Reply To: 1960 Scott 7.5 hp fuel purge?

[frankr]

I think I commented on this subject before, but here goes a recap.  As you are aware, that carb was designed to run on a chainsaw, so the saw can work in any position, even upside down.  But on a chainsaw, the gas tank is right tthere to supply a steady supply of gas.  Not so with an outboard with the tank several feet away.

Anyhoo, the diaphragm carb works much like a float feed carb.  The diaphragm chamber is air tight and as the engine sucks gas from the diaphragm chamber, it pulls the diaphragm toward the carb body, which opens the spring-loaded inlet valve, much as a float valve would be opened.  Diaphragm moves the other direction, allowing the inlet valve to close. Herte is the important difference.  The diaphragm chamber being airtight, there is no vent for an air bubble to get out, other than being pulled through the jets into the venturi.  And as the air is going through the jets into the venturi, gas is not going through, and the engine cannot run.  The only way to get the air out is pulling the starter 40 or 50 times, or purge what you can, thus the purge valve to get the air out of the line at least up to the carb entrance fitting.

If this is clear as mud, you can deduce that that system cannot tolerate air bubbles in the fuel supply.

Add to all that, the fuel pump is a lousy air pump.