the car is supposed to backfire on the "overrun". the purpose is to burn off
the unburned gases so they don't go out the tailpipe as nasty emissions. on
the over run, ie when you let off the gas and step on the clutch, ther is a
brief period of time when the carb is still putting a good fuel charge into the
cylinders. the air pump puts a shot of air into each of the cylinders via the
air rail. this air mixes with the unburned fuel and creates a combustion event,
thus burning off the fuel. this manifest itself as a backfire. it can, and
should, occur on any over run. the higher the engien speed, the more pronounced
the backfire. at this time, the diverter valve comes into play. this avalvae
allows only so much air from teh pump to go to the cylinders via the air rail.
the rest of the air is diverted to atmosphere. too large a volume of air could
keep the combustion event from happening or damage the engine. ted
Barry Schwartz wrote:
> >When I got my '80, the air pump hose was disconnected, and when I hooked
> >it back up, I got the same backfire.
> *********************************************
> Guys (and gals)
> If you get severe backfire right when you let off the gas (with exhaust air
> injected vehicles), check the vacuum operated diverter valve. The diverter
> valve controls the air flow to the air rail entering the exhaust manifold.
> If it malfunctions, you'll get a nasty backfire when you let off the gas.
> Sometimes even between shifts if your really stepping on the accelerator.
>
> Barry Schwartz (San Diego) bschwart@pacbell.net
>
> 72 FI, V6 Spitfire (daily driver)
> 70 GT6+ (when I don't drive the Spit)
> 70 Spitfire (long term project)
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