Carburetor pre adjustment

W

WillC

Member
Iv complete rebuilt the this Zenith carburetor and would like to know how many turn on the air and the fuel mix should be a good starting point for starting the engine. Thank you.
 
The adjustment screw on the carburetor is the idle/air mixture. It is set at 1 1/2 turns open as indicated above. The Gas Adjustment Valve is set with the choke rod in the cab. I always run mine at 1/4 open.
 
Hello Will,


I rebuilt the Zenith-2 too. I mounted new needles and yets. I can not open the idle scew more than 1/4 turn. If more, the fuel gets too fat. The gas adjusting knob inside I must open 1/4 turn always.
 
Use Initial setting of 1 1/2 turns from seated as indicated above. Also as indicated adjustment of the idle speed and idle mixture is typically set with GAV 1/4 turn open at motor idle, with the motor warmed up.. Adjust the two screws for best low speed idle. Most find the 1 1/2 turns the good setting for their As. This can vary with the A and carb. For some reason my truck runs best a 1 2/3. Maybe the carb has a jetting issue, do not know\. Am leaving it alone since the truck is running great as is.
 
Here is some info that may help. Zenith carb basics and the adjustment procedure from the Model A Instruction Manual.
I found that after establishing a desired idle with left lever fully up that fine tuning the fuel/air mixture needed to be done slowly allowing the engine to catch up to my adjustment. Take it slow and "listen" as directed in the manual.
Enjoy the adventure, Chap

http://modelabasics.com/carb basics 2.htm
http://motormayhem.net/wp-uploads/2009/04/Model-A-Instruction-Manual.pdf
Page 24
 
Remember the idle mixture screw on the carb regulates air not gas. Screwing it in, cuts down on air, richening the mixture.
 
If you haven't flow tested the Idle Jet, you really don't know where you are going to set the air Idle Adj. Screw. The published range is 44 - 48 ml/min. If your jet is at the low end, most likely your Adj. Screw will only be opened a half a turn. If you are at the upper end (48 ml/min.) you will find the screw is out 3-4 turns getting as much air as possible to mix with the higher idle gas flow. I find that purchased jets are most likely over sized and the idle circuit will perform miserably! If you are taking the time to clean and rebuild your Zenith Carb, have all the jets flow tested. Ed
 
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