Paul:

Your Zenith carburetors are basically fine pieces of equipment. While I have no evidence to support it, I suspect they are only suffering from either years of neglect and/or uninformed tinkering. Here is what I would do.

First, after the engine is warmed and running well, look into the air horns and see if there is any fuel dribbling from the center of the venturi. This can also cause the backfire. If so, the float in that carburetor may be too high. Go to this site for information on the Zenith rebuild process:

http://www.jaimekop.com/

Click on the carb manual link.

If the car runs pretty well after warm-up, I would check the timing to ensure it is at the factory specification; 35 degrees BTDC at 4,500 rpm with an initial setting between 3 and 13 degrees BTDC at 800 rpm. The condition of your points and plugs is immaterial at this point if it seems to otherwise run well.

If your timing checks out pretty close I think your next step should be to establish a good balance and idle speed setting for the car. This will probably fix the problem.

Remove the air filter assembly and make a temporary connection to the intake manifold from the cam breather hose if your engine is equipped with this PVC system. A length of 3/8 inch diameter rubber fuel hose will do the job so there are no vacuum leaks. Next, snug up the four base screws on each carburetor to reduce the chance of a vacuum leak. Now you are ready to balance the carburetors. Here is what I have written in the past. I think it is fairly easy to follow:

ZENITH ADJUSTMENT

1: Set the idle mixture screws at 1 1/2 turns. Fine adjusting the idle mixture can come later. The 1 1/2 turns is a good start setting for you.

2. Disconnect the linkage between both carburetors, but do not change its length, yet. It may never need to be adjusted after you get the carburetors set up, or it may need some tweaking. Later for it.

3. Make sure there is a hose connection from the breather tube to the inlet on the manifold otherwise the car won't run or idle properly. Mentioned this before, don't forget it.

4. Check the manifold vacuum line for the power brake to ensure it is tight and not leaking.

5. Identify the idle speed control dashpot on the rear carburetor. Locate the screw on the dashpot that contacts the little pad on the linkage and run in this screw until it does not touch the linkage at all. Get some clearance; try for 1/8 inch if possible. You will have to properly adjust this later to set idle speed with the auto transmission, if you have an automatic. You may not get clearance between the screw and linkage pad with the engine off, it may need to be running so vacuum will pull it back away from the linkage. This is a good time to replace the rubber vacuum hose.

6. At this point, try to get a vacuum gage on the vacuum line to the dashpot and see if you get a reading. Hopefully you should get 15”-18" of vacuum that will indicate no major vacuum leaks. (The vacuum reading will depend on your elevation; i.e. you get a lower vacuum reading in Denver than for the same engine in LA.)

Now you can concentrate on your next task which is to properly set the basic idle speed and balance. You will need some proper tools to balance Zeniths; a bonnet adaptor and a UniSyn flow meter. You can get them here:

http://www.baumtools.com/

Do a search on Zenith to locate them. I cannot say what they cost, I got mine cheap years ago in Vienna. If you have a UniSyn you can try to use a Cool Whip or similar food container to fit over the carburetor tops. There is usually a very close size that can be made to work in a pinch.

7. Attach a tachometer to the engine to measure idle speed.

8. The adaptor hood is placed on top of one carburetor and the UniSyn tool is place atop the adaptor. There is a rubber gasket on each carburetor where the air filter seals, leave it in place.

9. Press down firmly on the UniSyn/adaptor assembly to establish a seal against the gasket so the air can only flow into the carburetor through the UniSyn. While pressing down, adjust the UniSyn so the little gage ball is floating about midway in the column. Be sure the column is vertical and you adjust the UniSyn so the ball is floating near a marker line on the column. Check idle speed and adjust this carburetor to near the spec speed. This should be about 750 - 800 rpm for your car. Note the position of the gage ball in the UniSyn column. Mark it with a grease pencil if necessary.

10. Once done, move the whole adaptor/UniSyn as a unit to the adjacent carburetor. Do not readjust the UniSyn. Press down firmly, again, and observe where the ball is floating. A balanced pair of carburetors will have the ball floating at the same position on the second carburetor. If it is different, adjust the second carburetor to match the first.

11. Move back to the first carburetor. Check idle speed and position of gage ball in the UniSyn column. If idle speed changed out of spec or the gage ball is not in the same position as the adjacent carburetor, re-adjust by tweaking the carburetor adjustment slightly, up or down Do not adjust the UniSyn to alter the gage ball position.

This is a trial and error process, but once you make the first round of adjustment it all falls into place quickly at the second round of adjustment.

You now have balanced Zeniths and an idle speed of 750 - 800 rpm, and can now fine tune the idle mixture screws slightly, and evenly.

12. Slowly turn in one idle mixture screw until the engine falters and slowly turn it back out until it smoothes out again. Do the same on the adjacent carburetor. They should be pretty evenly adjusted, within 1/8 turn when you are done, and the engine idle speed should still be within the 750 - 800 rpm range. Try to keep track of the turns. That's all there is to it; nothing fancy, no secret handshake required.

13. Now re-connect the linkage between the carburetors; loosen the ball-end and adjust to fit if necessary. Once done, check the balance again to ensure it did not change with the cross linkage fitment. If so, quickly go through a final balance step with the UniSyn and you are done with balancing, no need to remove that piece of connecting linkage for this.

Next you need to set the curb idle and this is done with the dashpot you disabled at the start of the process. You need to initially adjust this dashpot to gain operating clearance between the screw head and the carburetor linkage when the engine is running at idle. You adjust this dashpot in two ways: running the screw in or out and adjusting tension on the big spring using the large nut or knurled adjusting wheel.

14. With the engine running and transmission in neutral, the dashpot screw needs to be set within .004" of the contact pad on the rear linkage. At this point you should transfer your interest to the adjustable spring on the dashpot. The spring tension needs to be set so that under vacuum the dashpot plunger assembly will withdraw the screw away from the linkage pad, increasing the clearance between the screw and the pad on the rear carburetor linkage; you want the spring just tight enough to pull back and bottom-out the plunger assembly, if possible. It may not be possible to get this piece to retract completely; it depends on your engine vacuum at idle. Your goal is to set spring tension that allows the plunger assembly to be pulled in completely with your available engine idle vacuum. But, you can get the spring too loose. If it looks like loosening it does not cause it to pull back, try tightening it a bit to the point the spring begins to extend. Again, this is somewhat trial and error. Once you have adjusted the spring so the dashpot plunger assembly has pulled back its maximum amount, go back to the screw and re-check the .004” clearance and re-set it if necessary. Use a thickness gauge to measure this.

15. Disconnect the dashpot vacuum line. This should cause the dashpot plunger and screw to extend and push against the contact pad raising the idle speed a bunch. If it passes this test, reconnect the vacuum line and then with the wheels blocked and emergency brake set, put the car in gear. The dashpot should extend enough to keep the idle speed at 600-700 rpm in gear. This prevents you from stalling the engine at stop lights and when you have the AC on. Check this further by turning on the AC (if equipped) and see if the dashpot boosts the idle up a bit more to keep the car from stalling. YOu might need to tinker this dashpot adjustment to get it optimum for your engine, but this should get you real close to perfect.

16. When you are done with this adjustment there should still be .004" clearance between the dashpot screw and the linkage pad with the engine idling in neutral. If not, re-set the clearance and re-check the idle speed in gear.

After all that you should have well balanced Zeniths that will remain so for a very long time. Install the air filter and then resist any urges to tinker with the carburetors.

Once you have reached this point you can adjust timing and replace points or plugs anytime without worrying about the carburetors.

Hope this helps,

230/8