[Healeys] Vapor lock gas

Michael Salter michaelsalter at gmail.com
Thu May 6 07:51:00 MDT 2021


Hi Ken, yes I think everyone with a 100 has encountered fuel delivery
issues with modern fuels. Methanol lowers the boiling point of petrol to a
point where it becomes a problem.
With the 100 there are 2 distinct issues.

Firstly, because the fuel pump and its associated lines are directly above
the exhaust pipe, hot air heats them. The fuel in the suction side of the
pump has, effectively, a lower boiling point than it would at atmospheric
pressure. This fuel tends to boil if there isn't sufficient air circulation
to keep the pump and lines cooled, as occurs while stopped in traffic on a
hot day. True vapor lock occurs because the pump will not pump  sufficient
volume of vapour to exceed the rate of its production by the boiling
process.

Secondly the carburetor float chambers and the small fuel passage to the
jet are very close to the exhaust manifold.
The manifold radiates lots of heat which heats them. Again when the car is
moving at a reasonable pace the incoming air through the grille and around
the radiator is sufficient to maintain the carburetor at a low enough
temperature to prohibit boiling but, again when stopped in traffic this
doesn't happen.

I have managed to eliminate the first problem by installing a insulating
sleeve from a modern car's AC hose over the fuel line from the tank to the
pump.

For the second I have made small heatshields from 24 gauge galvanized steel
to shield as much as possible of the carburetors from the radiated heat.
This has helped considerably but not completely eliminated the problem.

For the most part neither of these modifications are visible and I have
found that fuel delivery issues, although not completely eliminated, are
manageable.
I am considering installing a small blower and duct sourcing air from
behind the grille to see how much difference that makes but if course that
will be visible.
Hope that helps, others may have found better solutions.

M


On Mon., May 3, 2021, 2:58 p.m. Ken Fleming, <ahmg at aol.com> wrote:

> Michael , I thought you maybe have experience this ans have a solution .
> On my Bn1 which is M spec with carbs, advance distributed, LeMans Cam and
> Pistons has always suffered from what I call vapor lock of gas. Especially
> in hot weather .
> I am assuming the heat is from
> Manifold and it placement near carbs or fuel pump/ battery near exhaust
> play a role but I could be wrong . I have asked other owners and they seem
> most of them not to have same problem. I do have a heat sheild added with
> little result.
> I am at a loss as to
> What to do
> Next , but have you experienced this and have any suggestions. Are my
> assumptions
> Correct on heat causing the issue.  Could today’s quality if gas be a
> issue?
> I recall reading this was a common issue for 100’s to have vapor issues.
> Ken
>
> Sent from my iPhone
>
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