And the winner is, option 2. Fuel mixture is EXTREMELY rich, with
lots of excess fuel not burned in the cylinders pouring into the
exhaust system. This is a result of the emissions system doing
generaly what it is supposed to do while a faulty carburetor is
spewing excess fuel.
The air injection pump puts fresh air into the exhaust ports to help
complete combustion of unburned fuel (to reduce hydrocarbon
emissions). The gulp valve also introduces excess air into the
intake manifold under certain circumstances. A misbehaving gulp
valve might at times introduce too much air. Copious amounts of
fresh air introduced into the exhaust system combine with lots of
excess fuel from the faulty carburetor, and heat lights it off and
lets it burn in the exhaust system.
The burn commonly occurs in the catylitic converter, which is
supposed to run hot normally to perform exactly that function. When
there is far too much fuel available the cat-con may run red hot, and
more heat could also light up the pipe immediately after the
cat-con. All bad of course.
First order of business is to fix the faulty carburetor. Most likely
the float valve is sticking open resulting in liquid fuel pouring
into the engine. Another possibility is the choke mechanism, fuel
enrichment system intended to assist cold start, being stuck in the
full enrich condition, thereby overly enriching fuel mixture after
the engine is warmed up.
The simple answer is, if you don't want the fires of hell burning in
your exhaust system, stop pouring fuel on the fire. This is a common
problem with cars that spend a lot of time in storage. If someone
locked you up in a small room for 10 years, you might be a little
tight and cranky too.
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
At 02:27 PM 1/5/2010 -0800, Clayton Kirkwood wrote:
>I have a 79 B which I started up after 10 years.....
>
>.... The exhaust pipe started glowing bright red. Now, I've been
>told several solutions: 1) the mixture is too lean and therefore is
>burning in the exhaust, not in the cylinder; 2) the mixture is too
>rich and isn't burning completely in the cylinder, but is continuing
>to burn in the exhaust.
>....
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