Re: Anybody got the end all cure for vapor locking?

Posted by DavidM On 2022/1/4 8:08:42

I would assume that your 1929 Packard is in good running order under all normal conditions, as were my two 1929 Packards. Mine started, idled and ran well under all conditions except when it was particularly hot weather and then they sometimes vapor locked.
Vapor lock does not happen all the time, only under adverse conditions. When it happens the engine will run for a second then starve for fuel for a second and keep doing that until it either corrects itself or the engine stalls. It might happen on a long steep climb and be fine when it is back on the flat and not working so hard. It might happen in stop start conditions when there is less air flow through the radiator. It never happens in cool or cold weather.
The problem is caused by the fuel in the carburetor boiling. When that happens the fuel drawn into the engine is intermittently either normal atomised liquid or vapor from the boiling fuel. The vapor has little energy so the engine starves and misses.
The problem starts in the vacuum tank where the fuel is heated by its proximity over the exhaust manifold and from engine bay heat. The fuel will boil in the vacuum tank. I have proved this by removing the top when it has happened on my car. Even though it is boiling it flows under gravity to the carburetor where it continues to boil. The boiling fuel is drawn through the carburetor to the engine as a mixture of liquid and vapor which is what causes the erratic running. There is negligible energy in the vapor so the engine starves for fuel.
Fixing the problem is not entirely possible under worst case conditions. Everything possible must be done to reduce the temperature of the fuel and that is far from easy. A heat shield under the vacuum tank is essential. An electric pump bypassing the vacuum tank is probably the best solution as there will be far less fuel exposed to the very high engine bay temperatures so it will be cooler when it reaches the carburetor. An electric pump feeding the vacuum tank does nothing to solve the problem. Getting the fuel to the vacuum tank is not the problem.
Another solution is to raise the temperature of the boiling point of the fuel. The fuel does not have a single boiling point like water, it is a mixture with many different boiling points, the lowest being around ambient temperature on a hot day. Mixing kerosene with the fuel raises the average boiling point of the fuel mixture, it dies not change the boiling points of the fuel components, it simply provides more higher boiling point fuel. I have spoken to fuel companies who have confirmed its effectiveness and that it is not detrimental to the engine. However the fuel needs to have at least 10% kerosene to make any difference, 20% is better. Such quantities can be difficult to obtain.
Another solution, although hardly practical, is use Avgas, available at airports. Avgas has a much higher average boiling point for obvious reasons.
These comments only apply to the models with vacuum tanks for fuel delivery. The problem may be different on cars with mechanical fuel pumps,