It is normal for many high performance exhaust systems to moderately
backfire or pop when the throttle is closed from mid-to-high
rpm. In fact, one should expect a well-tuned high performance
engine to "pop" and "crackle" when the throttle
is closed at 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.
Why This (normally)
the throttle valve is in the idle position, fuel does not flow
out of the main system (needle, needle jet, main jet). Fuel is
only delivered to the engine by the pilot (idle) system.
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 throat.
these conditions the pilot (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.
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
stock Harleys do not do this until open-end mufflers, such as
the popular Screamin' Eagle slip-ons, are installed. The exhaust
must be both free-flowing and have an open exit for the popping
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.
A high temperature silicone sealant, as can be found in many
auto parts stores, may be used to seal the pipe/muffler junction.
While exhaust system popping may be considered normal, it is
certainly made worse by an overly lean idle circuit.
Be sure that your carburetor's pilot jet is the correct size
and that the idle air mixture screw is correctly adjusted before
looking for other causes of popping. The procedure for adjusting
the pilot circuit is covered in the Tuning
If exhaust system popping is very loud, 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). Look for failing high tension leads (plug
wires), failing ignition coil(s) and especially switches or connectors
as possible causes.