PETROL EVAPORATION - Gerald Walker
It was in the 1970s that we first experienced petrol evaporation problems with the Box Saloon. However, our old Austin Twenty never had such problems even when driven hard, for instance, flogging up Alpine passes or travelling several hundred miles in a day in very hot weather. It was therefore very disappointing to find that our "new" Twenty does suffer and we have finished several journeys with the bonnet on the back seat. Without the bonnet the car runs faultlessly, proving that underbonnet temperature is the cause.
Both Twenties have updraught carburettors fed by gravity from an Autovac on the scuttle. They have the same radiator and similar sized bonnet with no louvres. Every other Six we have seen has bonnet louvres but ours never had and we were reluctant to change this. Looking under the bonnet and pondering what to do I suddenly realised that there was one significant difference between the cars (apart from the number of cylinders). This difference is that the old four cylinder Twenty had the downpipe off the back of the exhaust manifold while the six cylinder has the downpipe at the front. This means that there is a large heat source immediately in front of the carburettor. The obvious answer is to insulate this pipe so I bought some insulation wrap, which I fitted to the vertical part of the pipe and then along as far as the carburettor.
Since then we have had one run down to Ipswich, but it was not really a hot enough day to prove anything. However, it was a hot day for the run to Mike and Gill's for the Car of the Year Show and again we had no problems, so I am feeling optimistic.
In addition to the insulation I removed the undershields between chassis and engine, to improve airflow (having spent hours making then since they were missing), and straightened out the petrol feed pipe, to eliminate any loops or high spots which could trap vapour.
The Austin Sevens have the same exhaust arrangement and might benefit. The exhaust insulation is expensive and only seems to be available in large rolls. I therefore have several yards left over which I am happy to sell to any member at £2 a metre if they want to experiment. You will just need some clips or wire to hold it in place.
