Carb questions and thoughts

G

Gino

Senior Member
Well I know the carb on my A is too rich, however when I lean it out the engine stalls when coming to a stop. I think the cure is to turn up the idle but then we have high rpms and this makes it more difficult to shift with zero grinding, especially when shifting into first from neutral. I believe there is a two bowl system one for idling and one for cruising, does this come into play and is there an easy answer. Thaks
 
I assume you have the Zenith carburetor. There is a complete description of the parts at http://model-a.org/components_full.html. There is a clever secondary well that performs the functions you describe. If the engine stalls when you come to a stop then there is something wrong with the carburetor. It could be as simple as a wrong float level or maybe the wrong size jets or the idle set too lean or too rich.

Or an intake leak as John suggested.

Bottom line is that this is not right and there is something wrong.
 
Yes many possible mechanical issues so before getting out the wrenches read your Owner's Manual for the correct procedure to adjust the idle.
 
Gino said:
Well I know the carb on my A is too rich, however when I lean it out the engine stalls when coming to a stop. I think the cure is to turn up the idle but then we have high rpms and this makes it more difficult to shift with zero grinding, especially when shifting into first from neutral. I believe there is a two bowl system one for idling and one for cruising, does this come into play and is there an easy answer. Thaks
Click to expand...

I just spent several weeks sorting out a car that is new to me. A couple days ago, as I was driving, it died rounding a corner. After a restart, it tried to die at the next few stop signs. I raised the idle on the steering column to limp home.

The new gasket I had placed between the carb to manifold attachment point had loosened. I had heard a slight squeal when the car started too, which will be a warning sign for me in the future. The squeal was intermittent and is now gone.

A 1/2" wrench and a couple turns on the bolts that hold the carburetor on resolved this issue.

I have read that many owners over tighten the carb attachment bolts and warp or crack the flange. (Zenith carb) As a rookie Model A mechanic, the question is often, "Without a torque spec, how tight is tight enough?"
 
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Rob Doe said:
I just spent several weeks sorting out a car that is new to me. A couple days ago, as I was driving, it died rounding a corner. After a restart, it tried to die at the next few stop signs. I raised the idle on the steering column to limp home.

The new gasket I had placed between the carb to manifold attachment point had loosened. I had heard a slight squeal when the car started too, which will be a warning sign for me in the future. The squeal was intermittent and is now gone.

A 1/2" wrench and a couple turns on the bolts that hold the carburetor on resolved this issue.

I have read that many owners over tighten the carb attachment bolts and warp or crack the flange. (Zenith carb) As a rookie Model A mechanic, the question is often, "Without a torque spec, how tight is tight enough?"
Click to expand...


That caution certainly applies to Pot metal carbs like Tillotson and others but not Zeniths.

BUT here is the most common way a Zenith carb flange gets broken.

When using a bolt that is too long to fasten carb to manifold.

Bolt should be no longer that 3/4 inch from bottom of head to end.

In other words 1 inch from end to end.


Click this URL to see photo:
https://www.fordbarn.com/forum/showpost.php?p=1763364&postcount=10

It should be noted that NOT every carb will be broken with long bolt.

Some do not have the boss below the hole.
 
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More than likely float - need & seat. I like using the tube some of the suppliers sell where you can set the fuel level instead of the float level.
 
Thanks, sounds easy, probably won’t be! I think I’ll fix the economy and the labor shortage then get at it.
 
There is a great site for Zenith carburetors at http://model-a.org/symptoms.html

Here is what they say about stalling at stops:

Engine dies when vehicle comes to a stop

Common Causes

  • Float is set too high or a pin hole absorbs gasoline into float.
  • Compensator Jet Orifice contains dirt or rust, or is too small.
  • Idling Jet Orifice contains dirt or rust, or is too small.
  • Idle Air Adjust Screw is mis-adjusted or too short, collapsing spring before it seats.
  • Throttle Idle Adjustment Screw is mis-adjusted.
  • Upper Casting Idle Jet Passage is restricted to Idle throat port.


There are other discussions about carburetor problems.
 
I also found that site very helpful. I recently experienced a couple of similar troubles and thought my experience may be of interest. I run a Zenith 2 carb and a 6 volt system. All was well and I bragged about how slow an idle I could drop it down to for that wonderful "potato-potato-potato" sound of the engine with it fully retarded and the throttle lever all the way up. Then, pretty suddenly, it started to stall out at stops. Simultaneously, I noticed very poor idling and a failure to adjust idle speed when I turned the gas-air adjustment screw on the carb either up or down. I did a little research, reading the trouble-shooting guides in the 'Book of Les' and on-line. I read about all those causes listed above and then some, including:

a short in the stop light switch
an air leak in the exhaust-manifold connection
a blown manifold gasket
a loose throttle plate shaft

I decided to test one thing at a time to eliminate or locate each cause. I started with the easy things first. There was no short in the stop light switch so I eliminated an electrical problem and knew it had to be in the fuel system somewhere. I checked for a leak in the gasket and at the muffler clamp, but didn't see a problem in either place, so was thinking the trouble had to be the carb. Next I removed just the lower half of the carb to examine the jets, the float and float valve. By removing only the bowl portion of the carb, you don't have to disturb the fuel line but can still get at the float valve and all the jets. I was pretty sure it wasn't the float valve because, unless a hole developed in the float allowing in gas thereby failing to shut of the gas supply to the carb, the float valve had already been properly tuned. The float itself was intact and there was no hole in it. I disassembled the float valve to make sure there was no detritus in there to prevent it from stopping the flow of gas. I was narrowing the trouble down to a plugged jet or a plugged passage. There is a small hole at the throttle plate that sucks in gas from the secondary well at idle. Sometimes this passage gets plugged with gunk causing poor idling. I first removed the idle jet. BINGO! When I held it up to a light and couldn't see through it, I knew it was plugged! The orifice of that jet is tiny. I used a piece of nylon fishing line to unplug it. And, while I had the bowl off, I checked all the other jets and passages as well. Nothing else was plugged. I reassembled everything and took a drive. Wow! It ran like it had just come off the showroom floor, and I could once again drop the idle way down. Amazing that such a speck of gunk can cause so much trouble.
 
29er.......I am experiencing the same - with my rebuilt Zenith. Stalling at intersections. I may have an idle jet problem. I don’t think it’s the float. Even so, the A runs pretty good out on the highway - it seems as though, some hesitation at about 50 MPH. I will double recheck the timing and points also.
Dave
 
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