Very common for exhaust leaks to cause this kind of backfire in a 1500
Midget. Look for casting problems in the exhaust manifold, I've seen pin
holes in them. Extreme heat that the stock catalytic converter put out back
when it was functioning can warp and crack the manifold as well. There is
an antibackfire valve in line from the smog pump. Also, the threads get
stripped pretty easily where the air pump pipe bolts to the exhaust
manifold. It only takes a small leak, one that might not even change the
exhaust sound. Air sucked in during rapid deceleration allows unburned
fuel in the exhaust to explode, causing the backfire you hear. You also
need to get a ZS carb tuning tool to lean out the carb beyond what the screw
on the side can do.
David Riker
74 Midget
63 Falcon
70 Torino
http://home.pacbell.net/davriker
----- Original Message -----
From <Wetmgs at aol.com>
To: <spridgets@autox.team.net>
Sent: Tuesday, May 29, 2001 6:27 PM
Subject: '79 Midget backfire
> 79 Midget built to US & Canada emissions standards.
> Distributor is a 41697B
> Carburetor is a 45DE4 with 44A Jet needle
> Runs rich and am unable to lean it down. Jet needle at end of adjustment.
> Violent backfires on sudden declaration at or above 2500 rpm.
>
> Catalytic convertor is hollow. The air pump appears to be working. Has a
new
> air filter.
> Timed at 10 degrees TDC. EGR valve seems to be working. Choke spring and
> valve appear to be working. The deceleration diaphragm is in tact.
>
> I can find no vacuum line leaks. The oil filler cap seals well, good valve
> cover gasket. Cannot find any air leaks between the carburetor & manifold
or
> in exhaust system
>
> Marc Derenzy
> '79 Midget
> 74 MGB
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