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I rebuilt the 2GC carb on my '69 S about 6 months ago and it's been running great. Decided to change the timing chain over Christmas and had the front of the engine disassembled (alternator, fuel pump, front cover, etc.). Everything went well and managed to accomplish the job with no oil leaks. After starting the car, I noticed an intermittent 'stumble' in the idle. Almost seemed like a miss. It freaked me out at first because my mind went straight to timing chain issues. The good thing about it was that it was intermittent. Cold start was fine but it would periodically stumble and hot start was difficult, which was never the case - even before the carb rebuild.
I ruled out any issues with the timing chain replacement and spent a few weeks chasing possible vacuum leaks, loose plug wires, etc. I was starting to worry I may have gotten some dirt in the carb since I had the air cleaner off during the timing chain work. Not likely but a small bit of dirt in the fuel bowl and periodically covering the jet seemed plausible. Before taking the carb off and stripping it down I decided to go to the CSM and look for any clues based on the symptoms. There's a good diagnostic matrix in the carb section and it jumped right out at me - the air vent valve.
If not for the CSM, I would have never thought to check this. I set the gap according to the CSM when I rebuilt the carb but it must have changed. I can only imagine someone who had to take their car to a shop for something like this. There's no telling how many parts would have been replaced and blind alleys chased - not to mention the money they'd spend (waste).
Well, after two minutes with a feeler gauge, the car is idling and running perfect. There's a small steel bracket over the valve to protect it as it's pretty delicate but, somehow, the clearance closed down just a little and it affected how the carb was 'breathing'. Posting this as it may help someone else in the future. Always check the CSM!
