Many of us will have experienced hot starting difficulties and cutting out in traffic in the past few years. I certainly have with Tattybogle, my 1932 RN saloon. I had blithely put everything down to that pesky ethanol and have used Ethanolmate since the Club started distributing it to members. The problem, however, has not gone away. I was therefore very interested to attend a 3-hour presentation on fuel problems with old cars. This was held by the MG Car Club and described the results of extensive testing at Manchester University by Paul Ireland on an MG XPAG engine. As much of the findings are applicable to all old engines including, of course, those of Austin Sevens, I shall try to extract what I understand to be the salient points that apply to our cars. Three major things that have been changed to petrol since our A7s chugged out of Longbridge:

Increased volatility – the real cause of the problem
The volatility of petrol has been increased to suit modern engines, but in old cars with carburettors this causes weak running, slow combustion and high under-bonnet temperatures. When we turn the engine off, or get stuck in traffic, the petrol in the carbs and pipes overheats and boils resulting in vapour lock which causes the hot start and cutting out problems. Modern engines with fuel injection do not suffer in the same way as the petrol is cooled by being continually recycled back to the tank. The Manchester University tests revealed the unexpected fact that it is hot gases escaping out of the cylinders to the carb, coupled with the heat sink of the engine block, that increases the temperature of the carburettor and fuel lines as much (if not more) than ambient under-bonnet temperatures.
So the watchword is to keep the fuel system as cool as possible and don’t rely on how Sir Herbert designed it to work back in the good old days. It is counter-productive to wrap the carbs and pipes with insulating material as this would prevent the heat ‘coming back out of the engine’ from dissipating. Far better to remember that heat rises and try to route pipes away from hot areas (not easy with the early A7’s gravity system). Increased cold air flow, baffles and carb flange insulating blocks would help, as would exhaust pipe lagging. Cooling systems must be working as efficiently as possible –although there is not much that can be done to improve a thermo-syphon system except keep the radiator and water jackets as clear as possible (inside and out). Electric fans could actually be seen as counter-productive as they suck hot air through the radiator and blow it over the engine bay at the very time you actually want cold air. One enterprising fellow has arranged his electric fan to be switchable so it can run backwards and suck hot air out of the engine bay. Does this work? He says so! I have fitted a 4-blade fan to Tattybogle but have yet to experience hot weather to see if it has made a difference.
But back to the petrol. The trick is to select the least volatile, but this is very difficult as there are actually THREE different specifications depending on the season: Winter (highest volatility) Spring/Autumn (medium) Summer (lowest).
Some have ethanol and some don’t – even the same brand and grade of fuel bought a few days apart. Unfortunately it is impossible to recommend one particular fuel as forecourts will be selling whatever their supplier decided was appropriate for the weather, and you have no way of telling what it is. If you fill up during a spell of inclement weather which is followed by a heatwave, you might be caught out with a tankful of high volatility fuel and a splutteringly reluctant Austin Seven. On the other hand, if you went away on the recent DA7C holiday in sunny Spain, you probably would not have suffered the hot start/traffic stall problem as all the petrol you would have bought there will have been of low volatility to suit the prevailing Iberian climate. Also, no two cars are the same. Even two Austin Sevens! So what works for me might not have any effect on your car. It is a matter of trial and error to see what is best for you. Volatility can be reduced by adding up to 10 % kerosene (paraffin). You can do this legally for cars built before 1956 but a licence from HM Customs and Revenue is required.

Lead was added in the 1920s to improve combustion and reduce pinking as compression ratios increased, but was phased out on environmental grounds at the end of the 20th Century. It has been shown that exhaust valve recession can occur when using unleaded petrol, but only usually on engines run at 3,000 rpm or more – so generally A7s are unaffected. If you are concerned, hardened valve seat inserts can be installed or a fuel additive used.
A few garages are still licenced to sell old-fashioned leaded petrol. We are lucky that one is Dawson Bros at Burley in the New Forest, although I understand supplies are a bit sporadic.

The ethanol which has been added to the chemical mix to improve petrol for modern cars has unwanted by-products for older engines. It attacks soldered joints, rubber pipes, seals and pump diaphragms. It also combines with moisture to cause acid corrosion to anything metal it comes into contact with.  Ethanol-proof rubber products are now available from specialist suppliers, and Burlen make a ‘Stay-Up’ carb float that is unaffected by modern fuels.  If possible the solution is to use an ethanol-free fuel, or otherwise an additive such as Ethanolmate which reduces the corrosive effect of the ethanol/petrol mix.
If a significant amount of water enters the fuel tank (from, say, washing the car rather than simply condensation over winter), the ethanol will combine with it and form a separate layer at the bottom of the tank below the petrol – this will cause rapid corrosion to the tank and no additive will prevent the effects of this ethanol/water mix.

Good news – Bad news
Finally, there is an ethanol-free fuel which is less volatile than pump petrol and performed best in the Manchester University tests. This is Sunoco Optima 98. The bad news is that it can only be purchased in 25 litre drums from the Anglo American Oil Co – but the good news is that they are just down the road in Poole!

Roger Bateman DA7C