The Infamous Zenith Fix - and a Valuable Metalworking Technique
Original Author: Alan Richer (OVLR)
As we all know, the poor heat treatment and insecure design of Zenith carburetors often causes the body halves to warp and twist - leading to air leaks, poor mileage and even worse running characteristics.
The classic symptom of the warped-body problem is an inability to adjust a rich idle mixture using the mixture adjust screw on the side of the carburetor base. If the engine won't stall with the screw wound all the way in, likely it's siphoning fuel past a bad gasket and seal between the halves.
The cure for this problem is simple and is called lapping - and the technique is in itself a very useful metalworking skill to acquire. In short, we need to restore the mating surfaces to a high level of flatness to eliminate the air leaks that the warping causes. We'll do this by hand grinding down the surfaces with abrasive on a known-flat base. This is an old machinist's trick for creating finely-finished gas-tight seals with a minimum of finicky measuring and tool work.
The supplies we will need are simple, and consist of:
A small sheet of plate glass (1/4" is better, but windowpane will do). At a minimum the piece should be the size of a sheet of paper, though larger is good as well. No specific dimensions here - anything you can persuade away from the gentlemen at the local glass shop will do.
Sheets of emery cloth - one of 120 grit or so, one of 220 or slightly finer. These are the cutting medium that we'll use to hand-grind the carburetor parts flat.
Oil - just about any light oil (not motor oil) will do here. We'll be using it to hold the sheets of emery to the glass and as a lubricant during the grinding process.
The technique here is simple. What we'll be doing is a backwards grinding process - we're going to be moving the part to be ground over a stationary abrasive on a known-flat surface. This way, the abrasive will remove any bumps or protrusions on the surface we want to flatten - leaving it smooth and as flat as the reference surface (the plate glass).
First off, disassemble the carburetor completely. When you're done, you should have just the carburetor casting halves in hand - each piece with all the jets, linkages, venturi parts and the rest removed.
Next, let's prepare the grinding workspace. Cover your work surface with paper (to keep the mess minimized) and place on that the sheet of glass. Wet the surface of the glass with the oil and smear it to a thin covering - this will hold the emery paper down and keep it from sliding around. Lay the coarse emery down on the glass, pressing it down to make sure it has even contact with the glass.
Now we're ready to start lapping - but first a word or two on what to look for. If you look at the gasket sealing surface on the carburetor top you'll see it's a uniform grayish color. This is going to change as we grind it flat - the surfaces will become bright and show a uniform pattern of scratches from the action of the emery. This is what we want - when the bright pattern and color covers the whole sealing surface then the part is as flat as the plate glass.
The process from here on is nothing more than simple hand work with a bit of attention to detail. Wet the surface of the emery with some of the oil - not a lot, but enough to give some slide to the part and float away the grinding debris. Place the part to be ground face-down on the emery sheet and move it about in a figure-8 pattern with even pressure for a half-dozen strokes - then pick it up and wipe the bottom face to remove the dust and oil from the sealing surface.
You'll notice that the bright areas are not covering the whole surface and are likely concentrated around the screw holes. This is what we're trying to correct and we're well on the way to doing so.
Continue the movement, making the figure-8 movement for a dozen or two strokes,wiping and inspecting. When the bright pattern completely covers the mating surfaces then this piece has been straightened and should be put aside for the next stage. Do both halves of the carburetor this way, then remove the coarse emery and clean the glass.
We have good flat surfaces now - but they still have the scratch patterns from the coarse paper. We can remove the scratch pattern with the fine paper, using the same technique as before.
Clean the parts of emery dust and oil thoroughly (I usually use carb cleaner in a spray can for this). Now, adhere the fine sheet of emery to the glass with oil. Using the same figure-8 motion and even pressure lap the carburetor halves on the fine emery, but just until the scratch pattern from the coarse paper is removed.
When the parts are evenly lapped, clean the carburetor parts thoroughly and reassemble the carburetor with new seals and gaskets. If assembled correctly and the lapping process was successful, you should now be able to adjust the mixture from rich to stalling using the mixture control screw on the side of the carburetor base.
The technique above (flattening metal objects by lapping) is a very handy technique that isn't simply applicable to carburetor parts. I've personally used this to flatten warped carburetor bodies as above, but also to remove wear from Range Rover oil-pump covers and straighten warped Land-Rover steering box covers.
It's a classic low-tech technique in model engineering for flattening up baseplates and other items that require a good surface for precise parts alignment.
In each case the technique was the same - even pressure and moving the object on an abrasive medium on a known flat reference surface. Plate glass is ideal for this type of work, as the process of its manufacture (floating on molten metal) leaves it with a very flat surface - ideal for our purposes.
To conclude, this is a good, inexpensive method for salvaging an otherwise unusable carburetor The materials are easily available, and doing so teaches a valuable skill that can be used on many items both on and off the car.