15 replies to this topic

[tiger99]

Fascinating, and I dare say that the Solex was better than well-worn,obsolete Strombergs, but SUs would have been better. (Some Strombergs were a sort of imitation of the SU, with a diaphragm on the piston, instead of a carefully dimensioned, free moving leakage path, but they seemed to go wrong more often.)

Fixed jet carburettors like the Solex and even the Weber can only provide the correct mixture at a few flow rates, and it will be off, richer or weaker, in between. The Solex will do better than most, because it (most models anyway) use an emulsion tube, with lots of carefully positioned holes, to make the fuel volume more closely match the airflow, whereas most others set the mixture at a few flow rates only, by a small number of calibrated jets. The SU can of course have any needle profile that you like, and so, in theory, can be made to provide correct mixture all the way from idle to full throttle.

As some of you will know, the problem with the carburettor starts because the pressure drop in the venturi is approximately proportional to the square of the flow rate, while the fuel flow through a small jet is proportional to the pressure drop. So if the mixture was correct at 1000rpm, at 2000rpm you would have twice the air and 4 times the fuel. Very, very bad! Some engineers are surprised that carburettors ever actually worked.

The difference in fuel consumption between cars equipped with SUs and those with Zeniths or whatever was very noticeable in the 1960s and 1970s, and it was all due to the profiled needle making near-perfect mixture possible.

Being an electronics design engineer, if fuel injection had not been invented I would have suggested measuring the airflow (easy) and using a small servomotor to control the jet orifice, so a very simple electronic system would achieve accurate mixture control, with fine adjustment derived from the lambda sensor. Much cheaper and simpler than a fuel injection system, just something like an SU with a servo-controlled needle. But the industry took the difficult option....