Re: Hot Restart Problem on 23rd Series

Posted by BH On 2008/6/16 13:48:17

In addition to vapor lock, heat can cause fuel percolation in the carburetor. This really nothing new (just forgotten knowledge) as I've seen references to the issue in shop manuals printed some 60 years ago, but fuel blends of more recent years may be more prone to this phenomenon.

Also known as a hot soak or fuel foaming, the condition occurs when fuel in the carburetor bowl gets hot emough to literally boil and turn into a foam, which spills out the bowl vents and down the throat of the intake manifold - resulting in a flooded condition.

In addition to the Reid Vapor Pressure (RVP) rating of the fuel (which is more of a problem with winter blends in recent decades), engine temperature actually rises for a period after shutdown. First and foremost, make sure your engine cooling system is in good working order - so that your engine isn't running too hot.

About 30-40 years ago, automakers further attempted to address the problem by using an insulator rather than just a gasket for the base of the carburetor. In another forum some years ago, I suggested that a V8 owner lay hands on a piece of phenolic board (such as what is used for printed circuit board), trim it to the shape of the gasket and sandwich that between two new carb base gaskets. It solved his problem. If phenolic board had not been available to him, I would have suggested a thin piece of maple wood.

Ultimately, high-pressure fuel pumps used with modern EFI systems rendered all of this moot.