Carburetor question

Bob from Northport

Bob from Northport

Well-known member
Thsi is a question for the carburetor guys that know the fuel flow of a zenith carb. When a Model A is ready to start, and the starter is pushed, where does the initial flow of fuel come from? Which jet or jets are the ones that the start fuel comes through.?? The pistons pull the fuel upward as the crank turns, and pulls first where??

Bob
 
Unless the choke is pulled, there's probably not enough air-flow/ vacuum to pull any fuel through the jets. If the choke is pulled, it probably pulls through all the jets to some degree.

Once the car has been running and is somewhat warm, if shut-down, there will probably be enough residual fuel in one of the cylinders to fire instantly, and spin the motor fast enough for some air-flow / vacuum to pull fuel form the jets w/o use of the choke.
 
The relatively low vacuum during cranking does not provide sufficient fuel through the idle port in the throttle bore of a Zenith. Some fuel is also necessary from the compensator jet. That requires more air flow, necessitating opening the throttle a few clicks, further reducing the ability of the idle port to deliver fuel during cranking. Zeniths are not known for first-crank no-choke cold starts. Other carbs like the Tilly and Marvel are better at delivery through the idle port during cranking. You may have noticed some later design carbs have either a flat spot or notch on their throttle plates where they close against the side of the bore next to the idle port. That is to make more air flow directly across the port when the throttle is closed. A very well designed carb does not need a choke plate. Many European and motorcycle designs used an enrichening circuit or device rather than a choke.
 
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