Zenith NDIX - How to swap the pump seal

Zenith NDIX - How to swap the pump seal
Zenith NDIX - How to replace the pump seal
On this page we will try and explain how to swap the (in?)famous seal on the pump piston of the Zenith NDIX carburettors.

The seal is the pink-ish part in the photograph here on the right.

This seal is made of leather, pressed into shape.

Should you find a seal of a different material on your piston, that would indicate that the seal has been replaced at least once before.

Because the seal is made of a natural material (leather) its effective life span is limited.

Aggressive modern fuel do the rest and before you know, the ancient seal is worn and crumbles to bits.

Then it's time to swap the seal as it is vital to the perfect operation of the carburetor and your engine.

Every time you depress the throttle pedal the piston injects a small amount of fuel in the air stream which prevents the engine from "drowning in air" and stalling.

When the seal no longer properly seals, the engine will lack this small amount of fuel and becomes almost undriveable.

The seal is the pink-ish part in the photograph here on the right

If you want to go the easy way, you could swap out the complete piston with seal.

If you want to go the easy way, you could replace the complete plunger with seal.

You can find those HERE.

But, of course, there's a price tag attached to it...


The lower-cost option is to replace just the seals.

For this purpose you can purchase the individual seals HERE.

Replacing the seals is not an easy job but it can be done by the home mechanic and is not too complicated once you get the hang of it.

On this page, we will explain how to swap them.

The lower-cost option is to replace just the seals

The tools required are a vice, a permanent marker, a metal saw, a forked pry-bar, and a hammer

The tools required are a vice, a permanent marker, a metal saw, a forked pry-bar, and a hammer.

You will also need a sturdy washer of about an inch wide.


In the coming instructions, we will always reference the photographs so please pay close attention to both the text as well as the images.

Please read the complete text several times in order to familiarize yourself with the procedure.


Here we go!

Fit the plunger upside-down in a sturdy vice, with the remainder of the seal at the top.

Pry away as much of the old seal material as you can, revealing a slot in the brass part.

We will refer to this slot as the "neck" of the plunger top.

Fit the plunger upside-down in a sturdy vice, with the remainder of the seal at the top.

Take a wide washer of a good thickness and mark the width of the "neck" of the brass part.

Take a wide washer of a good thickness and mark the width of the "neck" of the brass part.

Make sure you pick a sturdy washer as you will be exerting quite some force onto it later on.


You will now have to cut a slot in the washer, the width of the neck that you marked.

You will now have to cut a slot in the washer, the width of the neck that you marked

This is what the washer should look like after you cut the slot

This is what the washer should look like after you cut the slot.


Slide the washer onto the brass plunger top.

Slide the washer onto the brass plunger top

Make sure that the washer fits snugly onto the plunger top, as mentioned above, your will be exerting quite some force onto it and the snugger the fit, the better

Make sure that the washer fits snugly onto the plunger top.

As mentioned above, your will be exerting quite some force onto it and the snugger the fit, the better.


Now it's time to get out the pry-bar and the hammer.

Now it's time to get out the pry-bar and the hammer

Put the handle of the hammer onto the vice and lower the plunger to a convenient height in the vice

Put the handle of the hammer onto the vice and lower the plunger to a convenient height in the vice.

The objective is to create a leverage point for the pry-bar which you slide under the washer.

Try to position everything in such a manner that the exerted force is as vertical and upwards as possible.


Apply vertical, upwards, force as gentle as you can as it is easy to damage the plunger.

But... when done correctly, the plunger top will come free from the plunger stem.

Bear in mind that in most cases, the top has been in place for more than half a century so it might take some persuasion to come loose...

Applying steady force is highly favored over impart force!

Apply force as gentle as you can as it is easy to damage the plunger

Apply force as gentle as you can as it is easy to damage the plunger

You can now remove the remainder of the old seal material.

Place the new seal as shown in the picture.


Now place the plunger top back onto the stem, making sure that the splines line up perfectly and do not get damaged.

Gently tap the top home and there you have a refurbished Zenith NDIX pump plunger!

Gently tap the top home and there you have a refurbished Zenith NDIX pump plunger!

(91) Pump Piston Assembly for Zenith NDIX Carburetors

Zenith
NDIXPlunjer
9504783962551
Availability: In stock and ready to ship

(91A) Pump Piston Seal for Zenith NDIX Carburetors

Zenith
NDIXPlunjerSeal
9504785127484
Availability: In stock and ready to ship
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