Date: Fri Jul 10 18:27:30 1992   
Subject: Re: Vapor lock?

>My '71 240Z (L24, SU carbs) is experiencing what I think
>is vapor lock (in one of the carbs?); on hot days (over 90)
>with hot engine, when revs get to around 3K, engine begins
>to stagger, can't accelerate.  If I hold it steady at low
>revs for a little while, or coast in gear, it goes away,
>only to return at the next run-up.

Vapor lock is when the temperature of the gasoline in the gas line 
causes the vapor pressure to exceed the line pressure.  In other
words, it boils.  boiling gas won't fill the floats, needless to say.

Does your car have the factory vaporlock backfit?  This involves an 
electric fuelpump near the tank and lots of insulation on the gas lines.  
If you do, the next thing you need to do is run a return line to the tank
so you can circulate a continuous flow of fuel from the tank.  This is
how modern cars handle the problem.  The fuel line must be steel, 
3/8 brake line will work fine.  Simply tee into the evap control vent 
at the tank.  the fuel should run from the pump, across the manifold
where each carb flows from and through a restrictor orfice to the 
return line.  Select the restrictor such that there is adequate fuel
pressure when the carbs are at maximum consumption.  measuring
the pressure while dry floats are filling is a good test.  A 1/8" 
hole is about right.

John