Don't go removing stock pieces or playing with the mixture just yet. Unburnt
fuel can get into the exhaust if it wasn't burnt in the engine... I would
check your ignition system. If this car is new to you I would first do a
tune up. points, plugs, wires, and cap. then you will know where you are
starting from. Set your timing properly etc.
The dgv carb you have has an idle adjustment screw for adjusting the idle
mixture. The other circuits, transition, main and the emulsion tubes can be
changed but you probably don't have to do that. I don't think you could set
the idle mixture high enough to cause backfire on the dgv. Other things to
look at...
Get a compression tester and test your cylinders. This can tell you if you
have a compression problem. A common cause of backfire would be a burnt
exhaust valve which leaks raw fuel/air into the exhaust on the compression
stroke. A compression test will show a problem and a vacuum test can confirm
a valve problem.
Good luck,
Chris Reichle
creichle@nsc.msmail.miami.edu
P.S. removing the airpump would probably help fix the symptom but not the
problem and that would put you among the DPOs of the world...
----------
From: mgs-owner
To: mgs
Subject: Backfiring
Date: Tuesday, December 09, 1997 8:48AM
My recently-acquired '77 Midget has a backfiring problem when the
accelerator is released suddenly and the engine is winding down (is
"overrunning" the correct term?) Not being much of a engine mechanic, I'm
not real sure where to start, and the material I can find suggests about
25 things to check. Guess what I'm looking for is, if not a short cut, at
least some prioritization.
The engine has a Weber downdraft carb and Mallory dual-point ignition. The
air pump is still installed and functional. It starts readily and runs
nicely, other than the backfiring.
I assume backfiring is caused by an accumulation of unburned fuel mixture
in the exhaust manifold which suddenly ignites? Wouldn't that suggest that
the engine is getting more fuel than can be burned normally, and wouldn't
that suggest too rich a fuel mixture? The only adjustment I can find on the
Weber carb is what I assume to be the idle mixture screw at the bottom of
the carb. How does one adjust the mixture?
There's no smog check requirement here in SC and no vehicle inspection.
Should I remove the air pump? Might it, by supplying nice fresh O2 to the
exhaust manifold, be contributing to the backfiring?
Sorry for the "rank newbie" questions.
Tom
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