Carb running rich - how to correct

DavidKriegel

DavidKriegel

New member
I am brand new to Model A's after having worked on many Model T's and other classic cars

My engine stumbles runs rich despite mixture lever turn all the way in

Diagnosis when I shut off fuel with the fuel shut off valve, the engine turns from a stumble to a real nice purring kitten as the fuel bowl empties. Looking at the carb diagram it appears the air inlet in the bowl is being blocked by too high a fuel level. If this is correct diagnosis, what advice on bending the float to adjust the level correctly? I have not found this in my repair books
 
David, welcome to the Ford Barn!

The problem MAY be in the GAV fittings down at the carburetor. On mine I found that the fitting on the end of the GAV shaft was loose on the shaft. It should be tight - otherwise no adjustment occurs when you rotate the shaft to increase/decrease the mixture richness. I was running with the GAV turned all the way clockwise (or in) as you describe and it still ran rich, though not as bad as you describe. Worked a whole lot better when I corrected this!
 
Has it been sitting lately. My A sat for a long while recently and i couldnt get it to run right so i ordered a carburetor kit and adjusted the floats i dont have a problem now. Its not too expensive so instead of trying to fix it i just rebuilt it.
 
I suspect the needle seat is not closing off allowing additional fuel to flow to the secondary well. Remove upper housing of carb, plug off the cap jet and compensating jet, close needle seat, apply air to the secondary well; if the seat is closing off, no air should pass into the lower bowl. Some of the new needle threads bottom out before the point of the needle reaches the seat.
I would think the float level is not to high or you would have fuel flowing over the top of the cap and main jets with gas on and engine not running.
To plug off jet holes I cut the tip off some junk idle jets, flatter the tube at the end making a handle and plugging of the passage way.

Ron
 
One additional piece of info
The mixture choke knob in the cab will not turn by hand I had to turn it in the engine gompartment with a vice grips The car has been sitting since 1988 I will take carb apart today and find the problem and report back Thanks
 
I am assuming that your carburetor is a Zenith.

What can have a significant influence on fuel richness is the sizing of the individual jets. Most reproduction jets and a number or originals I have seen are not sized correctly. Usually the orifices are too large. To make a Zenith run correctly the jets should be flow tested and resized if necessary to the proper size.

As an example, if the comp jet is the same size or larger than the cap jet the GAV control has no affect.

it is also a good idea to set the float level using some kind of a jig where you can view the fuel level in a Glass.

For an understanding of the Zenith and some helpful photos, go to the Orange County Model A Ford Club web site at www.ocmafc.org. On the menu at left click on "tech articles". It will bring up a new menu. At the top is my name, click on it. It will bring up a series of articles. Scroll to the one on Zenith rebuilding and click. All the article are in an adobe file. They can be down loaded and printed and are free to use.

Tom Endy
 
DavidKriegel said:
I am brand new to Model A's after having worked on many Model T's and other classic cars

My engine stumbles runs rich despite mixture lever turn all the way in

Diagnosis when I shut off fuel with the fuel shut off valve, the engine turns from a stumble to a real nice purring kitten as the fuel bowl empties. Looking at the carb diagram it appears the air inlet in the bowl is being blocked by too high a fuel level. If this is correct diagnosis, what advice on bending the float to adjust the level correctly? I have not found this in my repair books
Click to expand...
Go to www.modelabasics.com for good carb info.
 
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