||The factory Evolution engine's
ignition can contribute backfiring through the carburetor.
||Long duration cams with early opening
intake valves can contribute to backfiring.
|Intake manifold air leak:
||A lean condition due to an intake
manifold air leak can cause backfiring.
||An overly-lean low-speed circuit,
non-functioning accelerator pump or clogged pilot jet can contribute
ignition systems have been ìdual fireî for decades.
Virtually all stock Evolution engines, Big Twin & Sportster,
have dual fire ignitions. The exceptions are the EFI touring
bikes and the 98 & later Sportster Sport models. All Twin
Cam engines are fitted with single fire ignitions. Under normal
conditions dual fire ignitions present no problems. However,
when combined with high performance long duration cams the stock
ignition can cause premature ignition of an air/fuel mixture
entering the rear cylinder. This, in turn, results in backfiring
through the open intake valve into the intake system.
Dual fire ignitions fire front and rear cylinder spark plugs
together. One of the sparks starts combustion while the other
is ìwastedî in other cylinder which is not on its
When the rear cylinder is getting a useful spark, the front cylinderís
spark is occurring near the middle of its exhaust stroke. There
is nothing to burn in the front cylinder at this time.
However, when the front cylinder is getting its useful spark,
the rear cylinder is on its intake stroke and a combustible mixture
may be present. If that mixture is ignited by the ìwastedî
spark, then a backfire occurs as the burning mixture forces its
way past the intake valve and out through the intake manifold
Single fire ignitions can often eliminate carburetor backfiring
since they do not produce a wasted spark in the rear cylinder.
In fact, single fire ignitions can generally eliminate backfiring
in any Harley. For instance, EFI and Twin Cam engines very seldom
backfire through their intakes; both have single fire ignition
the intake valve opens the more likely the dual fire ignition
will ignite air/fuel mixture in the rear cylinder. High performance
long duration cams open the intake valves earlier than the stock
one. This is the main reason why modified Harley engines tend
to backfire through the carburetor more frequently than stock
Intake manifold air leak:
and continuing problem with Harley engines is air leaks around
the junction of the manifold and the cylinder heads. Carburetor/manifold
leaks are much less common. An air leak can cause carburetor
Other symptoms of an air leak include a slow return to idle or
an irregular idle.
Excessively lean carburetor settings can contribute to backfiring.
If the mixture is too lean, it may burn very slowly and unevenly.
This condition, in turn, may result in burning mixture remaining
in the cylinder until the beginning of the next intake stroke
when it can ignite the incomming air/fuel mixture.
A too-small or partially blocked pilot jet can bring about this
An accelerator pump adjustment that starts the pump too late
can cause this problem.
A partial vacuum in the fuel tank can reduce fuel flow and bring
about a lean condition. The common factory Harley gas cap that
incorporates a one-way valve (for emission purposes) sometimes
restricts air flow into the tank. This restriction can result
in a partial vacuum and fuel flow restriction.
refer to the Tuning Manual for information about correcting a
lean condition. The manual is available on the ìtech pageî
of this website.