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zenith [G838 Wiki]



Two generic Zeniths version were used on M151s. On M151, M151A1 and early M151A2 the so called non-emission type was used. Identified by a normal idle mixture screw. Late M151A2 with emission equipment use a emission type Zenith, identified by a idle mixture screw burried in the carb body and sealed with a small cap. Both Zenith are very similar but not identical, they use an entirely different setting of the various jets. Shown in picture above is a late style 'emission' type carburetor.

The first type of Zenith carb used was the model 12848. Due to a different routing of the fuel internally, this carburetor can NOT be used in an 151A2 setup with a fuel return line.

When Ford began producing the M151A2, a different version of the Zenith was fitted, model 13841. this was also used on the non-emission AMGeneral M151A2 models. This carb was, once again, externally adjustable. Later, AMG introduced the “emission-controlled” vehicle and with it came a new carb, once again, a Zenith, with the designation of 13660. The “emission-controlled” vehicles tended to have problems with poor acceleration, need for some choke etc, so a modified version of the Zenith was introduced, designated the 13660B.

The earlier carb 12848 was no longer in the supply system so a replacement version was introduced, designated the 13660A. This latter type of carb was essentially an “emission-controlled” carb but with an externally-adjustable mixture screw.

Needs a little choke to run nicely

No real fix has been given yet, either accept that you need to pull a little choke or accept a lean spot while accelerating. Try a different carb on same engine.

Runs rich

Wrong float setting, defective pump diaphragm, wrong main jet (see rebuild kits)

Unstable idle

If carb has been rebuild properly, source could be a vacuum leak in the intake system. Check all vacuum lines, check the crankcase vent valve and if installed the fording valve for hairline cracks

Rebuild kits

Two kits are available, so called soft kit with gaskets and pump diaphgragm only and a hard kit with all soft kit contents plus various jets. Important to know however that only the hard kit includes the gaskets for the throttle and choke shafts. Especially the throttle shaft gaskets need to be almost always overhauled because they get really hot and by that very brittle over time. If they are bad, the carb will get false air and will not idle nicely. Some guys have used foam pads for those gaskets (like from mouse pads). The hard kit is available for early and late Zenith carbs and they are NOT exchangable. Having the wrong kit means you cannot use the main jet, idle jet and the pump diaphragm spring

Importance of brass angle fitting

For both the Holley and the Zenith always use the brass fitting between carburetor and fuel filter. Check the picture of the Holley carb in the Holley section. The fuel filter has a conical threads. Screwing the filter directly in the carburetor body will easily crack the carburetor. The brass fitting however has normal threads towards the carb and uses a special sealing washer and will withstand the conical threads of the metal fuel filter. The Zenith carb for the A1 has a 90degree brass fitting, the Zenith carb for the A2 has a 45degree brass fitting.

Recommendation Fuel Filter

It is recommended to use a clear plastic fuel filter in front of the original metal filter. The modern type plastic filter will not only have a much better filter effect but most important it is immediatly possible to check if any excess material like sand or rust is present in the fuel tank. Make sure you use a high quality filter out of durable plastic and install so the filter will not stressed by any excessive loads. Remember the filter is on the pressure side and close to the hot exhaust manifold.


Two good fuel filter options are:

1. Spectre / Clearview in-line fuel filter (available from Autozone or O’Reilly’s)

-Part #: 2369

-Alt Part #: 9706

It comes with fittings for 1/4, 5/16, and 3/8


2. MOELLER Ultra View In-Line Disposable Fuel Filter for 5/16 or 3/8 (obtained through a local marine parts store or online)

-5/16: Part # 033316-10


Setting the Float

1. Float upside down - gap should be 7.14mm (plus or minus 0.79mm)

2. Float drop…. at least 9.53mm (it can be more, but must not be less)

Tuning the Zenith carburetor

W. Winget's recommendation (edited): Place a piece of Mouse pad type foam material cut to fit on the outside of the throttle shaft (Air leaks trough here) A good o-ring may work as well, but I’ve had great luck with a scrap piece of the mouse pad foam.

