A common problem that most motorcycles face is an infiltration of air into the combustible air and fuel mixture that powers the engine. The excess air enters through a leak in the intake manifolds on either end of the carburetor, usually through a loose or damaged manifold. The imbalance within the air/fuel mixture presents several symptoms that, if left unchecked, can create poor running conditions or permanent engine damage.
Manifold Leak Symptoms
A carburetor manifold leak can display an array of symptoms at different times during the motorcycle's operation. The engine's idle speed is usually the first indicator of a manifold leak, preventing the engine from starting or creating an erratic engine idle speed that shifts between 500 to 2,000 rpm. This is normally caused by a smaller intake leak and is identified as an idle. The idle may stabilize as the rubber intake manifolds warm and expand. Carburetor manifold leaks can also cause frequent backfiring and popping or a power-loss that worsens over time.
The Effects of a Manifold Leak
Carburetor manifold leaks introduce more air into the mixture of air and fuel the engine needs to operate, creating a lean condition. The excess air generates more heat as the mixture is ignited within the engine, simultaneously raising the engine temperature and idle speed. As stated, a smaller leak will usually affect the motorcycle's performance and reliability. Larger leaks, however, can lean the air/fuel mixture to the point of over-heating the engine, and will cause severe internal damage.
Identifying Manifold Leaks
Start your motorcycle and let it idle until it is warm enough to recreate the problems that you have been experiencing. Spray a generous amount of penetrating oil onto the manifolds between the carburetors and the air box, one at a time, and listen for any changes in the engine idle speed. A slight, but sudden, decrease in the idle is an indication of a leaking manifold. If there is no change in the idle speed, spray the manifolds between the engine and the carburetors with penetrating oil. Stop the engine once you have identified the source of the intake leak.
Start with the obvious, looking for tears, cracks or holes within the manifolds' rubber bodies. Replace the damaged carburetor manifold, if there are any signs of damage. Next, check that the manifold is sealed completely around the carburetor flanges and tighten the manifold clamps with a screwdriver. Even the smallest of gaps can allow enough air into the mixture to create a problem. Finally, check that the engine-side manifolds are sealed against the engine's intake ports. Tighten the manifold mounting bolts or clamps against the engine or replace the entire manifold, if it will not seal.
- "Haynes Motorcycle Fuel Systems TechBook"; John Robison; 2000