Zenith Q - Do I have this right?

[JayJay]

All - reassembling the Zenith carb after cleaning (and it needed it!). Gotta question:

The throttle plate has a small notch which I presume is an air bypass if the plate gets completely closed (which shouldn't happen if the idle speed screw is adjusted properly, but I digress). Within the throttle throat the rear wall is smooth, and the front wall has a small passage that allows air to go down to the idle mixture chamber. Does this notch on the throttle plate go against the solid (rear) wall as shown in the picture below or does it go against the hole in the forward wall? It was against the rear wall when I took it apart, and it ran, but I have no assurance it was assembled correctly before me. The notch would line up with the hole if the plate was oriented so that the notch was forward.

Thanks all, once again.

[Benson]

The use of notch in throttle plate comes from one of the 2 or 3 versions of the Model B carb (I do not have documents available just now to tell you if it is B-1 or B-2 that used the notch).

Many years ago (more than 50 years ago) someone told me to use the notch on model A carbs.

I never could get the rebuilt carbs to idle correctly so I quit using the notch and they work much better.


I suggest that you get an other unmolested throttle plate.

Be sure that there is no damage to the FLAT (Angled) edge of the plate where it contacts the inside of throttle bore.

Every now and then a batch of new plates with un ground edges show up. Idle will be poor if plate does not seal correctly.

Also engine idle speed MUST be LESS than 500 rpm for the Idle Air Screw (AKA IAS) to affect the idle mixture.

If Idle Air Screw has no effect on idle mixture than Idle speed is too fast (more than 500 RPM).

I shoot for 425 to 475.

In addition if GAV affects the idle then idle speed is too fast.

When installing plate back off the throttle plate screw all the way before installing and adjusting the throttle plate. Throttle plate must not leak any air around the plate or the shaft where it goes through the casting.

Hold upper casting up to the light and adjust the plate until no light can be seen around the plate.

A small gap will work just minimize it as much as possible.

See here to find everything that you will want to know about Zeniths.

Chris Pelikan put a lot of work into this site.

http://modela.org/

[Gary WA]

Should be no notch in standard type Zenith

[ursus]

To maintain the idle upon a hard stop, some have made another modification which is to enlarge the idle port in the throat. This may have worked for that situation but it results in a fast idle that can be difficult to adjust. Lots of "fixes" have been done to Model A Zenith carbs in the past, so one should not assume they are dealing with a purely original carburetor.

[JayJay]

Thanks, all. It’s clear that this carb is far from untouched, sometime in the past someone put a helicoil insert into one of the manifold bolt holes. Other than that (and a worn throttle shaft hole I had to put a bushing in) it cleaned up nicely.

I found another Zenith carb I had kicking round and guess what, throttle plate doesn’t have a notch! So I’ll use that throttle plate and see if I can find another “unmolested” plate to replace it.

I sure wish we’d get back to having swap meets.

Stay healthy.

JayJay

[Benson]

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Originally Posted by ursus

To maintain the idle upon a hard stop, some have made another modification which is to enlarge the idle port in the throat. This may have worked for that situation but it results in a fast idle that can be difficult to adjust. Lots of "fixes" have been done to Model A Zenith carbs in the past, so one should not assume they are dealing with a purely original carburetor.

Good point, very true (see bold type above).

This damage is done many times when someone tries to drill out the BRASS plug behind this idle port in the throat.

They are careless and drill right on through into the throat destroying the SHAPE of the port.

In my book at this point the upper casting is junk.

The shape of port is important to idle operation.

When port is enlarged idle mixture is impossible to setup.