engine idle question

wallster

Member
I recently changed the carburetor on my '31 from a tillotson to a zenith and the car starts and runs great. The only issue I'm questioning is that when I'm cruising along and come to a stop, the idle drops down and then raises back up within a few seconds. If I adjust my idle down it will stall at when I stop. The float seems to be set correctly, the air mixture screw has been adjusted a few times and appears to be correct, the fuel flow doesn't appear to be restricted, and the GAV is set about a quarter turn out when warmed up. The car accelerates great and she runs smooth so I believe the timing is okay. Is this normal for the engine to idle lower right after a stop?

Wally
 
Look for an air leak. A little leak at idle means a lot. Make sure the flanges between carb and intake manifold are flat and not warped.
 
Some of this is reasonably "normal"! That's why Henry re-designed the carb, later. When stopping HARD, I just speed up the throttle a couple of knotches. In heavy stop & go traffic, I just LEAVE it speeded up a couple of knotches.
Bill W.
 
Ford recognized these Zenith carburetor problems after many complained of Model A engines dying at abrupt stops or when rounding a sharp curve.

To resolve this problem, he later changed the interior round carburetor bowl and round bowl float configuration; however,

A. With a dismantled round, smooth bowl Zenith carburetor, if one were to fill it half full of gas, one could witness how very easy it is for the gas to slosh around back and forth when the vehicle brakes are applied or sloshing side to side while negotiating a curve; however, when this bowl shape is compared to that of a later non-Ford Marvel carburetor, with distinct 90 degree corners around sides and bottom, one can see that with the rectangular shape of the Marvel bowl and float, that this different bowl shape prevents such easy back and forth and side to side sloshing of gas in the carburetor bowl.

B. Some may have experienced that Zenith carburetors with round bottom bowls appear to have this built-in engine killing feature at a red light or stop sign .....and that Marvel carburetors with a flatter bottom and flatter sides on both bowl and float does not have this engine killing fuel sloshing feature.

C. Most of us are old enough to remember metal ice trays in refrigerators used to make ice. Some may remember the difference in transporting water from the sink to the refrigerator "with" and "without" the metal ice cube baffle dividers and noticed a very distinct difference.

D. Just like the metal baffles in a Model A gas tank which were provided to prevent gas from easily and violently sloshing from side to side, and front to back, the metal plates in an ice tray prevented water from sloshing around in ice trays.

E. Furthermore, the Zenith's float bottom is smooth and convex to match the smooth concave carburetor bowl; hence, no obstructions.

F. Never experimented with this Zenith's smooth, round bowl and float allowing the sloshing of fuel phenomenon "yet" ............... but it appears that by experimenting with the addition of maybe soldered small copper wires and/or copper screen or similar type materials soldered to either the outer surfaces of the bowl or float, or creating small baffles, that this may prevent Model A's from dying at a red light when the engine is idling while the brakes are applied.

G. Not much space to work with between the bowl and float ..... but who knows?
 
Follow up: I drove the car yesterday (took a few pics while I was at it, see below) and with the idle slightly higher than normal (or what I think normal is) the engine will idle lower when stopping but does not stall and idles back within seconds. I'll probably snag a package of float bowl shims from Mac's when ever I go by there but the car runs fine the way it is now.

Thanks for all the responses. It's amazing how many quirky little nuances the model A has, never a dull moment.

Wally
 

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