[TR] Fuel percolation

[Terry Geiger]

I've seen percolation on several cars over the last year.  I agree with
Randall, I think it is a result of the fuel they are selling now; the
addition of the ethanol lowers the boiling point of the fuel.  I've seen
fuel literally boiling in the jets of an SU H6 equipped TR4 recently.  This
tends to happen after the car sits for about 15 to 20 minutes after shutdown
and the heat has enough time to "soak" the carb bowls and jets.  The TR4 in
question did have a heat shield and we had rerouted the fuel line away from
heat sources.  We were also running a low pressure electric fuel pump in the
boot so vapor lock in the pump wasn't the issue.  We also had a TD (SU H4,
same design as the H6 used in TR3/TR4) that was doing this recently (which
has a heat shield).  We insulated the fuel bowls and the bottom of the jets
which did help the situation but did not entirely cure it.

We haven't seen the problem on MGBs and big Healeys, but they have that nice
big heat shield with the cancer causing asbestos insulation to help keep the
carbs more insulated from the heat.

Regarding the heat shield, is there any insulation or is it a bare metal
shield?  I can see where an insulated heat shield would be work better, the
bare shield would tend to radiate the heat once it gets hot.  If you could
get your hands on some flat brake lining material and have it riveted or
bonded to the heat shield that might help.  Jet coating the exhaust manifold
I think would help.  Also, fiberglass wrap is used on exhaust components by
the racers to keep heat down under the bonnet.

Cars with fuel injection wouldn't have a problem, the fuel is under pressure
which raises the boiling point.  Also, unburned fuel is returned to the tank
which cools the fuel.  My 1982 Porsche 911 has the Bosch CIS mechanical
injection and we haven't seen any issues with the new fuel.

Terry Geiger
www.GeigerGarage.com


-----Original Message-----
From: triumphs-bounces at autox.team.net
[mailto:triumphs-bounces at autox.team.net] On Behalf Of Randall
Sent: Friday, June 10, 2011 9:25 AM
To: triumphs at autox.team.net
Subject: Re: [TR] BAck again - and with a question

> I thought vapor lock was starvation due to the fact that the fuel pump 
> can't pump vapor.

Me too.  But lots of folks confuse percolation (fuel boiling in the carbs)
with vapor lock, and it has become a common problem due to the witch's brew
they sell for gasoline these days.

And even just high underhood temperatures can cause mixture problems,
especially if your mixture is already on the rich side.

Tell us more about the heat shield.  Is it between the carbs & exhaust?  Did
you set the mixture on a hot day?

-- Randall