AUCKLAND MORRIS MINOR CAR CLUB INC
· Home · About Our Club · Club polo shirts / Badges
for Sale · Member Information · Club Library · Technical Pages · Past Events/Photos · Links · Other NZ Clubs · Minors for sale · Site Map · Series MM
· About Our Club
· Club polo shirts / Badges
· Member Information
· Club Library
· Technical Pages
· Past Events/Photos
· Other NZ Clubs
· Minors for sale
· Site Map
· Series MM
Technical Pages - FUEL SYSTEMThe SU Carburettor
S U Carburettor is a masterpiece of simple yet effective design- this is how it
It achieves this by drawing air through the carburettor throat over a small well of petrol (the jet). A constriction in the carburettor throat restricts the amount of air which can flow, creating a partial vacuum, which sucks petrol up into the air flow, creating the mixture, which is then drawn on into the combustion chambers.
The ratio of air to petrol must vary according to circumstances. As more air is drawn into the engine (as revolutions increase) then more petrol is needed to maintain the correct air/fuel ratio.
During normal running a ratio of 15 to 16 parts air to 1 of petrol (15:1 to 16:1) by weight is needed - air of course weighs very little. During cold starts a very rich mixture is needed of 1 part petrol to 1 part air (1:1) - roughly 15 times as wet as the normal running mixture. During acceleration ( such as it is with the Minor ) a richer mixture is temporarily needed.
Different carburettors deliver differing strengths of air-fuel mixture in different ways. Some, called fixed jet carburettors, have more than one jet - one for normal running, another for cold starts and a little pump to supply extra fuel when the throttle is depressed sharply. The SU carburettor achieves all this with just one jet and it's known as a variable jet or constant vacuum carburettor.
is how it works:
In the SU carburettor, the constriction in the throat which creates the vacuum is caused by the bottom part of the piston. Whenthe throttle pedal is pressed and the butterfly valve opens to allow more air into the combustion chambers, the strength of the vacuum is increased momentarily, but this pulls the piston upwards and so reduces the constriction in the throat and with it the strength of the vacuum- in this way a constant vacuum is achieved.
For more information about the type H SU carburettor, click on the following - type H carburettor, tuning carburettor, jet centring, cleaning, float chamber fuel level, needle size and position, faults
SU Carburettor Tuning
To adjust and tune the SU carburettor proceed as follows:
(1) Start and run the engine until it reaches normal operating temperature.