Fuel burn with 5.7 LX mercruiser, 24' Boat

[AKJohne]

In 2018 I and a buddy finished a rebuild on a 1982 24' Glasply, works great and having fun with it, have managed to put on 400+ hours in 2 seasons between the two of us.
1997 5.7 LX 4 bbl. AI GEN II

I had to have some work done on the outdrive this winter and all is working now, It looked great on the outside when it was all together, but where the water passage are, corrosion has taken a toll. I think it still has some life left in it but will need replacement at some point in the future.

My question is, how much if any gains are to be made with a DUO-PROP or EFI?

Curious what others are getting for fuel usage on these size boats?
This boat weighs 5400 unladen and expect it runs 7-8K loaded with fuel, humans, etc.

Typically average 2.1- 2.2 MPG... The boat/engine likes to cruise at 3100-3300 RPM. Usually in the mid to high twenty's.
(It actually seems to be doing a little better since I had the outdrive gone through, but due to so much trolling on the last trip I wont know until we get some time on under normal conditions)

Increasing RPM/speed doesn't seem to affect mileage very much up to 4K. But slowing down sure does.
I am running a 15P 3 blade aluminum prop on a Alpha 1 Gen II, 350 merc with 4 barrel.

I have has several people tell me a duo-prop and fuel injection are more efficient, just wondering if its true....?

 

[achris]

Dual prop is LESS, fuel in injection is more efficient. A 2 blade prop is more efficient than 3. The more blades, the less efficient, generally.

Here's a graph I did a while back off fuel usage on my boat with different props.



I know it looks like the 19" Vengeance is missing, but it's not. It's behind the 19" Black Max. They performed identically.
The 'Vengeance' props are 3 blade stainless steel, the Alpha4 (4 blade) and Black Max (3 blade) are aluminium.

It should also be noted that the same rpm does not mean the same boat speed. This is purely consumption vs rpm...

Chris........

 

[Scott Danforth]

you will burn 0.4 # of fuel per HP per hour.

however as Chris mentioned, there are more parts in the duo prop, so there is more drag (inefficiency) that will be offset slightly by the potentially more efficient EFI motor.

however with your current setup, you are carbed and Alpha drive. you wont notice enough increase in efficiency in EFI to offset the cost of adding it to your existing motor.

as for your drive, if you change the Alpha drive to a Bravo, you will see a slight increase in fuel consumption.

 

[AKJohne]

thanks guys, I completely missed your replies...! I guess I didn't have notification turned on...! I dinged a blade on the 15.25 X 15 I have been running and felt like fuel burn suffered slightly. I had a 16 X 16 repitched and I think he went to far, I am either going to go back with a 16 X 16 or try a 15.8 X 15 Solas. As for EFI, I think I will stick with the carb, I just dont think it is work the effort, the boat starts and runs very well except for the transition phase from idle to 1000 RPM. It doesnt stumble or hesitant when on advances the throttle normally. I dont typically firewall the throttle when talking off, just a steady push forward) . Where its noticeable is trolling and tying to just bump it slightly to increase speed.. I doubt there is anything I can do as its a transition phase in the carb. I have adjusted the mixture screws to highest idle and they are maybe just over a turn out.

 

[QBhoy]

I’d guess there it would definitely not be worth going efi over your carb. With the cost of what your need to do it. Even going to mpi it wouldn’t be worth the cost of it either. The cost of going DP wouldn’t be worth it either.
mid you getting mid to late 20’s at 3000 rpm on a boat that size. You must be close to the most efficient set up like it ever perhaps. Especially with props like that. Haven’t ran the numbers, but I’d almost bet they would show ridiculous numbers at that. Sounds like good going to me !

 

[AKJohne]

I’d guess there it would definitely not be worth going efi over your carb. With the cost of what your need to do it. Even going to mpi it wouldn’t be worth the cost of it either. The cost of going DP wouldn’t be worth it either.
mid you getting mid to late 20’s at 3000 rpm on a boat that size. You must be close to the most efficient set up like it ever perhaps. Especially with props like that. Haven’t ran the numbers, but I’d almost bet they would show ridiculous numbers at that. Sounds like good going to me !

That's encouraging ...! I did order a 15.8 X 15 Solas aluminum prop and will see how it does. On my virtual fuel gauge (Paddlewheel sensor and NMEA 2000) I frequently see MPG numbers at 2.3 and even better, however when I do the calcs at fill up it is typically between 2.1 and 2.2. I use 2.0 for planning purposes. I am trying to learn about Weber carbs and wondering if anything can be done to smooth out the off-idle transition?

 

[Scott06]

That's encouraging ...! I did order a 15.8 X 15 Solas aluminum prop and will see how it does. On my virtual fuel gauge (Paddlewheel sensor and NMEA 2000) I frequently see MPG numbers at 2.3 and even better, however when I do the calcs at fill up it is typically between 2.1 and 2.2. I use 2.0 for planning purposes. I am trying to learn about Weber carbs and wondering if anything can be done to smooth out the off-idle transition?

Rebuild the carb Weber and currently made edlebrock marine carbs are easy to tune. when was the last time carb was gone through float level set etc. usually what you are describing is a problem with the idle transfer slots maybe plugged up. should transition smoothly no issue if working properly.

