Fuel Flow Rate

Car Pit

Car Pit

Member
Troubleshooting a backfiring issue and after reading several posts, I want to better understand fuel starvation. Is there a known minimum flow rate for the gas going to the carb?

Right after I depress the gas pedal, it sounds like the carb is sucking air.
Once I reach higher RPM, backfiring.
When I return back to idle, it shuts off.

This is a fresh rebuild of everything except the carb. I heard some popping at higher RPM, and noticed that I was not very skillful in preventing leak paths at the manifold gasket. I changed it out, but now the problem seems to be worse.

One additional note, I'm running an electronic spark control.
 
I would check your float level, it maybe that the float is set too low.
At one time I did find a web site that had a chart that listed the flow rate / second, but I seem to have lost the link.
 
Hi Car,

This may sound simple & parochial, but usually technical, mechanical equations are difficult to remember.

With about a 3rd grade education which was common in 1930, many of the Model A owners I knew might have it explained this way:

1. I can easily travel 60 mph.

2. With dirty spark plugs, & no recent tune up, I can at least get at least 15 mpg.

3. Then, if it takes 1 gallon to travel 15 miles, it will take 4 gallons to travel 60 miles.

4. So if I want to travel 60 miles at 60 mph, I better have a "minimum" gas flow to the carburetor of at least 4 gallons of gasoline per hour.

5. Any less flow, I either need to either travel more slowly, or get an engine tune up.

But heck, what do I know about hydraulic equations, inches of mercury, gravity flow, & internal combustion?

Hope this helps.
 
when the problem is occuring richen it a tad with 2+2,, either through the carb airhorn or with a straw through the vacuum wiper port . then youll know if your running lean if the rpm's smooth out. it is also handy to have an inline spark tester to see the intensity and pattern.
you need to determine if its ignition or fuel related to take the next step.
 

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A QUICK test: When it won't go any FASTER, gradually inch out on the choke, if it then goes FASTER, you're starving for fuel. Also, at a fair speed, YANK out HARD on the choke several times & it "might" SUCK out an obstruction in a jet or passage in the carb. If that helps, it ain't a permanent FIX, but it's quick & cheep, for a diagnosis!
Bill W.
 

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BILL WILLIAMSON said:
A QUICK test: When it won't go any FASTER, gradually inch out on the choke, if it then goes FASTER, you're starving for fuel. Also, at a fair speed, YANK out HARD on the choke several times & it "might" SUCK out an obstruction in a jet or passage in the carb. If that helps, it ain't a permanent FIX, but it's quick & cheep, for a diagnosis!
Bill W.
Click to expand...

yep very true dad
half choke the chicken
 
1930 coupe said:
Take the gas line off the carb and put it in a can and open the gas valve. If you get a full stream you are good, if it drips or very weak dribble, something is stopped up.
Click to expand...
I don't know of any flow rates, but on Minerva, with a "pencil" filter in the top of the gas valve, a cast iron sediment bowl, & 1/2 tank of gas, she would flow a 6" stream of gas at the end of the carbuerator line!
Bill W.
 
Update...
I eyeballed the fuel flow. With about 3 gallons of gas in the tank, there is about a one inch stream of gas going to the carburetor, which is basically the same as what I see when I check it at the shutoff valve. Tank was cleaned and coated before reassembly and the pencil filter is clear. Gas is fresh.

I disassembled my Tillotson carburetor, sanded the warped seam as flat as possible, replaced the main jet and gaskets, reset the float to 1" below the seam, and confirmed the fuel level was 5/8" below the seam.
I then removed and wire brushed each spark plug and port hole and added a ground wire between the engine block and the body.

Same behavior... Starts fine, but before I can walk from the driver seat to the carb side of the engine, it starts backfiring into the air maze, speeds up slightly, and then shuts itself off. Can't even think about cycling the throttle without the car stalling and backfiring.
Tried everything from 1/4 turn to 3 full turns on the adjustment rod.
Adjusting the air control knob on the carburetor to anything but completely closed shuts the car off.
I adjusted the float to 7/8" below the seam and no improvement.

The local club is a couple hours away so it's kind of hard to call a buddy over to help diagnose things. Thanks for your ideas.
 
On my Tudor with tank half full, I measured rate of flow by disconnecting the fuel line at the carb, with tap open fuel emptied into the measuring cylinder at the rate of 360ml /min. ( I reckon this equates to around 2.5 mpg)
 
I have posted this before... but will again, check the ******** the points, that is the thin bendable part that follows the flat spring. Make sure it is not touching the case. I had one do that and it caused all kinds of intermittent problems. The issue was it was too long, I put a bend in it to take up the slack and the problem was solved. The ing. test light sown in the other posts above will help show these kinds of problems and help you know when you actually find the problem. As others have said folow logical trouble shooting procedures.
 
Sounds to me like a manifold air leak. When it is running, try to find a throttle/choke setting that runs the engine the smoothest, then take some WD40 or similar with the long tube on the nozzle, and spray around the manifold. If you have a leak you will hear the rpm change. Also check the carb to manifold area the same way.
 
Mitch,
Spraying carb cleaner into the air intake does keep the car from shutting off.
I bought a spark tester, but haven't had success with it yet as I can't keep the car running long enough to perform a good test.
What should we look at next?
 
Car Pit said:
Mitch,
Spraying carb cleaner into the air intake does keep the car from shutting off.
I bought a spark tester, but haven't had success with it yet as I can't keep the car running long enough to perform a good test.
What should we look at next?
Click to expand...

Car Pit,
if richening it with carb cleaner helps it run better thats telling you its running lean. possible causes are fuel flow, fuel line pushed in carb to far, carb problem, or vacuum leak at manifold gasket, carb base gasket, worn throttle shaft. ck for leak by spraying crc or carb cleaner around the gaskets. if you have nice engine paint use the crc.
it seems your diagnosis may have narrowed it down to which system may be at fault, the spark tester will come in handy one day money well spent.

sorry have been busy with the noreaster
 
I like to use an UNLIT propane torch to check for intake leaks. The engine speed should pick up and smooth out when the propane gets sucked into the air leak.
 
Hi Car,


When we humans are deeply concentrating on & continually looking hard for Bayer Aspirins at Wal-Mart, we never see the other headache remedies such as Advil, Tylenol, Alka-Seltzer, & BC Powder.

In your initial statement:

"One additional note, I'm running an electronic spark control."

Did you try a regular Model A distributor that worked fine on over 20 million Model A's?

Rural raccoon hunters are aware that an excellent coon hound usually taps several trees until he finds the "right" tree with the raccoon in it; while the rest of the hounds are "barking up the wrong tree."

Just sincerely hope this helps in your diagnostic endeavors.
 
So far everyone has been covering fuel and spark. What about valves? Has the head been off? Were the valves ground and not gaped? I this an old engine that you are getting running or a rebuild? What is your compression? Could the valves be acting sluggish and not closing all the way all the time. Have you checked your timing?
 
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