Low idle on hot engines

[KShortell]

OK, guys, I've got the same thing happening on a pair of GM engines.

Engine #1 (Suburban): 1986 350 SBC with HEI and 4-bbl. stock Rochester Quadrajet (electric choke).

Engine #2 (Bayliner): 1987 181/3.0L GM I-4 with Delco points distributor and 2-bbl. Rochester carb (mechanical manifold heat choke).


After running under load at speed (say 2,500 rpm+?) on the highway or cruising for at least 20 minutes and then coming back to idle (say, at the end of an Interstate off-ramp), I'll get a low idle problem. The choke light will come on in the Suburban. It also seems like the choke unloads too quickly in the Sub-- it'll hesitate at a stoplight after a mile or two of driving, but will be fine after that. No stumbling at startup or when warm. I've had no driveability problems on the open road and no lack of power in either engine.

The 181/3.0L will occasionally stall out when at full temp.

This used to be a really rare, once in a blue moon, affair for the Sub over the last 10 years. It's been more frequent lately. I actually had a shop do some work for me a couple of months ago, putting in a new electric choke. I had it back in the shop last week and it is even a little bit worse now.

Fortunately, there's no charge for last week's work. To be honest, they ought to have been ashamed to charge me anything if they had. Something didn't sound right so I popped the hood-- long story short-- I had to take it back to get my air cleaner lid! So much for me farming out work because I didn't have the time-- this is taking more time to sort out than if I'd have done it myself...

I'm starting to wonder if both these carbs are coming up for rebuilds. To be honest, it's been a long time since I've rebuilt a carb. Each time, they were Holleys on Slant 6's and I was pretty much helping Dad. I can't really remember the symptoms back then. So, I'm having a little trip down memory lane here.

Any thoughts as to why the low idle after continued running under load? Help refresh my memory on the typical symtoms of a carb getting ready for a rebuild?

THANKS!