Restore Manifold Drain Tube on 120's & Su8's
An interesting feature of thirties senior cars and some 120's is the intake manifold drain tube found on the underside. Four oval flanges are con-nected by tubing to a central descending tube terminating in a check valve. This check valve is a potential source of troule, not only because an overflowing carburetor can dump a noticeable amount of gasoline on the pavement under your Packard. More importantly, a malfunctioning check valve can be responsible for carburation ills which could be hard to diagnose if one were not aware of the check valve. In theory, the weight of even a modest amount of gasoline causes a spring to compress, allowing the ball valve to drop and release the gasoline. Manifold vacuum draws the ball into its seat when the engine starts, eliminating a possible source of extra air which could mean an intermittent lean mixture. In practise, the check ball and spring eventually rust. Accumulated condensaton, gum and crud may interfere with the proper function of the drain tube by depriving it of functioning as a relief for a flooded carburetor. A stuck ball valve may cause a vacuum leak, interfering with proper carburetor adjustment. The remedy is to remove the drain tube, clean it thoroughly, and replace defective or corroded parts.
Machine shop or hardware store assortments will generally yield both a suitable ball and spring. Keep in mind that the spring must compress under less than a halfounce of weight. As the check-valve seat is brass, a very fine abrasive such as polishing compound is adequate for refacing the seat by lapping the new ball in place. For this operation, you can manipulate the ball by super-gluing it to a 10d finnish nail, removing the glue with lacquer thinner when finished. From at least 1937 onward, no spring was used. The check ball was made of aluminum, light enough to rise into the valve seat under vacuum alone. For these applications, replacement of a badly corroded ball will necessitate a "retro-retro fit" to the spring-and-ball combination used earlier. A section of surgical rubber tubing can be forced over the end of the drain tube after re-installation and connected with a vacuum guage to test the repair for vacuum leaks. An aluminum ball can be lapped in the seat with toothpaste.
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