It is indeed common for 'performance' carbs and exhaust to cause
backfiring during overrun conditions (eg downshift and leave your foot
off the gas). But, it's most unlikely that would cause a burnt valve,
since the engine is not under load, EGT is low, and in fact the
backfiring is caused by a cylinder not firing at all. (The only way to
get backfiring in the exhaust is for one or more cylinders to dump out
unburnt mixture, generally due to a misfire.)
My suggestion : make an offer assuming the car needs a valve job, then
pat yourself on the back when you find out it doesn't !
IMO a valve that is burnt to the point of causing backfiring in the
exhaust will also cause a rough idle (the affected cylinder won't run at
idle).
Randall
> From: "Paul J. Burr" <tigerpb@ids.net>
>
> I need the advice of the collective wisdom. I looked at a gorgeous '74 TR6
> today, and I'm considering making an offer after I take it for a test drive
> later in the week. The owner seemed quite honest about the car- he's owned
> it 20 + years. He stated that he hasn't used the car much in the past 3
> years and really doesn't have the time for it due to kids, house, etc.
> He had the car set up with the dual down draft webers and tubular exhaust
> header with a dual exhaust system. The car also has a lummenition (sp?)
> electronic ignition. The car started right up from cold, no smoke, sounded
> nice and idled smoothly.
> Cut to the chase- the owner stated he had a slight backfire upon
> downshifting. He said that his mechanic had explained that the Weber setup
> and the freeflowing exhaust had caused the mixture to go lean, burning the
> exhaust valve on the #1. This, he was told was the source of his backfire.
> So, my questions are-
> 1.) How does one diagnose a burnt exhaust valve without taking the head off.
> This guys mechanic didn't, so I wondering if this is the source of the
> problem.
> 2.) Having no experience with the Weber carbs, could this be the source of
> the trouble? Any of you guys running these carbs have problems with the
> weber downdraft setup? Do they tend to backfire out the exhaust or through
> the carb?
> 3.) Could the electronic ignition be the source? How would a sticky or non
> functioning vacuum advance impact this type of ignition?
>
> Hopefully, by Friday I'll be armed with enough info to make an informed
> decision after the test drive. Thanks in advance!
> <
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