(1949 3600, 261 Engine and SM 420)
My Rochester B crapped out (signs of disintegration in the accelerator pump) after a 3 y.o. rebuild. Rather than wait around for parts I decided to throw on a spare Carter YF2101S, yeah I know it's not perfect for the 261 but I got work to do! Anyway, the Carter ran great but even with the throttle closed wouldn't slow below 2500 RPM. Must be leaking vacuum somewhere but I couldn't see where.
So I put on a shiny, never been used since it's rebuild in 1970, Zenith 228 that I bought once in case I wanted to go back to a manual choke. The Zenith starts easy and idles super smooth and then dies after about 20 seconds. If I choke it it will run longer, still smooth, but will idle at 1,000 RPM for about 15 seconds, then slowly decline to 500 RPM for 15 seconds or so, then back up to 1,000 RPM, but at some point will suddenly die like it's run out of gas.
I pulled the fuel line off and put it in a bottle and its pumping gas alright (don't know what the amount should be though). Played with the idle mixture to no avail. Took the top off the carb and the bowl was about 1/2 full. The float and needle were fine and moved freely. The accelerator pump worked fine.
Tomorrow I'm getting a new fuel pump and will take the carb apart to check that the galleries are clear in the idle circuit. Any other ideas of what I should be looking for?
