Zenith Installation Instructions

Zenith Installation instructions from John Jenkins

Please read instructions fully before beginning your installation and contact us immediately if you have any problems! We can only help you if parts and and basic settings are the same as when the carburetors were shipped. Once things have been changed, we are no longer responsible! you will also find additional information about carburetor balancing and engine tuning in an article by Jack Staggs here

Zeniths restored by 356 Carb Rescue had their float bowls and injection volumes set as per factory specifications. The injection volumes were adjusted very close to 0.25 cc for two pumps. This volume falls in the middle of the specifications for Normals and Supers and at the bottom of the specification for the 356C. Today’s fuel is more volatile so less injection volume is needed. The idling set screws were turned in such that the butterflies are open ten thousandths of an inch. The idle control screws or idle mixture screws were open 1 1/4 turns. This combination assures the engine will operate without the throttle linkage connected after you fire it up. Note: As of June 2009 these Zeniths were run on a test engine before shipping. A set received after this date would already be adjusted for idle. All you should have to do is slightly turn the idle mixture screws in/out to see if your engine idle speed improves, and slightly adjust the idle set screws for your engine.Warm the engine up first before making adjustments.

Important! Before you install the carburetors on your engine be sure the gasoline supply is free of particles and dirt. The float valves are very sensitive to particles and will not close properly which will cause the fuel to overflow in the bowls and flood your engine with gasoline. The setting of the float bowls was done with paint thinner, the specific gravity of which is very close to gasoline. This allows the float level to be set accurately and reveals any sticking float valves due to the manufacturing process or particles that have found their way into the top cover of the carburetor. Paint thinner is a lot safer, too. After the carburetor was set up correctly the paint thinner was removed, the residual blown dry, and then installed and run on the test engine. On the test engine all the idle mixture screws were checked for correct behavior, i.e., they work, and the idle set screws were set for the appropriate idle speed, e.g., 750 rpm for Normals. And, it was checked again for any leaks. FYI, if the carburetors set on the shelf for a long period of time (e.g., greater than six months) the float valve has a tendency to stick. Also, the leather on the accelerator plunger will shrink and may not push the gas into the injection nozzles. Beginning in June 2009 the float valve and the leather on the plunger were oiled to prevent these problems. If you need to swell the leather use denatured alcohol. 95 proof, not rubbing alcohol. Leave it in the alcohol for 15 minutes or so. Most often the float valve will stick because dirt or particles are introduced into the gas lines after a restoration or work has been done on the car. A gasoline filter is highly recommended in your car. Use a metal one and if you want to retain the originality of the gas lines on the engine then install it underneath the car somewhere after the gas line exits the tunnel.

Before you start you will need:

-Unisyn or Synchrometer (flow meter) -9 mm and 10 mm open end wrenches -flat blade screw driver for idle adjustment and mixture screws -12 mm box end wrench; short -(8) 8 mm nuts with 12 mm flats, not 13mm -(8) 8 mm wave washers -17 mm open end wrench -(4) red fiber washers for banjo bolts -(2) banjo bolts to connect gas lines to carb tops -(2) carburetor to manifold gaskets -more than two gallons of gas in your tank

Mount the carbs. A little past snug is enough. No need to tighten the nuts to 100 foot pounds. When you are using your 12 mm wrench to tighten the nuts be careful not to hit the idle mixture screws and bend them. The one most often bent is for number one cylinder. This is because it is directly in the path of the on coming wrench as you tighten the nut close by. Connect the gas lines with the banjo bolts. Do not try to tighten any more than is necessary. You can strip the threads. Get one "squeek" out of it and leave it alone until after the car is running. Check to see then if you need to tighten them any more. Check again after a day so. The fiber washers will compress. If a slight movement will not stop the leak then use another red fiber washer rather than over tighten the banjo bolt. Connect the throttle linkage for each carb so that they are close to even before you start. Have someone check to make sure the butterflies do not go past the full open position when the gas pedal is fully depressed. There must be a clearance of about 1 mm between the throttle stops and abutments on the carburetor bodies. Also make sure the throttle is not partially open when the gas pedal is fully depressed. Refer to the manual for the proper setting up of the linkage starting from the bell crank on the fan shroud to the gas pedal. Warning – if the linkage does not have a little play or slack in it then you will have a hard time getting the idle speed down because the butterflies will not be closing as they should.

Use the starter to begin filling up the carbs with gas. You may have to crank it for a minute or two. Once you hear the engine fire stop and check for leaks. The jet covers first and then the banjo bolts. These two areas are the most likely to leak. The jet covers were checked when the carbs were set up, but check them again to be safe. Then go down your gas line to make sure there are no leaks in your hoses or connections.

