[Rob Doe]

Good comments above.

I may be confused, but my GAV controls fuel to the Cap jet, which is the mid range jet, above idle up to about 35 mph. Or so I thought.

The Zenith carburetor hinges its float perpendicular to the center line of the car, and therefore when applying the brakes to stop, the float will dip and rise as gas sloshes in the float bowl. On my car, I sometimes run the GAV at 3/8 turn open rather than 1/4 turn to alleviate this. I run at 1/4 when at higher speeds.

I'd like to add a bit different description for using the spark control.

Count your notches/bumps on the control from top to bottom. There are 10 or 11. Each mark is 3.6-4 degrees of crankshaft timing advance, 40 degrees total. The lever on the distributor moves only 20 degrees properly adjusted because the distributor turns at 1/2 engine speed. A 4 cylinder engine fires a cylinder every 180 degrees of crank rotation, all 4 cylinders in 720 degrees or 360 x 2 complete revolutions.

The Ford information times the Model A engine at Top Dead Center on the compression stroke of cylinder #1. You place the spark control all the way up before setting this timing. This will be the maximum retard position.

You always start the car in this retarded position. This protects the starter and the teeth on the starter ring from kick back and possibly damaging one, the other or both. It also protects a rookie that misuses the hand crank from breaking his thumb, wrist or arm. It makes it easier for the starter and the battery as they don't have to overpower the compression of a rising piston.

After the engine starts I immediately advance the spark control 2 or 3 bumps. And from then on, when driving I use a formula given by an old salt that was an old salt older than myself. Eh, a long time ago, too. For each 5 mph driving speed, lower/advance the spark control 1 bump. 30 mph = 6, 40 = 8, etc.

It is my understanding that the Model A engine cannot make use of more than 32 degrees of timing advance, but that is beyond my knowledge level as to why. I might try a full 10 or 11 bumps if I was going 55 or 60 after say, robbing a bank ... and then only if coppers were hot on my tail and shooting.

In central Illinois it is pretty flat with only occasional hills. If climbing a fair grade in high gear at a low speed, I retard the timing a couple bumps to prevent spark knock as someone else stated in a previous reply.

Some folks time their engine with the spark lever advanced 1 or 2 bumps. When placed in the full up position then, the engine fires 4 to 8 degrees after TDC. This adds further protection for someone using a hand crank, and I think makes a very slow idle possible. I have not tried this myself to see about the idle. Someone else might comment on that.