 

[AKJohne]

Rebuild the carb Weber and currently made edlebrock marine carbs are easy to tune. when was the last time carb was gone through float level set etc. usually what you are describing is a problem with the idle transfer slots maybe plugged up. should transition smoothly no issue if working properly.

long story on this project, motor and drive came out of a boat that overheated and sat on a pallet for many years, motor was rebuilt and then I needed up with it and out it in the 24' Glasply. I have never had the carb rebuilt, as mentioned it starts and runs great, just the off-idle and only a issue when trolling. I hated to mess with it as other than that no complaints, but thinking about what you said and it potentially sitting for years it could use a good cleaning. I have looked at crab kit form 20 bucks to 150 bucks, mostly include some gaskets needles and seats and some have new floats. I dont think the housing/shaft needs to be addressed but will look at that if I have it apart. Any suggestions on kits?

 

[Scott Danforth]

OEM kit about $40

 

[AKJohne]

I ordered a OEM kit form Crowley, for a actual OEM OVERHAUL KIT 8M0120193, prices varied form $91.00 to $130.00, Crowleys not the cheapest but 4 bucks shipping, sometime to Alaska we get dinged pretty good. Hoping this solves the off idle issue.

 

[Scott06]

I’ve had good luck with mikes carb shop in WA.
the kit isn’t going to fix it, properly cleaning the carb body all the passages is where most go wrong, make sure to soak it in cleaner blow all passages out and stick a thin wire through them. I would suspect your idle transition slots are gummed up based on what you describe

 

[AKJohne]

I’ve had good luck with mikes carb shop in WA.
the kit isn’t going to fix it, properly cleaning the carb body all the passages is where most go wrong, make sure to soak it in cleaner blow all passages out and stick a thin wire through them. I would suspect your idle transition slots are gummed up based on what you describe

The big thing with the kit is new gaskets, which I will need to reassemble. But A new needle and seat will be ok, after all I think everything is 25 years old.

 

[Scott06]

The big thing with the kit is new gaskets, which I will need to reassemble. But A new needle and seat will be ok, after all I think everything is 25 years old.

Yes obviously the gaskets and needle and seat are important, I was just trying to reinforce that proper cleaning of the carb is the hardest thing for a home rebuilder to get right. You see lots of posts where people rebuilt the carb and still have carb issues, done it myself...also worth mentioning is your ignition system in good working order?
good luck.

 

[AKJohne]

Yes obviously the gaskets and needle and seat are important, I was just trying to reinforce that proper cleaning of the carb is the hardest thing for a home rebuilder to get right. You see lots of posts where people rebuilt the carb and still have carb issues, done it myself...also worth mentioning is your ignition system in good working order?
good luck.

I have a gallon of carb cleaner and will soak the parts overnight, I understand the importance of cleaning and not just throwing in new parts, its a valid point. Thanks.
Wondering about repainting... I do have some alodine and aluminum cleaner... it may be more trouble than its worth.

 

[Scott Danforth]

Make sure to chase every passage with a piece of monofilament or fine copper wire

The key to every carb rebuild is to verify the passages are clean

 

[AKJohne]

Carb rebuild done, kit arrived, pretty basic, new needles/seats, new accelerator pump and gaskets. I did a visual inspection and took plenty of pictures before. Then cleaned it with a electrostatic cleaner with the heat cranked up using NAPA (Berriman) carb cleaner. It did a excellent job and of course removed the paint. Then rinse with hot soapy water and blew out all the passages.

 

[AKJohne]

Then I flattened and surfaced the air horn and body to manifold using a piece of quartz countertop and some lapping compound. It did a excellent job of cleaning up the mating surfaces. Once done clean with brake clean and compressed air.

 

[AKJohne]

Throttle shafts were nice and snug in the body, so no need to remove them.
Floats were air tight, needle rods had what looked like some wear/corrosion near the tip. That's when I discovered merc used 3 step rods at some point. No longer variable and Mikes Carb did not have any advice on what exactly to use as a replacement if switching to a 2 step rod. Apparently the primaries jets are specific to 3 step rods as well. I wanted to replace them and was pondering what to do and found a NOS pair on ebay with new green springs. Done.... The new ones did mic slightly larger so I am glad I was able to find them. The primary and secondary jest were the correct size and since I could not get a new jet to match the three step rod I reused them.

 

[AKJohne]

Next I assembled it and taped it up so I could bead blast it. Then more cleaning with hot soapy water, compressed air, hot water, air and finally a good visual to make sure passages are were clear. Then final reassembly, tape if off again, clean with acetone, warm in the oven, spray with self etching primer and then paint, then back in the oven at 170, once in the oven I turned the heat off and just let it cool.

 

[AKJohne]

Then I installed the rest of the linkage, adjusted the accelerator pump and put the carb back on the motor, I cranked it a few times to fill the bowls and let is sit over night so I could check for leaks before I fired it up. The idle needles looked ok, but I cleaned them up with some 600 to remove any irregularities. Ste the choke back where it was as always worked fine. Started right up today, choke was good, completely off choke in about 3 and 1/2 minutes. Screwed in the idle and let it warm, grounded the dist and checked timing and set idle to 650, adjusted the mixture screws and I think its ready to go. I have a trip planned in April for winter kings, so will see if this made a improvement to the off idle throttle settings. I have a hunch its not gonna matter much... but am hoping, I just think that difficult transition spot for a carb. And only a issue when trying to increase speed in incrementally small amounts for trolling, other wise never was a issue. I do wonder if a 750 CFM carb is the right carb for a 350...? Its what came on it but I think it would have to turn 9K RPM before it would need a carb that big, I think a 600 would be more appropriate, however maybe it would be into the secondary's more...? If I get a chance I will check and see where the secondary's are when cruising at 3200. In any case I will report back... Thanks for listening...!