<VV> Vapor lock thmiking

[Padgett]

Bob - feel free to jump in but am going to take a stabatit.

In the corvair the fuel pump is a rubber diaphragm that pulls gas in by 
suction on the upstroke and out to the carbs on the downstroke. There are 
two one way valves that enforce the directionality of the gas.

The pump is designed for low (4-5 psi) pressure operation  but relies on 
the pressure of the reduction of volume (liquid gas is incompressible) on 
the outlet valve to open it.

My posit is that when vapor lock occurs, somewhere in the line to the tank 
has gotten hot enough for the gasoline to go from liquid to gas and form a 
bubble. If the bubble is large enough when it reaches the fuel pump (which 
is hot enough to maintain the gaseous state), on the down stroke the gas 
bubble does not exert enough pressure to unseat the discharge valve (can 
only figure that the stone must create some back pressure) and since it 
expands on the upstroke, no suction occurs, no liquid is brought into the 
chamber and the pump stops pumping.

This is why pouring a cold drink on the fuel pump will cure the problem, 
phe problem is at the pump

However it does not answer the issue of why a return line, being downstream 
of the pump at the tee, would be a cure. Simple reciculation of some of the 
gasoline would not make sense because when the pump stops pumping there is 
no recirculation and conditions for VL would be maximised when the engine 
is stopped.

Therefore it would seem that the return line serves to dissipate the bubble 
rather than simply recirculate the fuel.

In the Corvair the fuel line is mostly routed in cool areas with one 
exception: the area where it enters the shroud is directly above the 
exhaust manifold so a heat shield in that area might help.

The second is a little more involved but if you *really* do not want to add 
an electric pump then I suspect that a small temperature-activated fan that 
is powered when the key is turned off and pointed at the fuel pump might 
work. (my '86 Fiero V-6 had an electric fan directed on the distributor and 
the alternator to control heat, the fuel pump was in the tank so not an 
issue. )

Padgett