Miata Mailing List: August 1994, Message #15
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   From: mikef@mer-mail.ctron.com (Michael Friedman)
Subject: Re: Idle Problem/Popping the Plug
   Date: Mon, 1 Aug 1994 13:42:52 -0400

 > > My question regarding the backfire and subsequent idle problem is:
 > > What caused the backfire?
 > > It seems to me to be the root of the problem, and it is quite possible
 > > that other difficulties could arise due to backfires.  Is there any 
 > > input as to why the car backfired at all?
 > 
 > I am also interested in this, as my Miata also seems to backfire
 > when shifting from 1st-2nd and from 2nd-3rd.  It started a few
 > months ago, and I don't know why.  The timing and advance both
 > appear to be working properly.

Backfiring is frequently caused by a small exhaust leak in the 
header (manifold) or front of the exhaust pipe.  When you drop
RPM suddenly, there is a burst of unburnt fuel in the exhaust
which mixes with air siphoned in through the crack.  It can
ignite and cause an exhaust explosion which is sometimes
called a backfire.  

A "real" backfire occurs in the intake side, usually because the
cylinder fires before the intake valve is closed completely.  These
days with electronic fuel injection etc etc you don't get much
of either type of backfire because the mixture can react quickly
to sudden deceleration, and ignition timeing is rarely out of 
adjustment since the deletion of mechanical breaker points to
control timeing.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
On a related topic - my '90 has taken to dieing instead of ideling
during warmup on cool mornings (we haven't had many cool mornings
recently!).  I start the engine and drive about 3 blocks to my first
stop sign.  When I stop, the car dies!  I restart it, and the problem
does not re-occur.  Interestingly, when leaving work I do not experience
this problem, even though the distance to my first traffic light
is about the same as to the stop sign at home.  Usually, the ambient
temp is higher in the afternoon.  I've never experienced this sort
of problem in an injected car before.  Any ideas?  (I've also noted
a slight "surging" during acceleration under some circumstances,
but have not been able to nail down exactly when I does this.
Prehaps it's related?)

Mike Friedman