| I don't think I should
| explore as to why I think it happens mainly when you
| get off the throttle.
Because the engine keeps sucking air, and with the throttle plate
closed it creates very low pressure in the intake manifold behind
the carb plate. Onfortunately the carb jets are in that region so
gas gets sucked out of them pretty vigorously. The combination of
little air from the low manifold pressure and lots of gas makes the
mixture go way rich, leaving unburned (or at least incompletely
burned) gas available in the exhaust to ignite _if_ the exhaust
is still hot enough when it comes in contact with the air. A
normal street exhaust gives it enough time to cool before contacting
the air, but racing pipes or a worn out muffler often won't.
This off-throttle richness also produces a huge spike of
emissions, especially unburned hydrocarbons, and of course it happens
on every shift with a manual gearbox. Automatics shift under power,
so they are cleaner, at least in this respect. That's why little
delay cylinders like on a screen door appeared on throttle linkages of
many sports cars with sticks in the 70s, to flatten out this spike.
It's also why in some cars, notably the 70s Corvette, the hotter engine
was available only with the automatic.
This only applies to carbs, of course. Electronic fuel injection is
smart enough to shut off the fuel flow when the throttle is closed and
the rpm is above idle.
Dave Van Horn davevh@microsoft.com
|