Smooth all lines and ridges inside the throat of the carb with a dremel tool grinding them down then polishing them. (Do not increase the throat diameter of the Venturi ring (the smallest area in the passageway) as this would require re-jetting for more fuel)

On the backside of the throat I clean and polish the area with the dremel and make sure the two small holes for the idle mix and adjustment screw are open. (Do not widen them)

I smooth the leading edge of the choke and throttle plates like the leading edge of a plane wing. (Not razor sharp but ramped) I also do this to the backside slightly to ensure air trails off the edge well.

I use loctite to seat the throttle and Choke plate screws, (do not over tighten) then using the dremel, cut the excess off and smooth the ends flush with the shaft, as well as ramping the heads to allow air to flow. (Leave enough for potential disassembly in the future)

Finally I use a white tube Permatex® Form-A-Gasket® No. 2 non-hardening type (so far it’s worked fine) when mounting the assembling everything like the bowl gasket, diaphragm pump and carb base mounting gaskets (never silicon with gas, it won't work)

Now out of the 7 Carbs I disassembled from takeoff military service (never civy rebuilt) the accelerator pump diaphragm was backwards in 4 of them. Possibly from the factory…. The spring has to be on the outside, pushing the diaphragm inward. (I use the Permatex sealant for good measure as well but do not clog the hole in the lid!!!)

Once mounted and running, let her warm up, adjust the idle, then the air/fuel mixture screw (the one under the little metal plug you drilled out carefully at the base of the carb) and drop some finger nail polish in the hole to hold the setting.

Rebuilt 4 of them this way for different people, and in 5 years I’ve yet to hear of a Carb problem. Bad fuel yes, carb no.

Jet Size

To gain performance and smooth operation with the Zenith carburetor, the main jet AND the float jet must be marked with the number 30 on the face.

This jet size enriches the mixture just enough for great operation (everything else being equal and in good condition).

Jet marked 30 is .055 inch. A number 54 wire gauge size drill bit.

Jet marked 28 is .052 inch. A number 55 wire gauge size drill bit.

Purchase a number 54 wire gauge size drill bit and check the jet opening. Drill them out if the number 54 won't go through. Be careful, you don't want to bore too big an opening. You should clean the carburetor at this time. You could take this opportunity to replace all the gaskets too.

  • You can purchase the Zenith carburetor gasket kit (which includes the accelerator diaphragm) from most MV parts suppliers. Part # 2910-01-029-2796 or sometimes inventoried with a manufacturers number C181-350.

Also, if the carburetor you're working with has the idle adjustment screw capped (sealed), remove the metal cap (pry or carefully drill out) so you can then reach the adjustment screw (with a narrow blade screwdriver) to adjust the idle mixture. This will achieve a smooth idle. The mixture with the .055 jet opening will be richer; the factory pre-set (very lean) mix will result in a rough idle if you don't change it.

Performance may also be improved/smoothed by advancing the engine timing 2 or 3 degrees beyond the spec mark.

Another tuning tip from the forum, copied and pasted (contributor buttanic)

Non-emission Zenith # 12848 with adjustable idle screw:

I have had the Mutt for about 9 months. Since I bought it I have rebuilt the carburetor, set the dwell and replaced the plugs. It was running well but had a hesitation on initial application of throttle when accelerating. From what I have read it seems others have had the same issue and a slight bit of choke would reduce it, which it does to some extent. I decided that the problem lay in a lack of sufficient accelerator pump fuel delivery and a lean intermediate jet. Since the accelerator pump is vacuum operated the only solutions is to increase the pump discharge nozzle which I did, going from the stock .0250” (# 72 drill) to a .0260” (#71 drill). I also increased the intermediate jet size from .0260” (#71 drill) to .0280” (#70 drill). I left the main high speed jet at a #30. I also replaced the stock O ring on the idle tube with a -003 O ring, the original seemed too small to seal the idle jet in the housing and could have been allowing fuel to leak back into the bowl rather rather than going out the jet. This eliminated the hesitation during the initial test drive. I will see how it continues to perform before making any further changes and report back later.

Update, After several weeks of driving and there still being a very slight hesitation I again increased both the idle jet and the accelerator pump nozzle one size. The idle jet is now a #69 drill (.0290 ) and the Accelerator pump nozzle is a #70 (.0280 ). I find driveability much better, easier starting when cold and no down side.

zenith.txt · Last modified: 2014/09/14 11:10 by Horst
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