Is there a better MPG carb for Mercruiser 120?

[woodrat]

I have a 72 bellboy with a 120 mercruiser in it. It has a 2 barrell carb. It runs good and is so far very reliable, but I would sure like it to get better mileage! I went out in the ocean today and fished and ran 36 miles and used 11 gallons of gas. I know that's not terrible for a boat, but I sure would like it to be better.

Any ideas? Is there a progressive carb that would be suited for this application?

 

[Don S]

Re: Is there a better MPG carb for Mercruiser 120?

The carb was designed for your engine, but it may be wore out. Loose throttle plate shafts, worn passages. Perhaps a new one will help........ a little. Other than that, yearly tuneups at the beginning of the season and in general making sure everything on the engine works up to specs. will all help your MPG.

There is one other thing that may help.
One for each person on the boat. Choice of colors available on request

 

[DJ]

Re: Is there a better MPG carb for Mercruiser 120?

You won't find an engine that is much stingier than that one.

If you have it overpropped, you are lugging it and it will use more fuel.

What is your wide open throttle RPM?

Is the choke opening all the way?

 

[woodrat]

Re: Is there a better MPG carb for Mercruiser 120?

WOT rpm is about 4500. I believe the choke is opening all the way, but i will look at it again tomorrow. It doesn't sound or smell like it's running rich.

 

[Tail_Gunner]

Re: Is there a better MPG carb for Mercruiser 120?

The carb was designed for your engine, but it may be wore out. Loose throttle plate shafts, worn passages. Perhaps a new one will help........ a little. Other than that, yearly tuneups at the beginning of the season and in general making sure everything on the engine works up to specs. will all help your MPG.

There is one other thing that may help.
One for each person on the boat. Choice of colors available on request

No tail pipes I'm out.

 

[lilmandavis]

Re: Is there a better MPG carb for Mercruiser 120?

man hes complaining about 11 gals in 70 some miles? i burn 30 in an hour!!

 

[woodrat]

Re: Is there a better MPG carb for Mercruiser 120?

No, I burned 11 gallons in 36 miles. Which I know isn't bad, but I would like it to be better, since I'm not made of cash.

 

[QC]

Re: Is there a better MPG carb for Mercruiser 120?

woodrat,

What RPM and speed were you cruising at? If you're looking strictly for fuel economy I think you can make way more of a difference with how you operate her than playing with stuff unless it's broked.

Whatever you do I don't think you will exceed 4 MPG and you got 3.2 MPG. That difference would save you $6 per trip . . .

 

[bamadave]

Re: Is there a better MPG carb for Mercruiser 120?

For what it's worth, I have a Chaparral Sunnesta 232 with a 5.0 litre with an alpha 1 gen 2 drive. At around 3200 rpm, I get 3 MPG or $1.03 per mile at the moment.

 

[woodrat]

Re: Is there a better MPG carb for Mercruiser 120?

It will hit 23 knots at WOT on flat water. I was cruising mostly at 17-20 knots at hitting about 4000 rpm or so at 19-20kts.

 

[Boatist]

Re: Is there a better MPG carb for Mercruiser 120?

For best fuel mileage you want to run at a speed where you just came up on a clean plane. If your engine WOT RPM range is 4200 to 4600 and your turning 4500 then you could gain a little mileage by going up 1 or 2 inch in prop pitch.

I would expect that engine to get it best fuel mileage at 3000 to 3200 rpms but only if the boat is on a clean plane at that RPM.

How big is the boat, how much does it weigh and what type hull is it.

My boat is a 21 foot runabout closed bow with a V hull about 20 degrees dead rise and 2800 pounds dry. I have a 140 hp engine. 130 by the way they messure today.
I have two 3 blade stainless steel props both the same basic design.
The 17 inch pitch has a top speed of 35 MPH at 4600 RPMS.
The 19 inch pitch has a top speed of 38 MPH at 4200 RPMS.

When we go out to my favorite fishing spot which is 26 miles offshore the Loran C, GPS and Depthfinder will all indicate we traveled about 60 miles in the day. Now some of that distance is with the motor off and drifting. We have made this trip many times and almost allway the same mileage. Since my boat is pretty small so we only go on nice day with seas less than 5 feet. We run at 3000 RPMS and about 20 miles per hour. My boats natural planeing speed is 18 MPH with out the use of trim tabs. On the way back often run slower as the wind and seas have picked up. I adjust the trim tabs to keep us on plane unless it really gets bad.
I tried both props many times and when going to Cordell Banks always start with a full 34 gallon tank and refill when we get back for the next day.
With the 17 pitch prop I will use18 gallons of fuel over the 60 miles.
With the 19 pitch prop I will use 15 gallon of fuel over the 60 miles.
Swells on the way back will always be on the stern port quarter.

Anyway the 19 pitch prop will save me 3 gallons of fuel over the 17 pitch prop.

 

[QC]

Re: Is there a better MPG carb for Mercruiser 120?

Killer post Boatist!!! A coupla things, the 19 inch pitch gets better fuel economy because the throttle is more wide open. I know this sounds wrong to some, but it is why they run low numerical ratios in cars these days. Same hp (speed), lower RPM, wider throttle, better fuel economy.

