Atomic 4 Fuel Consumption

[Ball Racing]

Yeah, it will drive you crazy, I will go and make a speed pass, then come back to the dock and she is barely running good at idle,
I reset the idle mixture, and the main jet to get to smooth back out, then I turn right around and make another speed pass and I am slower

I think I need to change some of the circuit sizes in the carb, but am not ready to screw it up, and buy another couple of them yet.

[hanleyclifford]

It gets worse

Note that Don's instructions call for setting the main jet at cruise. Part of the problem is determining exactly what "cruise" rpm should be. Factors that influence are prop and vacuum. Assuming you are set at 17 degrees max timing BTDC and dwell within spec and that you desire to achieve optimal fuel efficiency at, say 9 inches of vacuum, you may have to settle for whatever hull speed those parameters may dictate. The goal of prop selection is to achieve hull speed at maximum fuel efficiency. The interdependency of the above factors, and perhaps some others not mentioned, is a subject of great interest to cruisers. In my case I found that at 1900 rpm and 9" of vacuum with main jet set for 1:14.7 fuel/air ratio according to my "analog" gauge, I could only get 5.5 kts in flat water. Clearly I need to shave the prop to get the nominal 6 kts.

[Sony2000]

In a perfect world to adjust the carb you first need a prop that gives you hull speed from a well timed engine running at 75% power. Now lean the mixture to get the highest exhaust tempature in degrees F, followed by enriching the mixture until the temp is 50 degrees cooler. You've arrived. Your there. Don't change it.

[Jesse Delanoy]

My first A4 had an adjustable main jet. When it gave up the ghost and I repowered with a Moyer-rebuilt unit, it had a fixed main jet. I always got between 3/4 and 1 gallon per hour, never felt the need to install an adjustable.

[hanleyclifford]

Question

In a perfect world to adjust the carb you first need a prop that gives you hull speed from a well timed engine running at 75% power. Now lean the mixture to get the highest exhaust tempature in degrees F, followed by enriching the mixture until the temp is 50 degrees cooler. You've arrived. Your there. Don't change it.

I would be interested to know what "75% power" is on the A4?

[Sony2000]

The Atomic 4 is rated 30hp at 3500 rpm. So she can run all day at 75%, or 2625 rpm and is the most efficient, fuel wise and power wise, under the above conditions. So backing up, if you are at hull speed, with 2600 rpm you are the nearly the most fuel efficient, and can only adjust the mixture by exhaust temperature to eek out a tiny bit more mpg, leaving sparks plugs with that tanned color.

[Ball Racing]

But, 75% of the rated 3,500rpm
is different because some people may be at 2,600 rpm max WOT which in turn is 100% of their motor in that set-up.And if you 75% that rpm it may not be at hull speed.
Or rather if you prop it to turn 3,500 it will take more than 75% throttle to get it to hull speed.
And 75% of 3,500 is great for load-and plugs- and fuel use IF you still could top out 3,500 rpms.
Kinda like climbing a mountain in your truck in ovedrive only turning 1,800rpms but you are using 90-100% of the throttle, by the rpm numbers you are fine, but the amount of strain, and throttle you are given it is over laboring it.

Tricky stuff, numbers that is..

[Sony2000]

Yes you are right. Don't be concerned with hull speed. All the boats are different. And anyone operating an Atomic 4 at WOT, and turning 2600 rpm, is dumb, and possibly stupid.

[ndutton]

And anyone operating an Atomic 4 at WOT, and turning 2600 rpm, is dumb, and possibly stupid.

Oh-oh, looks like I'm a charter member of the dumb and stupid club. I max out at somewhere around 2200 RPM. My normal cruising RPM is a little less, like 2000. She seems to purr along quite contentedly there.

[hanleyclifford]

Regroup

Don't forget what Dave has written about vacuum. I believe this is a critical parameter and I have some proof that it affects fuel efficiency.

[Ball Racing]

Yes you are right. Don't be concerned with hull speed. All the boats are different. And anyone operating an Atomic 4 at WOT, and turning 2600 rpm, is dumb, and possibly stupid.

So, what is your WOT RPM?

[roadnsky]

Oh-oh, looks like I'm a charter member of the dumb and stupid club. I max out at somewhere around 2200 RPM. My normal cruising RPM is a little less, like 2000. She seems to purr along quite contentedly there.

Me too!

Purring along cruise @ 1900 = 5-5.5K (Vacuum @ 7.5")
Max out around 2300

[hanleyclifford]

Me too!

Purring along cruise @ 1900 = 5-5.5K (Vacuum @ 7.5")
Max out around 2300

Jerry, My numbers are not too different from yours except I have more like 9" of vacuum. I think we are both slightly over propped.

[Sony2000]

Hdutton please don't be affended, I was aswering Ball Racings exaggeratons with exaggerations. The Atomics WOT should be 3500 rpm as you know, and we play with transmission reductions and prop sizes to maintain a 3500 rpm WOT. A WOT of less then the specs means a number of things.The engine is working too hard and everyone is operating on the "back side of the power curve", and into the zones of ineffiencies for the Atomic 4.

[Ball Racing]

Hdutton please don't be affended, I was aswering Ball Racings exaggeratons with exaggerations. The Atomics WOT should be 3500 rpm as you know, and we play with transmission reductions and prop sizes to maintain a 3500 rpm WOT. A WOT of less then the specs means a number of things.The engine is working too hard and everyone is operating on the "back side of the power curve", and into the zones of ineffiencies for the Atomic 4.

My exaggerations?
Please note them
Quick poll:
Who has their A4 able to maintain 3,500 rpm under load?
Sony, what RPM do you run at?