How much gas should be delivered to A carb?

[Benson]

How much gas should flow from the fuel line to maintain the 5/8ths inch fuel level under load?

Several places say "check for good fuel flow" what is "good fuel flow" in ounces per minute or ??

Thank you

[MCHinson]

I have no idea how many ounces per minute, but if you disconnect the fuel line at the carburetor and open the valve, you should see a steady full flow from the end of the fuel line. You will know it when you see it.

[Ron in Quincy]

The float and float valve controls the amount of gas in the bowl of the carburetor; that why its important to properly set float level.

Ron

[Richard Lorenz]

Suppose that at 50 mph full throttle you are getting 5 miles per gallon. That translates to 0.17 gallons per minute. If my assumption and math are correct, it is the same as 1 cup of gasoline in 22 seconds. Will someone else please check this calculation.

[steve s]

Originally Posted by MCHinson

I have no idea how many ounces per minute, but if you disconnect the fuel line at the carburetor and open the valve, you should see a steady full flow from the end of the fuel line. You will know it when you see it.

I have been surprised how easy it is to think you know it when you're not really seeing it. Any steady stream of gasoline can look a little scary and make you think, surely that is enough. It doesn't take much to idle or run slowly, but it takes a surprisingly strong stream to get up to 45-50 mph. I've seen an in-bowl fuel filter let a good looking stream through but not be sufficient for high-way speed; take out the filter and he was off to the races.

The best way I know to describe it is that the flow should look like a young man proudly writing his name in the snow, not an old man laboring to keep his shoes dry.

Steve

[steve s]

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Originally Posted by Richard Lorenz

Suppose that at 50 mph full throttle you are getting 5 miles per gallon. That translates to 0.17 gallons per minute. If my assumption and math are correct, it is the same as 1 cup of gasoline in 22 seconds. Will someone else please check this calculation.

Richard,
I calculate 22.5 seconds/cup. Close enough.

Steve

[Peter J]

But how do I keep my shoes dry?

[gweilbaker]

The best way I know to describe it is that the flow should look like a young man proudly writing his name in the snow, not an old man laboring to keep his shoes dry.

Steve,
You forgot to add;
And in her hand writing!

GW

[MikeK]

How to Calculate Required Fuel Delivery

Under full load (near zero intake vacuum) at the horsepower peak a well tuned gasoline engine produces 11.30 hp-hr/gal. Converting that to fuel used per horsepower, you get 0.0885 gal/hp-hr. A stock 40 H.P. engine will require 40 times that.

40.0 hp x 0.0885 gal/hp-hr = 3.54 gallons per hour (hp cancels hp in the math)

1 hour = 60 minutes, so 3.54 gal/hr = 3.54 gal/60 min

Now divide by 60min = 0.059 gallons per minute required for 40 horsepower

You probably have no way of measuring 59 thousanths of a gallon, so convert to fluid ounces which you can easily measure:

1 gallon = 128 fluid ounces, so 0.059 gal/min = 7.55 oz/min

Now, add a little rough justice: The Model A engine is probably not tuned/cammed as efficiently as the 0.0885 gal/hp-hr figure we started with, so add 10%, and you probably want another 20% so you do not ever run at the absolute limit of fuel delivery.

7.55 oz/min + 30% = 9.82 oz/min.

You are looking for fuel delivery of almost 10 oz/min. for a 40 HP engine.

BUT THERE IS A PROBLEM !! You need to get the fuel past a partially open float valve. The biggest restriction/ smallest opening in a clean Model A fuel system is the seat in the float valve. The calculated fuel delivery does no good if the carb float needs to be wide open to get it. Realistically, you want that flow rate into the carb with the float only 1/8 - 3/16" down. Carbs suffer when the level is 1/4 or more down. You need to measure the flow through the float valve with the float locked a measured distance down.

