Fuel burn with 5.7 LX mercruiser, 24' Boat

[Scott Danforth]

the beauty of the carb is it will flow only what the motor needs. on top of that, keep in mind the 4.3's get 600-650 CFM carbs, the 5.0's get 650-700 CFM carbs, the 5.7's get 700-750 CFM carbs and the 7.4's and 8.2's got 800-1050 CFM carbs

your fine on the carb

 

[QBhoy]

the beauty of the carb is it will flow only what the motor needs. on top of that, keep in mind the 4.3's get 600-650 CFM carbs, the 5.0's get 650-700 CFM carbs, the 5.7's get 700-750 CFM carbs and the 7.4's and 8.2's got 800-1050 CFM carbs

your fine on the carb

Scott. Just thought I’d ask. I’ve a friend who is building a 383 stroker (not a common thing here). What the min carb he would need on that ? You reckon ? It’s getting built for around or just shy of 400 hp, he reckons. Think the cam is for 5800 rpm or so. Internals all built too.
cheers Scott.

 

[Bondo]

Scott. Just thought I’d ask. I’ve a friend who is building a 383 stroker (not a common thing here). What the min carb he would need on that ? You reckon ? It’s getting built for around or just shy of 400 hp, he reckons. Think the cam is for 5800 rpm or so. Internals all built too.
cheers Scott.

Ayuh,..... He could go with the 1409, 'n fatten it up abit, or the 1410,.....

Run the numbers to be sure,..... spinnin' 5800 rpms is whinin' it pretty tight,....
Alpha drives ain't gonna like it,....

 

[QBhoy]

Ayuh,..... He could go with the 1409, 'n fatten it up abit, or the 1410,.....

Run the numbers to be sure,..... spinnin' 5800 rpms is whinin' it pretty tight,....
Alpha drives ain't gonna like it,....

Cheers bondo. It’s a bravo 3 he has on her. Appreciate the advice. Brilliant.

 

[Scott06]

Scott. Just thought I’d ask. I’ve a friend who is building a 383 stroker (not a common thing here). What the min carb he would need on that ? You reckon ? It’s getting built for around or just shy of 400 hp, he reckons. Think the cam is for 5800 rpm or so. Internals all built too.
cheers Scott.

On a 383 spinning to 5800 and warmed over id do the 750 Cfm 1410

the OP with a stock 5.7 600 cfm/1409 is fine. I found better results with my idle transition on my 1409 fattening up the idle mix a bit. Best idle was fairly lean. i did goto a bigger accelerator pump nozzle that cured a bog in WOT holeshot. Id read your plugs and see how they look at the rpm you have an issue with. Run it for like 10 min shut it off and pull a few plugs see how they look. It may go away with the rebuild if you had an idle transition slot clogged etc.

what rpm r u having stumble with ? 1200 or so?

 

[AKJohne]

On a 383 spinning to 5800 and warmed over id do the 750 Cfm 1410

the OP with a stock 5.7 600 cfm/1409 is fine. I found better results with my idle transition on my 1409 fattening up the idle mix a bit. Best idle was fairly lean. i did goto a bigger accelerator pump nozzle that cured a bog in WOT holeshot. Id read your plugs and see how they look at the rpm you have an issue with. Run it for like 10 min shut it off and pull a few plugs see how they look. It may go away with the rebuild if you had an idle transition slot clogged etc.

what rpm r u having stumble with ? 1200 or so?

Idle at 650 and increasing RPM to under 1000 RPM is the difficult spot to move just enough to change speed, currents are pretty viscous in lower cook inlet and typically try to fish slack but sometimes we keep fishing through a tide change and often adjusting speed slightly, or trying to, especially of your not catching anything. Usually trolling around 700/800 RPM, I end up bumping the binnacle with my palm to make small moves on the throttle.. Binnacle and cables were all new two years ago, so no issue with things moving, just the fact that the change needs to be so small. I could troll with the kicker and may do more of that this next trip.

The carb was in really pretty good shape, float bowls were clean, it was good to go thru it, but I would be very surprised if it changed. The motor has been rebuilt and ~ 280/300 HP, from the online calculator's I have looked at that motor would need to turn around 7K to exceed 6000 CFM. I am propped for ~ 4600 and was wondering if a smaller carb might cure this issue and be more efficient. Not in a hurry to go speed $500 bucks... will see how this rebuild works out it takes gas to make HP, so in some ways if it runs right sits gonna burn so much a hour no matter what. !

 

[AKJohne]

Finally put a few hours on the carb rebuild last weekend, ran a few hundred miles and hours of trolling. I would say it is/was a success as the csrb seems to handle incremental throttle inputs nicely and can raise the rpm a small amount at a time. Suspect the new needles are helping with that. Starting procedure is slightly different as i thought i flooded it only to find out it needed more fuel. Temps were in the 30s, thinking I need a hair more choke initially. Figured I should report back. I had some other issue's will ask about on another thread.

 

[craig newton]

If that is the 260 hp engine and all stock at 3000 rpm it burns 8.7 gallons per hour. At 3500 rpm it burns 9.8 gallons per hour. It burns 21.0 gallons per hour at WOT which is approx 4800 rpm. https://www.boat-fuel-economy.com/outboard-fuel-consumption-us-gallons

 

[AKJohne]

If that is the 260 hp engine and all stock at 3000 rpm it burns 8.7 gallons per hour. At 3500 rpm it burns 9.8 gallons per hour. It burns 21.0 gallons per hour at WOT which is approx 4800 rpm. https://www.boat-fuel-economy.com/outboard-fuel-consumption-us-gallons

Closer to 300 HP, but those numbers are pretty close. conditions day to day can make a big difference on fuel use. Have not seen 4800, 4600 but not 4800 and GPH is not 21 GPH. Typically average 2.2 MPG with the boat. Cruising 3200/3800. Interestingly enough more RPM/MPH does not seem to greatly affect the MPG. Going slow sure does!

