Kevin,
I have recently subscribed so I've probably missed a bunch of your
postings, but I raced a Ford Fiesta for many years with a Weber 32/36
dfav, much like a formula ford has. If I understand correctly, when
you "stab" the gas, it spits back and has little top end power. You are
describing classic over-carburetion, something not found with variable
venturi (SU and Stromberg) carbs.
A motor is just a big pump. Air and fuel in, exhaust out. Pushing
exhaust out is resisted only by any blockages in the exhaust system
which is why we put "empty" mufflers and larger and more efficient pipes
on. However, the fuel / air mixture - without going to far into it -
must get "sucked in" and what gets sucked in must be of a certain
mixture, ie betwee, 12:1 and about 16:1. With me so far?
Classic CV (constant velocity) carbs were designed to provide a
"constant velocity" of fuel / air mixture, that is the speed at which
the mix is sucked into the cylinders is the same at low and high RPM.
This is done by making the motor think the carb is very small at idle,
yet very large at high RPM. That's why the piston moves up and down.
Also, the mixtire needle is tailored to admit a precise amount of fuel
for a given piston position, and that needle would be specifically
tailored to your spitfire's needs as produced by the factory.
The Weber is different. At about 2/3 throttle, a second throttle
buterfly opens. A bigger opening all of a sudden means a slower air
speed. Think about how fast you can pour out a milk jug versus a beer
bottle (2 faavorite liquids). Unless you changed the motor's fuel needs
- ie made it more efficient - it will such the same amount of fuel / air
but at a much slower speed through the carb. Slow "velocity" lets the
fuel settle out of the air stream into pools in the manifold, and some
raw gas may even puddle into the cylinders. This is a BAD thing for
performance since raw gas doesn't burn.
The Weber also has an accelerator pump jet which sprays (?!? squirts
?!?) a SMALL amount of raw gas into the carb venturis to compensate for
sudden need. With slow velocity, this gas too will end up in a puddle
somewhere.
Now tha I'm finished writing my book which may or may not make perfect
sense, here's what to do...
-DO NOT stab at the throttle. Opening past 2/3 at low RPM is almost
certainly a majot part of your problem. This carb is capable of 8000
rpm in a 1600 cc kent block. That'a a bigger motor making over 100 bhp
wit ha rally cam. Don't put out a candle with a fire hose.
-Look for a twin SU manifold from a 1300 cc mk 3 spitfire and fir 2 1.25
or better 1.5" carbs. Throttle will be better and so will economy but
they might be hard to find.
-Or, finish the job. Get a header and free flow exhaust system. Pull
the head off and make sure the valves are CLEAN (they're probably all
carboned up from rich fuel and oil burning) so that the maximum mixture
gets into the cylinders. Upgrade to a road or road rally cam designed
for 2000 - 6000 rpm. Then watch your spitfire fly!
By the way, I may have missed something. Is there any chance that
either your ignition or cam timing is way out of whack? If you changed
the cam and the timing is retarded from the correct spec (never trust
the marks on the cam wheel) you will have absolutely no bottom end and
the motor will pull to chainsaw type speeds. Advancing the cam (a few
degrees at most) does the opposite.
So ends the lesson. I hope I covered some new ground that will help.
And, one final word... if you're frustrated, take a breather and come
back to it fresh in a week or two and start at the start. You may be
surprised at what you find with a different mindset.
Dave Terrick
...been there, done that.
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