I
thought the problem with the flooding carburettor and the consequent
and damaging fire to GDP 38 was an experience now confined to history.
Sadly, this has not been the case, but fortunately I have no further
fires to report.
| The
flooding problem had gone the day after the fire, the car drove
perfectly up to the paintworks man in Chesterfield and back
a few days later. A few trips out in the car appeared to have
proved the point that the disaster was behind me. A week later,
a cold start resulted in the same problem; gushing petrol fumes
from the exhaust pipe. Since the symptoms cleared themselves
after the day of the disaster, I concluded that it would do
so on this particular day and so I took the car out for a good
drive in the hope that loads of accelerator would clear the
beast's throat. After 20 minutes the poor car failed to proceed
at a traffic light and with a high degree of embarrassment I
had to be pushed to prevent a major traffic jam. A considerable
amount of fuel had been consumed in that short journey and having
removed the spark plugs to hack off the soot, the car did fire
up with some reluctance and then it drove like a dream for the
rest of my journey. This state of compliance on the part of
the carburettor prevailed for a further week and a number of
one to two hour pleasure trips. Sorted once more and hopefully
for good, I thought. |
|
Defiance
Prevails, Headaches Develop
Come
the weekend of July 15th, The Demon returned. My frustration is
difficult to put into words. I needed help. Based on my understanding
of the problem, centreing around the needle jet in the float chamber,
a call to Fiennes restoration resulted in a new jet and seating
to replace the suspect pair I installed just 18 months ago. The
parts were with me within 24 hours and Jason Fox came down from
Chesterfield to investigate the matter. The new components were
installed and the seal was perfect. I fired up the engine, but the
problem persisted. The carburettor was removed from the engine for
further examination. Was it the ventura, the slow speed jet or the
high speed jet? These seemed to be in good condition following Jason's
rebuild of the carburettor some time ago and so the unit was reinstalled
on to the engine. I fired her up again, but no change was evident.
It
was by now almost bed time. We knew that the carburettor on GAU
15 was a fine affair and so a decision to remove it and install
it on to GDP 38 was made. It was with tears in our eyes that we
yet again witnessed the high levels of fuel in the exhaust output.
We called it a day. We could both sleep on the problem (not that
I could sleep with such a matter in my mind) and come to some sort
of rational conclusion the following morning. Essentially, the true
problem lay not with the carburettor but something which either
controlled it i.e. rods and levers or something else. It suddenly
dawned upon me that the excess fuel in the exhaust was not even
passing through the carburettor but straight into the cylinders
via the inlet manifold. The culprit had to be the Autovac. Over
the telephone, Jason agreed and gave me some instructions for further
investigation.
Carburettor:
Not Guilty
If
the current suspicion was that the Autovac was flooding fuel directly
into the inlet manifold, a tell tale sign would be the union between
the Autovac vacuum pipe and the manifold. On loosening this, the
evidence was in fact there; petrol was dripping from it. I then
removed the brass head to the Autovac. As I was undoing the cheesehead
bolts which seal and secure it, I could hear the gurgling of fuel
from the upper to lower chamber as the airtight seal was being broken
and on removing the head with its attached float and sprung-loaded
closing valve, everything fell into sharp focus. The float slurped
with petrol within. The float had been 'waterlogged' or rather 'petrollogged'
and was therefore less of a floater, more of a sinker! In consequence
it was incapable of rising sufficiently within the cylinder to close
off the inflow valve and as soon as the engine was doing its job,
the suction force generated by the inlet manifold was drawing uncontrolled
amounts of fuel into the upper chamber, filling it with the consequential
inevitability of the stuff pouring down the vacuum pipe, thus overwhelming
the manifold and then the six cylinders with a dire result.
On
close inspection of the Autovac float, it was clear that the bottom
solder seam was perforated and in a suitably angled position it
drained of its internal fluid within 20 minutes. These new facts
were reported to Jason who had a quiet sigh of relief over the telephone.
He suggested a call to Autovac Spares of Moulton, near Northampton
to ascertain if they could help and within 24 hours, thanks to Michael
Wyndham-Grice, the company owner there, I had a replacement float.
I quickly opened the parcel, switched the two floats and within
two minutes the Autovac head and unions were secure. There was then
a few minutes of suspense and a cup of tea. With some degree of
intrepidation, I turned on the fuel tap, set the quadrant controls
and fired up the engine. Bingo !...No cloud of petrol gushing from
the exhaust tail-pipe and although running a bit on the rich side,
the problem appeared to be overcome. The spark plugs were removed
and the soot brushed off and an evening out with the hood down to
the lovely village of Crich in Derbyshire (the home of The National
Tramway Museum) to meet up with section members for our annual visit
to the fish and chips restaurant there (The Cardale Fish Restaurant
in the TV series, "Peak Practice"). What a story to tell
!
Closer
Inspection For Other Matters.
Within
a few days, the Autovac head was off again for a visit to Autovac
Spares for Michael to replace the petrol inlet valve and to conduct
a general overhaul of the system, replacing parts as necessary.
I had been suffering for two years or more from a frustration caused
by what appeared to be fuel starvation manifesting itself as 'kangaroo'
power losses and surges, lasting perhaps 30 seconds, not only on
hot days but also during the winter. I never resorted to the aluminium
foil 'remedy' because I have little faith in it. I assumed an Autovac
problem but more associated with the old float possibly catching
and sticking at an angle on the cylinder sides. On removing the
float, it seemed clear that the soft solder seam had been poorly
repaired at some point in the past and was very rough. Perhaps this
was causing friction, thus preventing free excursion of the float.
Using a carefully administered fine file followed by lapping paste,
I smoothed out most of the offending protuberances. With hindsight
now, I believe I had rubbed down on a particularly thin area of
solder and created that fine perforation responsible for the less
than buoyant float described above.
A
Retrospective Analysis
How
did the petrol find its way into the float? The perforation was
minute to say the least, but with rises in temperature, both climatic
and engine-induced, expanding air would have been forced out of
the float and liquid petrol sucked in later as the Autovac and ambient
air cooled. This slow process would have been a creeping affair
over a period of many weeks until a critical mass of fuel in there
rendered the float incapable of ascending high enough to shut off
the valve on the one hand and on the other descending too far, too
early, possibly restricting the gravity-fed flow into the lower
chamber.
Why
the flooding carburettor? In my opinion, the fuel vapour/air mixture
generated by the carburettor passing into the inlet manifold system
would have been meeting an overwhelming force of liquid petrol now
evaporating partially and expanding rapidly on account of a rising
engine temperature and preventing free flow of the carburetion mixture,
so much so that I can only think of the analogy of the force of
a Tsunami. It is therefore likely that the carburetion mix was being
forced back whence it came and the pressure thus created preventing
further activity of the carburettor.
The
Autovac: Another Moving Part for Regular Maintenance.
One
thing I did learn from my visit to Michael Wyndham-Grice's workshop
in Moulton is that the Autovac is yet another item of engine support
that must be inspected and parts replaced every couple of years
or so. It might be prudent for members to remove the Autovac head
and send the thing, complete with float, to Michael for him to cast
his eye over it, strip it down and get it up to scratch.
Michael
was most helpful and generous in his dealings with me. I can recommend
him to all enthusiasts with cars reliant on an Auotovac. He has
a good stock of replacement parts.
Michael's
address and telephone are as follows:
36,
Oundle Drive
Moulton,
Northamptonshire,
UK
NN3 7DB
Tel:
01604 647181
