1976 BMW R90/6 - Mikuni Carbs. back firing

Light flywheel, 2 into 1 Luftmeister race pipe, 34mm Mikuni round slides. Leaned out the jetting, using #195 mains...

Category:

Autos & Vehicles

Tags:

• motorcycles
• BMW
• R90/6
• airhead
• back firing

[arcanacelestia]

If it was "back-firing" the detonation flame would be coming out of the air intake - NOT the exhaust pipe.

[rockgin]

@arcanacelestia - not true. "Back firing" is a generic term that relates to secondary combustion at the intake or at the exhaust. In a rich condition (air/fuel or jetting mixture) or if the timing is off, the the back fire can occur in the exhaust pipe. Intake back fires can occur if there is an airleak at the intake manifold.

[kriegdouch]

haha. i'd be doin that with a group of harleys behind me with their underpowered v twins. i was reading motorcycle magazine then & now with a 76 bmw r90s as a super bike. i don't really see how the germans did it though with its opposed cylinder. respect for the bike though

[rockgin]

@kriegdouch - the balance of opposing twin cylinder design is great and with a few modifications (lightened flywheel, headwork, bigger round slide carbs., etc) they haul ass. The drive shaft take some getting used to, but they can definitely roll.

[arcanacelestia]

@rockgin In normal engine operation the exhaust valve will always be opened after the detonation has raised the cylinder pressure twelve fold, therefore secondary explosions in the exhaust results from an excessively rich mixture leaving residual unburnt fuel accumulating in the exhaust pipe. This is not called back-firing because the expanding gasflow goes forward in the normal direction of the engine's gas flow.

[arcanacelestia]

@rockgin A backfire through the carb can only occur if the exhaust valve is not properly seated/sealed, or the ignition timing is so far advanced that the fuel/air mixture in the cylinder detonates before the intake valve has fully closed, or an over heated glowing spark plug caused by too lean a mixture is causing pre-ignition before the intake valve has seated and sealed. This is called "back-firing" because the gas fires backwards against the normal forward direction of engine gas flow.