How to Check and Adjust Fuel Level on AMAL Pre-Monobloc Carburettors

Andy Marks
2 days ago
3 min read

Correct fuel level is one of the most important factors when setting up an AMAL pre-monobloc carburettor. If the fuel height is wrong, changing jets, slides or needle positions can become a frustrating exercise. This guide explains how to check the fuel level in an AMAL Standard carburettor, identify common faults and make any adjustments required.

For a lot of later carbs you will find data on what the fuel level should be set at, usually as a distance below the top of the float bowl. You won't find any data for a pre-monobloc AMAL though. They were set up in the factory and they don't need adjusting once correct.

That being said, the monobloc was introduced in 1954 and the pre-monobloc, or 'Standard' carbs were phased out over the next few years. So that's at least 70 years for someone to mix and match parts, or bend the tangs on the top of the float.

Over the years we have had reason to check and adjust a few of these, so the following article shows you how we do it.

What Fuel Level Should an AMAL Pre-Monobloc Carburettor Have?

This is the first issue, as I have already said there is no data officially published. The usual approach is to set it just below the bottom of the pilot jet drilling, this coincides nicely with the top of the brass blanking plug on the side of the carb body, offering a useful visual reference.

Close-up of an AMAL carburettor with a white arrow pointing to a small blanking plug.

Checking Fuel Level Using a Sight Glass

The easiest way we have found of doing this is to make up a sight glass which can be screwed in to replace the blanking plug on the end of float chamber. You can then turn the fuel on and see where it stops.

As an aside it is worth letting it come up until it stops, and then keeping an eye on it for a couple of minutes. If it creeps up you either have a leaky float, or the needle in the float chamber isn't sealing properly. Both of these things will need checking and correcting to get proper running.

Common Causes of Incorrect Fuel Level

It isn't technically adjustable, so if it is wrong, it's something that a previous owner has changed in the last 70 years. Things I would check are:

The float not being attached to the needle correctly

If the ears are not properly engaged in the groove in the needle it is free to slide up and down

Brass carburettor float beside a long brass needle on a black background, isolated and still.

A leaking float

If the float has a small hole in it, it will fill up with petrol and sink, causing the fuel to continue flowing in. This can be checked by placing the float in some warm water and checking for bubbles.

Hand holding a carburettor float with a metal loop inside a white plastic cup of water.

A worn needle

If the needle is not seating properly on the bottom of the float chamber, it will be allowing fuel to leak past, raising the level in the float chamber.

Mixed or incorrect parts

Not always obvious but after over 70 years who knows what people have done. Replacement parts are readily available if anything looks incorrect or has been modified.

Adjusting the Float Level

As I have already alluded to, if the level needs adjusting there is a good chance that parts have been 'mixed and matched' or are faulty. So inspect and replace if possible. Both Burlen and Hitchcocks carry very good stocks of carb spares.

If you need to tweak it slightly you can do this in one or more of the following ways:

Add an extra fibre washer under the carb body to lower the entire float chamber
Bend the 'ears' on top of float to lift or lower the needle relative to the float
Machine a new groove in the float needle - this is drastic and, although I have seen it done, I would advise against it. If you need to alter your float level by this much you probably have something else amiss.

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