 

[AKJohne]

I put some more hours on it last weekend, weather was decent so I got the mixture screws and idle a little tighter. Was actually sunny and good water conditions so I had a chance to see what RPM's the secondary's came in at. At 4200 that start flowing fuel, the active throttle plate opens slightly before there is enough air flow to create a vacuum for fuel to flow. Since it was nice we decided to check for WOT, we saw 4850/4900 RPM before we shut it down. Right at 39.3 MPH according to Mr. Garmin, it was still pulling and it wasn't completely trimmed out, i think we could have seen 40. The real surprise was the RPM as I had changed to a slightly larger prop, I went from a 15.25 X 15 to a 15.8 X 15. I was expecting 4600 not 4850.... I know every prop is a little different.......... However there was one other change that I made and I wonder if it made a difference? Last winter when I had it inside and doing the service I noticed the exhaust bellows had come loose form the outdrive end. I contemplated redoing this and decided to just trim the end of the bellows slightly so It wouldn't buckle when the drive was all the way down/forward. Which is where it typically ends up as we always need some trim tab unless running WOT. IN any case I wonder if there is a possibility the trimmed bellows helped with exhaust flow....? I suppose the only way to know would be to put the old prop back on. However it had a ding form last season, which is one of the reasons I changed it. The 15,25 would typically max out just over 4600, any thoughts on the trimmed bellow...?

 

[ccoon520]

The rebuild and cleaning may have just opened up the fuel passages more allowing it to push to the higher RPMs. With the exhaust bellows being underwater and already disconnected from the outdrive I don't think there would be any gains there as the backpressure wouldn't have changed trimming it a bit.

Before the carb could have also been tuned a little rich on the top end causing it to rev a little lower as well.

 

[AKJohne]

The rebuild and cleaning may have just opened up the fuel passages more allowing it to push to the higher RPMs. With the exhaust bellows being underwater and already disconnected from the outdrive I don't think there would be any gains there as the backpressure wouldn't have changed trimming it a bit.

Before the carb could have also been tuned a little rich on the top end causing it to rev a little lower as well.

The mains did not change, reused the original jet, which is typical.

 

[Scott06]

I put some more hours on it last weekend, weather was decent so I got the mixture screws and idle a little tighter. Was actually sunny and good water conditions so I had a chance to see what RPM's the secondary's came in at. At 4200 that start flowing fuel, the active throttle plate opens slightly before there is enough air flow to create a vacuum for fuel to flow. Since it was nice we decided to check for WOT, we saw 4850/4900 RPM before we shut it down. Right at 39.3 MPH according to Mr. Garmin, it was still pulling and it wasn't completely trimmed out, i think we could have seen 40. The real surprise was the RPM as I had changed to a slightly larger prop, I went from a 15.25 X 15 to a 15.8 X 15. I was expecting 4600 not 4850.... I know every prop is a little different.......... However there was one other change that I made and I wonder if it made a difference? Last winter when I had it inside and doing the service I noticed the exhaust bellows had come loose form the outdrive end. I contemplated redoing this and decided to just trim the end of the bellows slightly so It wouldn't buckle when the drive was all the way down/forward. Which is where it typically ends up as we always need some trim tab unless running WOT. IN any case I wonder if there is a possibility the trimmed bellows helped with exhaust flow....? I suppose the only way to know would be to put the old prop back on. However it had a ding form last season, which is one of the reasons I changed it. The 15,25 would typically max out just over 4600, any thoughts on the trimmed bellow...?

bellow trimming wont make much of a difference in that combo.

The rpm difference could be more related tot he blade design specifically how much cup it has. you may have had less slip with the old one vs the new one.

 

[AKJohne]

bellow trimming wont make much of a difference in that combo.

The rpm difference could be more related tot he blade design specifically how much cup it has. you may have had less slip with the old one vs the new one.

Very true, and so it is with props, I am always on the hunt for the perfect prop... LOL Fuel mileage doesn't seem to change much, even at high RPM is still runs 2.2 MPG. It may be slightly underpropped with this new prop. But just slightly and acceleration is OK when getting on step. Its a pretty heavy boat. I may try a 16 X 16 again, I had one before and seemed like it was over propped, as I was looking for 4800 RPM at WOT... I know think 4600 would be ok at WOT. I think its better to be slightly under rather than over.. IMHO.

 

[Scott06]

Very true, and so it is with props, I am always on the hunt for the perfect prop... LOL Fuel mileage doesn't seem to change much, even at high RPM is still runs 2.2 MPG. It may be slightly underpropped with this new prop. But just slightly and acceleration is OK when getting on step. Its a pretty heavy boat. I may try a 16 X 16 again, I had one before and seemed like it was over propped, as I was looking for 4800 RPM at WOT... I know think 4600 would be ok at WOT. I think its better to be slightly under rather than over.. IMHO.

4600 is ok but you dont want to go lower in RPM. If you are hitting 4800-4900 this is perfect.

Actually you are better being slightly underproped than over propped. While yes you have to watch the WOT rpm when lightly loaded, you wont risk lugging the engine, and general performance will be better in most phases of operation.