Ok, my son's K5 is driving me nuts. I don't think there was ANYTHING set right. It’s a 77 400 SB with lots of miles but seems generally sound. No smog anything, no EGR, no cats, no canister, nice and clean. Originally, it wouldn't even try to run. Got it running by advancing the timing about 15 degrees, and plugging several vac leaks and found idle air screws run out 6 FULL turns on one side, 5.5 on the other.. Vacume was around 10 in hg but it was running as long as I kept the choke closed. That is with all vac ports plugged including PB, PCV, vac advance, and modulator. Did compression test. #1 @ 110. 2 cylinders @ 120, the rest at 130. This is a 77 400 with high miles so, this looks reasonable, not great, but not OH MY GOD I'VE GOT TO REBUILD! Plugs are black and mostly chalky, though the engine seems to be running lean. There appears to be no oil fouling. Took off carb. Bottom plate inverted screws (going up) were sloppy loose, 2 turns to snug. Several other needed snuging. Isolator was hard and severely deformed. Looks like over tightened. Checked base which appears to be within 0.05 of flat so it should seal, but I didn’t actually use a feeler gauge to check, just visual under a straight edge. Also noticed bad but not severe wear in the throttle plate bushings. Other than that, carb seems ok, looks like a reman with a few years on it. Replaced isolator with new FelPro and vac is up to oscillating between 13 and just under 15 in hg. Now it runs much better, but still not great. Idle air screws are run all the way in, opening up has little effect. It idles rough, and I know it will never idle right with the worn butterflies, but it should be better. After warm up, it runs best with about 1/4 - 3/8” air gap on the choke. I blocked the linkage to keep it from opening past that. Now vac comes up to around 20 and it runs great. Also picks up roughly 100 rpms over an open choke plate. I’ve been over everything again and again, I can find no more leaks. Looks like the factory manifold has never been removed, so I am hoping (praying) that it’s not leaking on the bottom of the gasket. I went around the thing all over with Propane and it never settled down, there was no effect at all. At this point I’m thinking I should just replace the carb with a reman. I frankly don’t have time to rebuild right now, and the bushings are a pain. With the worn bushings, something needs to be done about the carb anyway, that that *may* fix the problem. But I don’t like taking pot shots. Does anyone have any ideas what I may have missed or should check? I haven’t done any significant engine work or diagnostic since somewhere in the late 80s… For about 10 years I joined the “don’t even change my own oil” crowd so I have forgotten allot of what I once knew. I only mention that so that you won’t assume I’ve checked the obvious, since it may have slipped my mind. But just thinking through the whole “how an engine works” thing seems to indicate there aren’t many possibilities remaining. Any suggestions/testing tips will be appreciated.