Running very rich

Chris Van

Chris Van

Member
Hi Boffins

Please help: My 31 Town Sedan is running very rich. I have leaned out the mixture to the max (Johannesburg, South Africa is at 6000 feet above sea level) and have installed a carburetor kit. The problem is that within a month the spark plugs have all sooted up and the engine starts to misfire until I remove the plugs and clean them. Am running unleaded fuel as the lead replacement petrol available here is of a substandard quality. Apparently I need to adjust the float level to let the engine run leaner. How does one set the float level on an A so that it is correct?

Would very much appreciate your assistance as I use the car as a daily driver and this is unbearable, apart from the heavy fuel consumption of course.

Many thanks

Chris
 
you are aware that the adjustment screw controls the air not the fuel? seat the needle and it is full rich...
 
I can't see how float level will have anything to do with whether it runs rich or not . If you raise the float level high enough it will cause the engine to starve for fuel under load. With the float too high it would run lean and backfire but it would not run lean and smooth . If the float level is too low it would cause leaks and stalling apon braking or coming to a stop but wouldn't cause the engine to run rich as you describe. If the running rich problem began after installing a carburetor kit, that will more than likely be the problem. I would try replacing the jets that you removed. The jets in the kit may be too large. The knob on the choke rod controls how rich or lean the engine runs. Make sure that the knob on the choke rod is tight and not just spinning on the choke rod shaft. Turn the knob clockwise to lean or counter clockwise to richen the mix . Zenith carbs usually run best with the GAV knob turned 1/4 turn off the seat when warm. The adjustment screw with the spring on the front of the carburetor housing controls the air mix for the idle circuit . Tightening the screw richens the idle circuit, loosening the screw leans. The idle air adjust screw usually idles best with the screw one and one half turns off the seat.

I wonder what spark plug that you are using. I had similiar problems when running Motorcraft TT 10 or Champion W 16 Y spark plugs on short stop and go drives. Champion W 18 spark plugs are hotter and run cleaner for most model A short around twon drives. I would take a look at the air gaps between the rotor tip and the contacts in the distributor cap. An air gap of .035 will give you a hotter spark at the spark plugs and help better ignite the fuel and burn off the soot. If the coil is weak and the heat range of the spark plugs is too cold, the spark plugs will continue to run black and sooty . Most times running rich gets blamed on the carburetor . I have found the problem to be weak ignition in most cases. ust my thoughts.
 
Jim Mason said:
you are aware that the adjustment screw controls the air not the fuel? seat the needle and it is full rich...
Click to expand...

Just to add to this,

On a properly working carb, one sets the mechanical idle so the engine runs slow then screws in the adjustment until the engine starts to stumble then count the number of turns as you go the other way until the engine starts to stumble. Go back 1/2 the number of turns and it is set.

EX Clockwise, stumble, counterclockwise 2 turns, stumble, clockwise 1 turn.
As for the actual problem, jet flow is off, need flow tested jets.
 
If you have no way to measure the flow in your carb jets, then I would second Mitch's suggestion on obtaining jets from Renner's. In my recent experience of flow testing 30 original and new jets, I found only three complete sets that were just barely close enough to specs to use. Your problem lies in your jet size or your GAV adjustment; the air adjustment screw only affects idle, and works in conjunction with the idle jet and the secondary well. It is part of the idle system, and shouldn't impact how rich the engine runs at driving speeds. That is controlled with the GAV, as noted above by others.
 
Ray is right, if you're 6000 feet altitude, then you're going to need smaller jets. A good A carb will run 1/4 open around sea level. That way when you go up to high altitude, you can close down the GAV and still have the correct air/fuel mixture.
 
700rpm said:
If you have no way to measure the flow in your carb jets, then I would second Mitch's suggestion on obtaining jets from Renner's. In my recent experience of flow testing 30 original and new jets, I found only three complete sets that were just barely close enough to specs to use. Your problem lies in your jet size or your GAV adjustment; the air adjustment screw only affects idle, and works in conjunction with the idle jet and the secondary well. It is part of the idle system, and shouldn't impact how rich the engine runs at driving speeds. That is controlled with the GAV, as noted above by others.
Click to expand...

He lives in south africa so getting parts takes time and money.

If you do a search for flow testing you should find info on how to test and size the jets here on the barn.
 
