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This was a LONG time coming. I finally found the source of the exhaust noise coming from the engine. I was in the engine bay inspecting vacuum lines with the engine running when I found the leak. This motor has a stove pipe that heats up the choke. Exhaust gases cross through the intake manifold, under the carburettor. This cross over passage for exhaust gasses incorporates a pipe through which the engine pulls vacuum, taking clean air from the carb through the pipe, into the heat exchanger in the exhaust gas cross over, and up to the choke. The hot air causes the choke to close.
Frankly, this design is dumb. It does operate the choke in a cost effective way, and it does warm up the carb from cold faster. Thats all the good there is. On the bad side however, you have exhaust gases heating up your intake air, boiling fuel in your carb for vapor lock, and failed intake manifold gaskets can lead to burning the oil out of the top of the block. Oh, and it can also lead you spending a bunch of money on headers that fail to fix an exhaust leak.
Oh well, live and learn. I will block off that exhaust cross over passage in the near future. That will require I go to an electric choke, which I happen to have in hand from my new to me Olds 455 motor.
After replacing the gasket and fixing that exhaust leak, I adjusted the choke. The motor has been REALLY hard to start ever since I owned it. Turns out the choke adjustment is supposed to be held down by two tight screws, and I only had one loose one. It was very easy to adjust the choke back into spec and tighten down that one remaining screw. Much easier starting now!
Next I replaced the cracked vacuum line going to the vacuum reservoir that feeds the heating and air conditioning controls. The dash controls themselves leak vacuum like crazy, but at least I can now turn off the dash vents thus preventing cold air from constantly blowing on me in the winter time.
Finally, I installed a cheapo mechanical Harbor Freight vacuum/boost gauge. Using this gauge I was able to verify I have no serious vacuum leaks remaining. The olds 403 is supposed to pull about 18 inches of mercury at idle in park, and 14 at idle in drive. That is right where I am, so I think all the major vacuum leaks have been resolved. Pictures
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| | | | | | engine warmed up at idle, in park | | | | | engine warmed up at idle, now in drive (but not driving) | | | 