The other bigee is your trim tab comment. "I adjust the trim tabs to keep us on plane unless it really gets bad."
This is why those of us with adjustable tabs keep harping on them. We can adjust the tabs for the conditions. Huge feature!

 

[bruceb58]

Re: Is there a better MPG carb for Mercruiser 120?

I have a 7.4L Carb Volvo DuoProp on a 24' boat. The RPM that I run at make a huge difference in MPG. I have a GPS sounder that has a fuel flow meter integrated into it to give me instantaneous MPG. I find my MPG at 3500 RPM is 1.7 and at 3000 is 2.3. That is a 25% improvement for 500 RPM.

I believe that a lot of that is due to my engine developing peak torque at a fairly low RPM. I see an even larger fuel usage improvement at even lower RPMs but don't like to cruise that slow.

Your 4cyl engine probably develops its peak torque at a higher RPM than mine. The closer you get to that RPM number, will increase your mileage.

 

[QC]

Re: Is there a better MPG carb for Mercruiser 120?

I believe that a lot of that is due to my engine developing peak torque at a fairly low RPM.

Fuel economy in a boat is mostly about hull efficiency, definitely not peak torque. Marine engines are rarely, if ever, at peak torque. When you're boat is up on plane, the only time it is at a full lug (max torque for that RPM) is at WOT. I know this is confusing and opposed to what we are all told, but it is a fact. Peak torque simply does not come into play with marine engines except if they won't or barely plane. The only way that you could be at peak torque at 3000 RPM is if you also had the throttle wide open. I know you said peak torque RPM, not necessarily peak torque, but trust me, it is about the hull, not the engine.

Almost all I/O boats are most efficient around 3000 -3500 RPM, the fastest ones are usually best around 2500. This is because that RPM corresponds to the speed just when it goes onto an efficient plane. As you noted, they are even more efficient at idle, but who wants to go that slow. This also proves how much hull efficiency has to do with it, because spark ignited, throttled engines are the least efficient at idle . . .

 

[bruceb58]

Re: Is there a better MPG carb for Mercruiser 120?

QC,

I see what you are saying and generally agree with you.

What explains identical hulls with different size engines with the larger displacement engine getting the better fuel economy? Generally, it is the larger engine that can generate the same anount of torque at the lower RPM which also tends to be closer to where it's peak torque is generated hence where it is most efficient.

 

[woodrat]

Re: Is there a better MPG carb for Mercruiser 120?

Boatist:

Thanks, that is a really helpful response. My boat is a 19' fiberglass bellboy, closed bow runabout, built in 72. I'm embarassed to say that I have no idea what it weighs. I haven't taken it across the scales and I can't find any original specs for it. I am in the process actually of shopping for a new prop anyway, since my bro banged up mine a couple of days ago trying to manouever next to the dock in a side wind with current added. So I do need to study prop sizing and try to find a new one; your post is very helpful. Is there a chart or guide to general prop sizing?

I should state that some of my miles travelled are trolling with a yamaha 9.9 kicker, and that is going to give me all kinds of false information about the Mercruiser motor itself. Tomorrow we are going upriver and will just run in and tie up to fish, so I will get a better read on just the I/O part of the mileage picture.

 

[QC]

Re: Is there a better MPG carb for Mercruiser 120?

You're real close bruce, especially with the lower RPM comment. This translates to a bigger prop, which is more load, which is a wider open throttle too. But the fact is that the smaller engine is actually more efficient in your example if it is setup correctly. Check out this test:

Small block vs. Big block

It also notes that the big block is quicker to plane which is where the higher peak torque rating shows up. In every other category, the small block wins out.

 

[bruceb58]

Re: Is there a better MPG carb for Mercruiser 120?

Too bad they didn't test the MAG 7.4 vs the MAG 5.7 in that test. They used the most powerful 5.7 vs the least powerful 7.4. Also, they don't quote any pewrformance tables which shows RPM vs Speed and MPG which they usually do when they run those tests.

 

[QC]

Re: Is there a better MPG carb for Mercruiser 120?

The 300 vs. the 310 is close to valid from a bhp standpoint (advantage to the big-block). With that said, None of these tests are valid unless they are performed under controlled conditions. That data is sometimes available off the individual spec sheets. The test cell data cannot account for the weight benefits of the small block though. That alone explains some of the fuel economy advantage; the higher compression muddies the water even further. IMHO the wider open throttle is the reason for the majority of the benefit though.

 

[bigbrownbuku]

Re: Is there a better MPG carb for Mercruiser 120?

I have a 7.4L Carb Volvo DuoProp on a 24' boat. The RPM that I run at make a huge difference in MPG. I have a GPS sounder that has a fuel flow meter integrated into it to give me instantaneous MPG. I find my MPG at 3500 RPM is 1.7 and at 3000 is 2.3. That is a 25% improvement for 500 RPM.

I believe that a lot of that is due to my engine developing peak torque at a fairly low RPM. I see an even larger fuel usage improvement at even lower RPMs but don't like to cruise that slow.

Your 4cyl engine probably develops its peak torque at a higher RPM than mine. The closer you get to that RPM number, will increase your mileage.

thats because your vacuum secondaries open right around 3k rpm. if you have ever looked down the throat of a carb when this happens youd buy shares in oil.