quote:Originally posted by Bosswrench:
FOAM air cleaners are dangerous on Webers regardless of tune, but paper ones are fine. K & Ns are maybe OK. What happens in IR intake manifolds is, very strong air pulses go back and forth from carb tip to intake valve, and carburetors are dumb devices: they will meter gas in either direction. So at resonant rpms, you get a small cloud of gas hanging above the carb ram tubes (sometimes called 'reversion'). The cloud soaks foam air cleaners; now all you need is a small backfire on starting and the whole thing bursts into flame. ALWAYS carry a fire extinguisher when running Webers! I've had 2 Weber-fires and my a... was saved the first time by a passing trucker with an extinguisher; the second time I had my own... The nice foam cleaners wound up in a ditch.
Next thing that happens if you don't get the fire out pronto is, the 8 small aux venturis that are in the middle of each carb throat are made of pot-metal, which melts at low temp in the fire and runs down the short intakes to the cylinder. If an intake valve happens to be open, molten metal runs into that cylinder and hardens on the piston. Guess what happens when you try to restart the motor? Time for a couple of new pistons!
FWIW, EFI does not meter fuel in reverse so this doesn't happen; just another little-known advantage of EFI.
WOW! I can see that happening, I had lots of backfires with the Webers but thank god never a fire. I guess I lucked out. Thanks for posting.