Dave Lemon owner of Mazdatrix discovered a strange phenomenon.
Apparently a Weber carb on the end of tuned p-port manifold
can enter a unstable condition where the mixture obviously
is changing rapidly from way too lean to way too rich.
The engine goes maximum rough and a ton of fuel air mist
eight feet high appears over the carb intakes. We are
not sure what causes this. It could be a mechanical resonant
condition of the carb intake system shaking the float bowl
or it could be something to do with rapidly reversing airflow
confusing the venturi sensor. It happens about the tuned
RPM or peak torque RPM. One can pass through it. It seems to
be a function of the jetting. This is a new one on me.
We will know more in a couple of days.
I was pleasantly surprised to find out the 2nd gen front cover
water pump and ignition distributor fits the RX8 engine with a change
to a 2nd gen drive gear on the nose of the RX8 e-shaft.
That makes for a very simple stand alone ignition system.
--
Paul Lamar ...No rotor no motor.
Paul,
I saw this years ago when I was running Holley carbs. I do believe it is
caused by the resonance frequency at a particular rpm and runner length. On
short runner manifolds we ran inverted power valves as this reversion was
causing a very rich area. The air flowing up through the carb backwards
pulled additional fuel causing the over rich condition. The reverse power
valve closed at low manifold pressure instead of opening and allowed us to
get a better fuel mixture across the rpm spread. Way too much fun to tune.
Any system using a venturi will have the issue.
This is why I recommended using a .062 inside diameter hose 10 feet long
coiled up to attach any manifold pressure sensor. The air friction is better
at dampening the pulses so a computer doesn't get spooked.
Larry
Thanks Larry. Is this something we can do to a 51 MM Weber?
What is an "inverted power valve"? Got a sketch?
--
Paul Lamar ...No rotor no motor.
Paul,
The normal power valve on a Holley is an enrichment valve that opens at very
low manifold pressure and can be purchased to operate at a variety of
pressures. This allows you to tune beyond what can be tuned with just the
main jets. The reverse power valve just has the spring on the opposite side
of the diaphragm causing it to be open at high manifold pressure and closed
at low pressure. I think it was originally designed for use with dual 2
barrel carbs on racing VW engines, but worked well on rotaries. I cannot
remember if they were available for the Webbers or not. I will check my
Webber book tonight when I get home.
Larry
Thanks Larry. I'll pass this on to Dave and perhaps it will ring some Webber
tunning bells. Here is the dyno test data.
--
Paul Lamar ...No rotor no motor.
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