[TR] TR3A fuel evaporation

[Randall]

> Question, not an answer:  Is 80 degrees F. really hot enough to cause
> vapor lock without something else?

Looks like the list must be down, as I only got a direct copy of this.

The short answer is "maybe".  The slightly longer answer is "I don't know".

First of all, we aren't talking about true "vapor lock", since carb heat
shields won't do anything to lower the temperature of fuel between the tank
and fuel pump (which is where vapor lock occurs).

Instead, the shield should help reduce the temperature of fuel in the
carburetors, both the jet and the float bowl.  These get heated directly by
radiant heat from the exhaust manifold, as well as hot air from the cooling
system and conduction from the engine.  There are several things that can
happen when the fuel gets hot, none of them are good.  Joe's shield helps
block radiant heat from the manifold, which of course depends on how hot the
manifold gets; more a function of engine state of tune than ambient
temperature.  If you've ever stopped on a hot day and tried to stick your
hand down there, you'll have a good idea of just how hot it gets!

The most severe problem is "percolation", where the fuel in the bottom of
the jet boils and pushes liquid fuel out the top of the jet (similar to the
way a percolator coffee pot works).  The liquid fuel runs out into the
intake manifold, where it can easily leave the mixture so rich that the
engine will not start (or run).

Anyway, although not every car will have problems even in high ambient
temperature, it seems clear that many do; and likely more will have trouble
with today's E10 gasoline than did before they started adding ethanol.  

Perhaps Don Elliott will repeat his story of how his TR3A ran fine, until he
made the mistake of buying a tank of US-spec fuel.

-- Randall