Jeff asked about misting fuel at the carb throat.
This is a problem caused by a number of factors.
Igntion timing isn't one of them!! Aaron correctly
mentioned one of them: cam overlap. The other major
factor is the design of the exhaust system you are
using.
Many people mistakenly refer to "back pressure" in the
exhaust system as a good thing. It isn't. The
unimpeded flow of exhaust gasses is a always a good
thing. The problem with modified exhaust system is
that they can cause reversion waves. This causes a
pulse effect back up the exhasut system. If you are
running a cam with a lot of overlap you can end up
with this pulse running right through the combustion
chamber and having the effect of pushing some
vaporized fuel out of the carb. Then it's sucked back
into the carb causing an over rich mixture.
Typically this will only happen over a small amount of
the rpm range. There are many cures for this including
exhaust crossovers (used on current Mustangs for
example, often referred to as H pipes or X pipes);
exhaust venturi "cones" which sit in the exhaust
header next to the port and prevent the reversion wave
getting into the combustion chamber. Another technique
is a "brooklands" style (aka collector) exhaust
system.
By using these techniques racers can optimize
reversion characteristics to suit their needs. A great
example are Harley drag bikes. Unbaffled drag pipes
are a potential reversion nightmare. By cutting the
pipes incrementally shorter you can tune the reversion
out of the rpm ranged used. In fact one can tune the
reversion wave to help scavenge the combustion
chamber. The typical street bike is probably giving up
as much as 10hp by running those loud pipes....
HTH
Neil
Jeff wrote:
Another lister mentioned something that I've noticed
with my carbs -- the slight pushing out of air with
some fuel misting from the carb throat.
I started with a MkIV engine, but it's been highly
modified from the head on down, including a fast road
cam from Ted Schumacher (.425 lift).
Is this just a very fine tuning timing issue? Or
should I be concerned about something else?
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