David Parks wrote:
Here is a problem with my spitfire that I have spent much time restoring.
SOMETIMES, mostly after driving for awhile, I get missing at high
RPM under moderate to high acceleration load. When this happens I
notice that the plastic fuel filter that is mounted between the supply and
the fuel pump, is almost empty of fuel. If I tap on the filter while
the engine is running the missing problem goes away.
I have checked all the connections for tightness, and when the
filter looks empty if you remove it where the imput goes into the pump
a substancial amount of fuel comes out the line even though the
filter looks emplty. I am going to try another larger filter. But my
question is? With new hoses, fuel line, clamps, fuel pump etc. where is
this air coming from in the filter.
Any help would be very appreciated. When the filter is full the car
runs great. When it is empty it runs very poorly.
++++++++++++++
David - I have a similar problem with my Mk I Spit. I assume yours has the
dual SU HS2 carbs, too? I can think of a couple of possibilities. (1)
Fuel percolation due to excessive underhood heat (2) bad fuel filter or (3)
cracked/split diaphram in the fuel pump letting air into the lines.
As to item (1), it would be consistent with my own observations in my Spit
where it starts missing as it gets hotter under the hood under mid to full
throttle in the upper rev ranges. I've thought about fabricating some
aluminum heat shields for the carbs, but I am not sure that will solve it.
Frankly, I can't believe the car would need heat shields today if it did
not need them 33 years ago. The other option is to re-route fuel lines and
wrap them in foil to reflect as much heat as possible. Also, check your
timing. Barry Schwartz was a great help to me when he suggested I advance
the timing a little to decrease heat build up in the engine - worked great
in terms of the coolant temp., but I still have the high rpm missing at
part throttle to deal with, although no where near as bad as it was before.
As to (2), have you tried some other fuel filters? Maybe the filter you
are using has a restriction in the intake tube or output tube that you
can't see. Have you tried blowing through the filter to be sure it is not
clogged somewhere not obvious to the naked eye?
As to (3), this is the least likely, but worth checking. I don't know if a
cracked diaphram would allow air to enter the input side, but hey, it's a
WAG and just might be right!
Good luck, and let us know the solution if you find it - maybe it'll help
both of us.
Ross D. Vincenti - Costa Mesa, CA
64 Spitfire 4
64 Porsche 356C Coupe
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