Hi Guys
Many thanks for your comments. I am going to work through everything carefully but will start by replacing the jets and also putting in a new needle and seat.
Will keep you posted.
With warm regards
Chris
 
Chris , If all fails look into a new Zenith 13922 carb. I had similar problems that you have and enlisted the help of my " A ' experts and rebuilt my existing carb. then bought one online ..Problem resolved with the 13922 + no more GAV adjusting + better gas mileage .....Ed
 
ed s. said:
Chris , If all fails look into a new Zenith 13922 carb. I had similar problems that you have and enlisted the help of my " A ' experts and rebuilt my existing carb. then bought one online ..Problem resolved with the 13922 + no more GAV adjusting + better gas mileage .....Ed
Click to expand...

if you find that you do not have to adjust the GAV as it doesn't make a difference then your carb is not performing properly
 
Are you by any chance running an air filter? I was and it was not perfectly clean and sooted up the plugs just like you described.
 
My experience was what Purdy said. After re-building the carb I still had fouled plugs and found I had a weak ignition system. I replaced the coil (with a PerTronix "Flame Thrower") and coil wire; rebuilt the distributor using old-school points gapped at 0.22; added an original-type AL condenser and a Bill Stipe "B" cam, installed Champion W18 plugs and gapped them at 0.035. I have a 6:1 head, and find the extra spark of those adjustments makes my 29er really purr! (THANKS PURDY!)
 
29er said:
My experience was what Purdy said. After re-building the carb I still had fouled plugs and found I had a weak ignition system. I replaced the coil (with a PerTronix "Flame Thrower") and coil wire; rebuilt the distributor using old-school points gapped at 0.22; added an original-type AL condenser and a Bill Stipe "B" cam, installed Champion W18 plugs and gapped them at 0.035. I have a 6:1 head, and find the extra spark of those adjustments makes my 29er really purr! (THANKS PURDY!)
Click to expand...

Thanks 29er. Many have weak ignition and just don't know it. Thanks for the mention and I'm glad that it worked for you !!!!!!!
 
Purdy Swoft said:
I can't see how float level will have anything to do with whether it runs rich or not . If you raise the float level high enough it will cause the engine to starve for fuel under load. With the float too high it would run lean and backfire but it would not run lean and smooth . If the float level is too low it would cause leaks and stalling apon braking or coming to a stop but wouldn't cause the engine to run rich as you describe. If the running rich problem began after installing a carburetor kit, that will more than likely be the problem. I would try replacing the jets that you removed. The jets in the kit may be too large. The knob on the choke rod controls how rich or lean the engine runs. Make sure that the knob on the choke rod is tight and not just spinning on the choke rod shaft. Turn the knob clockwise to lean or counter clockwise to richen the mix . Zenith carbs usually run best with the GAV knob turned 1/4 turn off the seat when warm. The adjustment screw with the spring on the front of the carburetor housing controls the air mix for the idle circuit . Tightening the screw richens the idle circuit, loosening the screw leans. The idle air adjust screw usually idles best with the screw one and one half turns off the seat.

I wonder what spark plug that you are using. I had similiar problems when running Motorcraft TT 10 or Champion W 16 Y spark plugs on short stop and go drives. Champion W 18 spark plugs are hotter and run cleaner for most model A short around twon drives. I would take a look at the air gaps between the rotor tip and the contacts in the distributor cap. An air gap of .035 will give you a hotter spark at the spark plugs and help better ignite the fuel and burn off the soot. If the coil is weak and the heat range of the spark plugs is too cold, the spark plugs will continue to run black and sooty . Most times running rich gets blamed on the carburetor . I have found the problem to be weak ignition in most cases. ust my thoughts.
Click to expand...

PURDY:
Great Write Up Thanks
Harley03joe
 
Besides the jets/flow rates I would double check the GAV needle assembly to make sure that you have the correct assembly for your carb. Last spring I had Renner rebuild my carb and he installed a "B" needle assembly in my Zenith "A" carb. I fought a rich running condition for most of last summer trying to find the problem until a local Model A buddy found the improper needle. The "B" assembly looked exactly the same although too short and allowed fuel to flow freely. Even Renner didn't see the difference after a second look at his own work.

Good Luck
 
When rebuilding a Zenith carburetor for a Model A the first order of business is to use a set of jets that have been flow-tested. Repo jets on the market are all over the map in size. Most original jets around have been molested by people pushing welding tip cleaners through them.

Flow testing is a time consuming and tedious task because you have to solder the end of the jet closed and re-drill it to the size that gives you the desired flow rate for a particular jet.

Someone above mentioned their is a supplier that offers re-sized jets. Unless you have a flow tester set up that would be the best way to go.

Once the carburetor has been properly rebuilt, with a known good set of flow tested jets, the GAV control is what controls the richness\leanness when driving. Normally the GAV is cracked open about a quarter turn to half turn. The actual adjustment of the GAV depends on the individual carburetor and where in the flow rate window the cap and comp jets are sized to.

The air\idle adjust screw only controls the air to fuel mixture at idle and has no bearing on the car running rich or lean.

Tom Endy
 
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