4.1 - Tracking down fuel odors ( Clark Westneat,  June 14, 2005 )

You must isolate where the fumes are coming from. There are
essentially three areas to look -- Boot , bonnet and underneath. 
I have had all 3 so here are a few suggestions.

After you run the car and at a point when it smells the worst,
simply stop and open the boot, stick your head in and take a smell.
If it is a strong odor, then this is where you start.  If you do
the same to the bonnet area and it is a strong smell than that is
where you start. You must really get into it at this point to try
to isolate it by where the smell is strongest.

If you do both and you don't get the strong aroma then you need
to check the fuel line that runs from the boot to the bonnet as
well as the tank(s). I just found that my line was broken and it
was pumping petrol into the boot where it was soaked up by the carpet.
Because it was soaked into the carpet, I never found the leak until
recently when I lifted the car and shook the line.

If you suspect the boot area, then what you need to do is simply
turn on the ignition and check for leaks.  I had two leaks on my system.
One was where the plastic line went into the tanks (I have 2 tanks on the 420)
and the other was the connectors on the SU pumps. I replaced the plastic
line with metal flare stubs into the tank and I replaced all of the lines
with new rubber in the boot.  This seems to have eliminated most of the
smell from the boot. Carefully check where the lines connect onto the SUs.
Sometimes the petrol seeps past the Stainless clamps.

In the bonnet, you will occasionally get a stuck float (or a
Saturated float) which causes the smell.  When I had this problem, I took
the carbs off and I used sand paper to clean the rod the float slides on
as well as smoothing the inside hole in the floats. I found that after 35
years the rod had gotten a bit rough and I felt this was not helping the
operation. Also check to see if the arm the float contacts is set properly. If
one of the legs is skewed to the other it will cant the operation and the
petrol will not shut off properly. With the domes and pistons out of the
carbs, you should be able to turn the ignition on, have the pumps pump and
the flow of petrol stop at the level of the jets.  If this is not happening
tha this may be your problem

The basic operation is simple: you will only get a smell if the
petrol is exposed to the air.  You must go over the entire delivery system
to see where petrol is meeting air in an unauthorized place and manner.

Addition from George Camp, June 14, 2005

Do not forget the 3.8 MKX had an extra area to watch.
There is a valve on the rear bulkhead to prevent drain back from one
pump to the other.

I am actually referring to the ''Non-return valve in the
Rear bulkhead. If you look at J32 (parts manual) p 279 item 33 you
will see what I mean. The later external pumps had built in non-
return valves so the part was eliminated but the Lucas E-2FP had no
such valve and indeed the first external pumps also had none. Without
the valve they would transfer fuel to the unused tank. I do not
have time to look it up but there was a tech. or parts bulletin
that covered the change. The point of my post is this valve is
often overlooked and is prone to leaking in its aged condition.

Paul Scott adds on June 17th, 2005:

I would like to add the following possibilities, although I
am not sure if they all will relate to the Mk10:
Banjo fittings on the carbs and fuel pump can weep, they may
need re-flattening (lapping on emery on a mirror), I have
never really been able to get the fiber O rings on these
banjo fittings to seal properly and have had to resort to
adding fuel proof gasket sealer. On the 340 and Mk2’s the
fuel filler cap should not be vented, the tank is vented
through an overflow pipe that comes out of the side of the
filler pipe and is vented out under the car, clipped to the
bottom of the petrol tank. If the cap is vented then the
fumes can find their way into the boot. The Auxiliary
Enrichment Carburetor (automatic choke) is open to the air,
the fuel level is the same as the level in the float chamber
and the petrol sits in the bottom of the air intake.
I converted to manual choke as on the E-types, which has
vastly improved but not eradicated my fume problem, starts
from cold much better though.