After all the leaks are repaired then start the engine. Check the banjo bolts again now that the gasoline pressure is higher. Look down the throat of the carbs and see if gasoline is leaking out of the bowl into the throat. If you see gas dripping into the throat then the float is stuck open. Shut it off. Take off the top of the carb and make sure the float is moving freely. Then check the float valve to make sure no particles are stuck in the valve. Blow it out well with compressed air. There are no other reasons for the bowl to overflow. Well, there is one more. The float can have a leak, though these were checked. If you think the float is leaking then shake it to hear gasoline in it. You can also weigh it. Each float should weigh about 5.2 grams. After all is OK then warm up the engine. Don't be afraid to use the idle set screws to keep it running.

Disconnect the throttle linkage at the carburetor levers (L/R) and leave the throttle linkage (length) adjusting nuts loose. Using your Synchrometer or Unisyn set the idle set screws such that the engine is running at about 1000 rpm and each carb is flowing equally at the same rpm. Now, one at a time, screw in the idle mixture or volume screws to just before they seat (engine runs slower) then run them back to about 1 1/2 turns. (Note: if your set of Zeniths was run on a test engine you will not need to turn them in all the way. All you need to do is turn them slightly each way to see if you can improve the idle speed for your engine. They had already worked fine on the test engine.)If the idle mixture screws do not affect the idle speed of the engine as you turn them in then check your float level. It is probably too high. Determine if the float level is correct before you continue. Check the fuel pressure. A high fuel pressure (e.g., greater than five or six psi)will force the float valve open and raise your float level. If you are not using new idle mixture screws check to see if one or more of them has any grooves cut in them from someone turning them in way too tight, or, if the needle part of the idle mixture screw is a little bent. If they are not exactly centered then you’ll have little luck getting them in the correct spot for the proper idle mixture. Turn them a little in and out again until you find the spot at which the engine is running smoothly and highest rpm. I watch the synchrometer dial. If you have a timing light that gives you rpm then good for you. Use it. Do this again for both carbs with the idle screws set so the idle is at 1000 to 1100 Rpm. Adjust each idle mixture screw in or out until the highest rpm is achieved. You will probably need to reset the idle set screws after you have obtained a higher idle speed with the idle mixture or volume screws. You need to keep the rpms down so the distributor does not advance. That really messes you up. After you have gone around the idle mixture screws a couple of times and are satisfied you have them “just right” then adjust the idle set screws so that the engine is turning at 750 to 950 rpm. Normals should be about 750, Supers and C’s 850. This is a reiterative process. Do it again. Balance the carbs with your flow meter so they are drawing air evenly. Reset to your preferred idle speed if necessary. This should be a real easy process. It only took about fifteen minutes on the test engine to dial in a set of Zeniths…..unless they have a problem. This is why they were run on a test engine, so you don’t have those elusive problems.

Connect the throttle linkage. Make sure there is no tension on the throttle rods when connected. Increase and hold the engine speed at about 1500 rpm. A friend really helps here. I place a short piece of wood against my front seat to hold the pedal steady while I do this adjustment. The manner is up to you but the rpm needs to be steady. Moving your flow meter from carb to carb, adjust the two throttle linkages so that the flow is even for both carburetors. If the flow changes as you go higher in rpm then chances are good that part of your throttle linkage is worn. Suspect the pivot ball opening on the driver's side of the cross shaft that goes across the back of the fan shroud and connects to both throttle rods. This end tends to wear out first. Keep them greased well.

If you have difficulty setting the idling mixture correctly then suspect ignition or mechanical problems. The butterflies in these Zeniths were replaced if they showed the slightest wear. The throttle spindles were also checked to be sure they were not twisted, so both butterflies will open and close at the same time. The idle mixture screws, of course, were new so they are not suspect.

When you are satisfied with the settings take a short trip around the block to clean it out. Then rev up the engine to 3000 rpm and suddenly release the throttle. If the engine dies or falters the idling mixture is too rich. Turn the idle mixture screws in a very little. E.g., an eighth of a turn. You are better to have the idle mixture a little lean rather than foul plugs over time. If the idle mixture is too lean you will hear some backfiring. As mentioned above, the injection volume has been kept low as well to avoid plug fouling.

Happy motoring. Don't hesitate to e-mail or call if you have problems. 928-204- 0507 or 619-224-3566

This is a copy of the instructions that came with the restored carburetors. These carbs had been extensively engine tested before shipping, so there was really nothing to do but bolt them on, fill them with gas, let the gas sit for a while to soak into the leather skirt of the accelerator pump and start 'er up! maybe adjust the volume control screws.