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3: Backfires Through Carburetor 
    
Common Causes:

Ignition: The factory Evolution engine's ignition can contribute backfiring through the carburetor.
Cam design:  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.
Carburetor jetting: An overly-lean low-speed circuit, non-functioning accelerator pump or clogged pilot jet can contribute to backfiring.


Ignition:

Harley 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 firing stroke.

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 and carburetor.

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 systems.

Cam design:

The earlier 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 engines. 

Intake manifold air leak:

A common 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 backfiring.

Other symptoms of an air leak include a slow return to idle or an irregular idle. 
 
Carburetor jetting:

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 condition.

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.

NOTE: Please refer to the Tuning Manual for information about correcting a lean condition. The manual is available on the ìtech pageî of this website.

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