I have a new strange problem with my big cam 451. Holley blue pump with regulator. Normal behavior: Idles at about 13.5:1 AFR on the gauge at a fairly steady 1200 rpm, 8" Hg. If the speed ever drops below 1000, the vacuum signal also drops and there is not enough fuel to maintain a decent idle, so it spirals in and dies. The Contour fan load kicking in will drop the idle 100 rpm, which is why I keep it at 1200 (fans off) so it won't fall off the cliff below 1000 when they kick on.
It has now started idling as above for 10 seconds or so, but then starts to actually get richer on the gauge, and by the time it reaches 12.5:1 the idle is slowing down as expected when too-rich, and as before, once it drops enough it dies in 5 more seconds. If I give it some throttle and rev it up to 2000 or 3000 for a few seconds, it clears out and will idle normally again... until the cycle repeats. I can hold it at 2000 and it won't slow down and die. The other night it died for the first time at a stoplight and I couldn't get it to restart for several minutes - also thinking it's flooding.
The wideband AFR is actually measuring O2 in the exhaust - so a misfire (e.g. plugs loading up) will cause the gauge to read falsely lean which it's not doing. That's why I think this has to be in the carb.
I've been racking my brain trying to think of anything other than a slow leak in the needle/seat causing the bowl level to rise. Fuel pressure is a steady 4.5 psi at the carb. I don't see gas pouring out of the boosters when it dies. A ruptured PV diaphragm would do it all the time, not intermittently. Likewise a piece of debris in the IAB's (this is an old 4780-2 Holley 800 DP that I converted to adjustable brass).
Any thoughts? Pulling my hair out!
thanks.