Back-fire

It’s probably backfiring because it starving for gas so you probably only have one problem not two seperate ones. There should be a screen in the gas tank, a screen in the sediment bowl and a screen in the Zenith. To service the screen in the tank you must drain the tank and remove the valve. You can look through the fill hole with a flashlight and see if that screen is still there, frequently they are missing. If there is no screen then you don’t need to worry about removing the valve. Disconnect the gas line from the carb and put it in a gas can and open the valve to check for flow (this would not tell you if the screen in the carb is plugged).
 
Yep, could be a blockage in A) the fuel supply to carb or B) within the carb itself. For A) Check there is a good flow of petrol from the tank by opening the petrol shut off valve to allow petrol to run through the pipe into a jam jar. –My Tudor empties at 350 ml per minute when half full. I sometimes take this opportunity to back flush the pipe with air a couple of times with the petrol tank valve open so air bubbles through into the tank- then recheck the sediment bowl for debris. For B) dis-assemble and clean as needed. .. (It's usually something simple, fortunately) .....Oh!... nearly forgot... C) check the gas cap is venting OK !!
 
This slide may help you. Check the easy to do things first, such as look for one or more vacuum leaks. The engine should idle at a manifold vacuum of 20+ Inches of Mercury with a steady vacuum gauge needle. Check the ignition system. Install a known good condenser. Check the ignition timing. See if the ignition is getting a consistent spark at the points and at the plugs. Do a compression test to check the valves. Take the side cover off of the timing gear cavity and inspect the gear. If the car ran good before the backfiring, leave the carburetor alone!!!!
 

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Did the problem exist for a long time or is this something new? Knowing that could help a lot. Do you know if it’s backfiring out of the carb or out of the tail pipe?
 
He said that it is backfiring at carb. It could be timing but he said that it was also starving for fuel, so likely a clog somewhere in the carb. The Zenith was built to be easy to fix. Just drop the bottom half with the one nut, after disconnecting the GAV, and clean all the passages and jets with spray carburetor cleaner. This is assuming good flow from the tank. While the bottom half is off turn on the fuel to check that there is not blockage at the float valve. Do that outside with a bucket and fire extinguisher handy.
 
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