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Re: Alternator and Airpump follies

To: Mike Lishego <>
Subject: Re: Alternator and Airpump follies
From: Bill Schooler <>
Date: Mon, 20 Apr 1998 18:30:40 -0400
Mike Lishego wrote:
> I had a friend with a Chevy Luv truck that did that same funny backfire
> thing.  We didn't know why, but we chalked it up to bad vacuum hose
> routing.  Do you have stock emissions plumbing, or was it replaced at some
> time?  BTW, our fix was the same as yours - pull it all off!
> "Take time to smell the roadsters."
> Michael S. Lishego
> . Mirable dictu, but the car does not backfire anymore when I shift.
> > Could those of you out there that know more about this backfiring problem
> > inform me as to what has changed so that I now no longer backfire every
> > time I let off the accelerator (as when shifting).


The reason your emission-control equipped MGB backfires upon
deceleration is due to a malfunctioning gulp valve.  Under normal
conditions, with all the smog stuff installed, your air pump is putting
fresh air into the exhaust system, so as to allow further combustion of
the spent fuel air mixture coming out the exhaust.  Normally, it just
burns.  However, when you decelerate, this creates an extraordinarily
rich mixture condition being presented to the exhaust system.  The gulp
valve, upon deceleration, opens and dumps air (also from the air
pump-note the two hoses coming from the pump) into the intake manifold
thereby leaning out the mixture. When it malfunctions (doesn't dump) the
fresh air coming from the air pump meets a fuel-rich mixture coming out
the exhaust valve and - kapow! 

Guess what.  By removing the air pump you have removed the source of all
that fresh, combustible air going into the exhaust.  No more "kapow".  

Recommend you purchase and read John Twist's Technical Manual.  Contains
an excellent explanation of what is going on in the pollution control
system - and the proper way to disable it.  I'll tell you if you'd like,
but I really do recommend purchase of Twist's booklet.  It's got a great
deal of useful information on many subjects (that keep coming up on this

*Bill Schooler      *Check the MGCC Wash DC Centre Web Page
*Woodbridge, VA     *
* *Editor of The Spark
*69 B/GT, 53 TD     *Web Page Coordinator
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