At 06:57 PM 4/2/01 +0000, you wrote:
> Any theories as to why exhaust leaks would cause popping? Unburnt fuel
>mixing with fresh outside oxygen perhaps? -Jon-
Yep. It is normal for many high performance exhaust systems to moderately
backfire
or pop after the throttle is closed from mid-to-high rpm. The popping is a
result of the
air/fuel mixture becoming very lean when the throttle is closed and the
engine is rotating
well above idle speed. It is also necessary that the exhaust system have
rather open
mufflers.
The combined effect of the closed throttle and elevated engine rpm is to
create a fairly
strong vacuum in the intake manifold. This vacuum, in turn, causes a high
air flow rate
through the small gap formed by the throttle valve and carburetor/tb throat.
Under these conditions the idle system cannot deliver enough fuel to create
a normal,
combustible air/fuel ratio. The mixture becomes too lean to burn reliably
in the combustion
chamber. It gets sent into the exhaust system unburned and collects there.
When the odd firing of the lean mixture does occur, it is sent, still
burning, into the exhaust
system where it sometimes ignites the raw mixture that has collected ----
the exhaust then
pops or backfires.
Other Causes:
Air Leaks:
Any source of fresh air into the exhaust system can create or worsen
the conditions that bring about exhaust backfiring. The most common
entry
point is the junction of the header pipes and mufflers. Even a
small air leak
can dramatically increase the intensity or likelihood of exhaust
system backfiring.
Lean A/F Mixture:
While (high performance) exhaust system popping may be considered
normal,
it is certainly made worse by and overly lean idle circuit.
Ignition:
If exhaust system popping is very loud, very irregular and
accompanied by loss of power,
then you should suspect that the ignition system is not performing
as it should. If, for some
reason, the ignition sometimes fires at the wrong time, then
exhaust popping can become very
energetic (loud).
BTW, on second thought, I had intended to use cement muffler to plug the
small leak
but decided to use a product called Liquid Metal. It is a thick, pasty
mixture coming
in a tube, dries and cures quickly into a hardened metal which is resistant
to gasoline,
water and oil.
Bob
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