 3 gallons of gas in 4 hrs?

 

[airshot]

You do realize how old that 4 cylinder design is don't you? And hard to believe it only used 3 gallons of gas in 4hrs of hard running. If your top Rpm at full throttle is only 4700-4800 rpm you might need a different prop.

No, that is not wot...just cruising speed. Never keep them at wot for long periods of time. Just cant believe how much better on fuel economy !! And it idles right down to a crawl....

 

[jimmbo]

Seriously nobody is going to comment on the 3 gallons of gas in 4 hrs?

I assumed the engine was idling...

 

[airshot]

I assumed the engine was idling...

No, really, I topped off the tank before going out, gas gage didnt hardly move, so I stopped and topped it off again thinking the fuel gage was off. It took 3.1 gallons, I was shocked, tried putting in more thinking it was an air bubble but just ended up with fuel all over my well. Had to idle across the no wake zone out and back for a total of 1 mile each way, other wise up on plane 4200 up to 4800 rpm. I will be checking this again, but was quite surprised. !!

 

[flyingscott]

So that crossflow 2 strk designed in the 50s gets better fuel mileage than a new 4 4strk?

 

[airshot]

So that crossflow 2 strk designed in the 50s gets better fuel mileage than a new 4 4strk?

I really dont know gow to explain it. Just purchased this outfit used of course, the orevioys owner said he was amazed at how good it was on fuel. I though, yea sure, but...will be doing some additional testing. Been around boats for more than 50 years, I know something must be off, but not sure where. It is 15 miles from my launch ramp to Toledo lighthouse according to my gps, been out and back twice, plus a bunch of other running around over a few days time. So time well tell what is going on...

 

[flashback]

Maybe someone installed one of those oldie carbs from jc Whitney that claim 200 miles a gallon! LOL

 

[QBhoy]

Is this from the old 4 cylinder classic 40/45/50 motor or the 3 cylinder 40 that would be more common in 1992 ? Sounds very economical. The best and most efficient of modern engines around that size will likely use 4 gallons or more an hour at full throttle.

 

[airshot]

Is this from the old 4 cylinder classic 40/45/50 motor or the 3 cylinder 40 that would be more common in 1992 ? Sounds very economical. The best and most efficient of modern engines around that size will likely use 4 gallons or more an hour at full throttle.

It is the 4 cylinder, 2 stroke oil injected classic model. From all I have read most claim 2-3 gallons per hour at cruising speed and about 4 gal per hour at wot. I have not been running at wot except for a few short bursts. I have no idea just yet what the optimum fuel efficiency rpm will be. It seems to like 4200-4600 rpm and keeps on plane nicely. My old 3 cylinder 70 hp burned 6 gallons an hour or a splash more. That would drop down to 4.5-5 gph at cruising speed. Will be doing more testing with this next tank of fuel....

 

[JimS123]

Yes, how much they have improved!

I have a place I go fishing and the route is always the same. Part ways no wake zone and part ways open river. Over the last 40 years, here's a rough comparison:

2 Strokes:
1952 Big Twin 25 HP - 7 gallons
1974 'Rude 18 HP - 6 gallons
1970 9.5 Rude - 5 gallons
2009 Yamaha 25 HP - 4 gallons
(all on the same boat)

4 Strokes:
2017 Merc 60 HP - 3 gallons
2019 Merc 150 - 4 gallons

If you don't have a performance curve to determine optimum cruising speed (best fuel economy) run at about 70% +- of max rpm. That's assuming that the prop will provide near full rpm spec.

 

[airshot]

While I continue to test, I am beginning to think I had an airlock in the tank/ filler neck that prevented me from getting more fuel in the tank. I have a piece of plastic pipe that fits down the fill neck into the tank under the floor. After filling the tank and letting it set for a day, it showed much lower on my tube. Further testing will let me know if this is true. .....

 

[jimmbo]

The tank vent should prevent that

 

[airshot]

The tank vent should prevent that

You are correct, but I need to prove it first. Dont have any other reasons as to why the outlandish fuel economy...checked the outer vent outside the hull, I can blow thru it, so not clogged.

 

[jimmbo]

Doesn't mean/prove it is connected to the tank

 

[airshot]

Doesn't mean/prove it is connected to the tank

Your right, that is my next project, remove the floor panel and see if all is good...

 

[airshot]

Thought I would post an update to my OB with awesome fuel economy...just filled the tank again and it only took 5 gallons for almost 5 hrs of run time. Seemsvto goodvto be true I know....checked the fuel level in the fill pipe and it was full, parked it in the barn overnight and checked the fill tube again...yes it dropped down considerably. Apparently it is trapping an air bubble that keeps me from filling the tank completely unless I go slow as possible. I was able to add another gallon plus of fuel and checked it this morning and it still shows full. I appear to have figgered this out, but..in all honesty, I am still impressed with the performance and fuel economy of this motor !! I owned a 2 cylinder 40 hp 2 stroke Merc some years back that was dissapointing in performance, I replaced it with a 40 hp Jonnyrude which performed better, much better. Fuel economy was as advertised around 4-5 gallons per hour running fairly hard. I was skeptical about this current model, based on my past history, but after several recomendations bought it. Very impressed with this motor in both performance and fuel economy. Neighbor has similiar 16' SC as mine but lighter in weight and he has a 50 hp Honda 4 stroke. Yes his fuel economy is better but not as much as I thought it would be. His is about 30% better as near as we can figure. However, mine will outperform his by 5 mph and my boat is about 300 lbs heavier than his. I understand the technology in this 1992 motor is supposed to be from the 50-60's, but if so, I will take it anyday... According to my local marine mechanic, these motors have far less problems than the newer models with all the electrical gizmos

 

[flyingscott]

Thought I would post an update to my OB with awesome fuel economy...just filled the tank again and it only took 5 gallons for almost 5 hrs of run time. Seemsvto goodvto be true I know....checked the fuel level in the fill pipe and it was full, parked it in the barn overnight and checked the fill tube again...yes it dropped down considerably. Apparently it is trapping an air bubble that keeps me from filling the tank completely unless I go slow as possible. I was able to add another gallon plus of fuel and checked it this morning and it still shows full. I appear to have figgered this out, but..in all honesty, I am still impressed with the performance and fuel economy of this motor !! I owned a 2 cylinder 40 hp 2 stroke Merc some years back that was dissapointing in performance, I replaced it with a 40 hp Jonnyrude which performed better, much better. Fuel economy was as advertised around 4-5 gallons per hour running fairly hard. I was skeptical about this current model, based on my past history, but after several recomendations bought it. Very impressed with this motor in both performance and fuel economy. Neighbor has similiar 16' SC as mine but lighter in weight and he has a 50 hp Honda 4 stroke. Yes his fuel economy is better but not as much as I thought it would be. His is about 30% better as near as we can figure. However, mine will outperform his by 5 mph and my boat is about 300 lbs heavier than his. I understand the technology in this 1992 motor is supposed to be from the 50-60's, but if so, I will take it anyday... According to my local marine mechanic, these motors have far less problems than the newer models with all the electrical gizmos

Maybe your motor is from an alternate dimension where physics and combustion technology don't apply? Was the salesman hinky looking at all?