Last night I was converting a fuel selector valve to no vacuum. I came across the same failure again. There is a rubber diaphragm type check valve in the vacuum circuit. This prevents air being blown into the fuel line. It had turned to a gooey substance, similar to silicone, which had almost sealed the vacuum port closed. It is probably damaged by gas, acting as a solvent, being blown into it and causing it to degrade. The only way to see it is to remove a pressed in plastic retaining disk.

From what I have seen, this is a possible attributing factor for those having starvation problems and need to run in prime. One problem with running in prime is that the prime circuit hole diameter is small than any of the other ports. It is only about 1/8" diameter where the others are about 3/16" diameter.


For those interested in the conversion, here is the link:

http://gstwins.com/gsboard/index.php?topic=19363.0

I do another modification to the valve beyond what was posted. I drill out the center post since it is not needed anymore. Plus it should help with the fuel flow. Too bad the linked thread has not been added to the FAQ forum.