With a Zenith, it's easy. One bolt drops the bottom of the carb, leaving the fuel line, float and valve in place. With a piece of mechanic's wire set the float so it hangs down 1/8- 3/16" from the point where the valve closes. Put a catch pan underneath, open the fuel valve, time it and figure the fuel delivery rate.
Now, wasn't that easy.

[Wilky]

Some years ago I flow tested several arrangements of mine (cars that ran fast with no difficulty) - using different fuel lines (originals vs. repros) and different fuel levels in the gas tank, and with fine-mesh brass filters installed in the shut-off valves. I did the flow testing with the fuel line removed from the carb and caught the fuel at that point. I also first verified that the gas caps were properly vented. I found some (~20%) variations, but in general the gasoline flowed to the carb at about 1 oz. every 2 seconds. John

[steve s]

Originally Posted by MikeK

How to Calculate Required Fuel Delivery

... You are looking for fuel delivery of almost 10 oz/min. for a 40 HP engine.

BUT THERE IS A PROBLEM !! ...

One more little detail, in case you're not using a scale but a measuring cup calibrated in ounces. In the US, a fluid ounce is about 1.04 avoirdupois ounce (a little more if it's for nutritional labeling). But, the bigger problem is that gasoline is less dense than water. "A pint's a pound the world around", as long as it's a pint of water. The specific gravity of gasoline is 0.72 - 0.74, so you need about one-third more volume of gas to get the correct weight if you're using a measuring cup calibrated for fluid ounces.

Steve

[MikeK]

Originally Posted by steve s

One more little detail, in case you're not using a scale but a measuring cup calibrated in ounces. In the US, a fluid ounce is about 1.04 avoirdupois ounce (a little more if it's for nutritional labeling). But, the bigger problem is that gasoline is less dense than water. "A pint's a pound the world around", as long as it's a pint of water. The specific gravity of gasoline is 0.72 - 0.74, so you need about one-third more volume of gas to get the correct weight if you're using a measuring cup calibrated for fluid ounces.

Steve

Steve, Thanks, but I think I already took that into account. If you look back you will find this line in my post:

1 gallon = 128 fluid ounces, so 0.059 gal/min = 7.55 oz/min

My calculations are for fluid oz, not mass or weight by avoir or troy ounces.

If you want to calculate a multiplier for the btu penalty for E10, you could add that.
1 gallon of ethanol = 76,100 btu. 1 gallon base gasoline averages 114,000 btu.
Thus a 10%/90% mix (E10) would be?? ___ btu. That would be a ___% addition. Have fun.

[steve s]

Originally Posted by MikeK

Steve, Thanks, but I think I already took that into account. If you look back you will find this line in my post:

1 gallon = 128 fluid ounces, so 0.059 gal/min = 7.55 oz/min

My calculations are for fluid oz, not mass or weight by avoir or troy ounces. ...

You're right. A fluid ounce is a measure of volume, independent of its density--the sort of unit that makes physical chemists crazy.

Steve

[MikeK]

Originally Posted by steve s

You're right. A fluid ounce is a measure of volume, independent of its density--the sort of unit that makes physical chemists crazy.

Steve

Steve, thanks for the crazy confirmation. Wife has Ph.D. in biological chemistry. She sez my mission is to drive her crazy. Must be the chemist thing. I'll have to ask her to calculate the pee-in-the-snow flow rate for a comparison.
I always loved P-chem in undergrad school.

[Tom Wesenberg]

Mike,
I think your equation for ethanol is wrong.

From my research: 1 gallon of ethanol = 10 lbs. of cow dung

[MikeK]

Originally Posted by Tom Wesenberg

Mike,
I think your equation for ethanol is wrong.

From my research: 1 gallon of ethanol = 10 lbs. of cow dung

I'll pour myself a shot of bourbon (proper ethanol use) while I ponder that conversion constant. At least you can get methane from the dung. I wonder how many btu's per lb? Don't let agri-business or politicians know, we'll end up with E15+5, 15% corn squeezin's + 5% butt squeezin's. That oughta foul any tank.

[Benson]

Thank you all for your posts and info.