SETTING UPDRAUGHT CARBURETTOR LEVELS

In reply to Brian Shaw’s question (PWA7C) as to whether one can measure the level of petrol in up-draught carburettors without removing them from the car, I would like to offer the following method which I dreamed up one night. I am sure others will be using it, but I certainly have not seen it documented. Here goes:

Before attacking petrol levels, check that the carburettor needle and its seat are in good order and the levers and collar on the needle allow free symmetrical movements. (See figure 1.)

YOU WILL NEED about one dozen wooden cocktail sticks (virgin); a well sharpened pencil or fine knife blade; a bright torch (preferably small) plus soft soldering facilities if altering fluid level by moving collar if necessary. The job is best done in darkish surroundings.

 

 

METHOD

Place car on level ground, remove emulsion tube, turn on petrol at tank, looking down the hole where the emulsion tube used to be you will see the petrol and a tiny reflection of yourself looking back up! The trick is to measure the reference point at the top of this “well” how far down the petrol is when it is (a) at the top of the main jet and (b) when the float chamber needle closes. (i.e. normal float chamber level). The suggested difference quoted in the Zenith Manual is 3mm.

TO MEASURE (a), flood the carburettor so that petrol drips out of the air intake then turn off the tank. Allow to equilibrate for half a minute. The petrol will now be at the level of the main jet through which it has flooded. Pass a wooden cocktail stick slowly down the “well”, keeping the tip of it centrally whilst viewing with torch held in other hand. It is very easy to see when the stick touches the centre of the meniscus as the reflected torchlight changes completely. DO NOT withdraw the stick when this happens. Your reference point is when the stick touches the centre of the meniscus. Keeping the stick steady is also easy as one can rest the hand and stick on the edge of the top of the “well” to exclude shake. Without moving the stick, mark it with a pencil or sharp blade horizontally as shown in figure 2. Repeat the procedure twice at least with same or new sticks to confirm identical depths. Measure tip to mark accurately.

TO MEASURE Normal Float Chamber Level (b) suck petrol out of “well” draining carburettor until it is below the normal float level (needle will be seen and felt to have risen). Turn on petrol and allow a few seconds to equilibrate then turn off at tank. Repeat the measurement in exactly the same way. The difference between (a) and (b) is the measurement you are after.

Bonus 1. The performance of the needle valve can be checked by repeating measurement (b) at about one-minute intervals with the petrol turned on. A rising level means leakage.

Bonus 2. Unless you alter the main jet measurement (a) will not change so write it down if you are happy with its accuracy.

Bonus 3. To compare the performance at slightly different levels I have tried weighting the float with little brass washers which I just chuck on top of the float so that the needle passes through their centres. This saves moving the collar. The effect on the level of different weight washers can be measured as above for future reference. If one has to move the collar on the needle the alteration in petrol level should theoretically be exactly the same as the movement of the collar. I have found it best to hold the needle in a vice with soft aluminium jaws and move collar against the jaw when heat allows using the jaw as a reference stop (see figure 3). Movement up the needle lowers the level but I still get this wrong. Beware moving the level too close to the main jet level. The car may flood uncontrollably when stopped facing uphill.

You will never burn rubber at a green light with an up-draught carburettor and all the above tinkering may have little benefit, but I suppose we all feel happier if we know what is going on inside, and that was the original question - wasn’t it?

Hillary Bagshaw

Solent Austin Seven Club

(With many thanks to both Hillary for a well-researched article and the PWA7C from which I copied the article for Dorset members’ enlightenment—Ed.)