Time for a new furnace?

I have a very old, very large Bryant furnace (circa 1938) in my basement. Last spring, near the end of the heating season, I noticed a gassy smell when the furnace first fired.
Today I had the unit examined by a technician from a local (reputable) repair company. When we turned the furnace on, his CO meter went insane as we approached the front of the unit.
Now, the trouble is that he couldn't find the source of the "leak." It seemed to him that the burned gases weren't reaching the flue properly. It could be that there's a blockage in the heat exchanger or the exhaust pipes. Or there could be a crack in the heat exchanger itself. (It's just not possible to get a full view of the exchanger without taking part of the furnace apart.)
I'm trying to decide on options. Should I just get a new furnace? Or should I pay the furnace company a fair bundle to disassemble the unit and look for a block or leak? Will exchangers that are approaching their 70th birthday even come apart, so we can look for blockage?
If a new unit is need, what's the ballpark price in the Pittsburgh region? (The old, 1938, ratings were 200,000 BTS in and 160,000 BTUs out. But several furnace folks have told me that a new unit wouldn't need to be anywhere near as robust, because newer furnaces lose less of their heat up the chimney.)
Thanks.
----------- Brian E. Clark
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Or it could be a bird nest on the of the flue.

At that age, it is risky to try to disassemble anything. Rust, corrosion, frozen bolts,

I can't help you.
(The old, 1938, ratings were 200,000 BTS in

True. Your heating contractor can do the calculations to determine just how big a unit you need.
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On Wed, 21 Sep 2005 22:08:48 -0400, Brian E. Clark
67 years for any piece of equipment has already more than earned its keep. Replace it with a new unit, and with that, the ability to find parts to upkeep it in top working order. This also allows you to check the heating ducts and trunking for integrity and clean them. Repairing the old unit only delays the inevitable.
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How long do you expect to remain at this place? Ignoring repair costs, just the savings in energy will justify a new furnace over not too many years. This furnace may be something like 60% efficient and replacing it with a 92% efficient furnace would mean of savings of 1/3 your yearly energy cost. [Can someone give a better estimate of how much he would save on energy?]
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wrote:

Inserted.
Sixty percent efficient? It may have been that when it was installed.

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I think 60% efficient is extremely optomistic. Emphasis on extreme.
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