troubleshoot 1bl carb racing intermittently

[hplacecf]

hi.

newbie here.

I have a 1977 Chevrolet c10.
l6 250.
1bl carb.


looking for suggestions on how to troubleshoot the following issue that I have had since I purchased the truck couple of months back.

sometimes when I start it - engine can be warm, cold, or steaming hot - the engine races like it is in a very, very, very high idle. much higher than just a 'normal' high idle.

the way I 'fix' it is to shut it off, then restart it, and all is well.

when this happens I have looked under the hood at the carb while it is happening. I checked all of the linkages, etc. I can increase the gas as if I was pushing on the pedal all the way, or tapping it as if I wanted to get it off of a normal high idle - but, none of this does any good. only shutting it off 'resets' it.

I have sprayed the inside and outside of the carb with carb cleaner and checked that all of the linkages are not catching up anywhere.

i have never seen this before.

any ideas?

thanks!


note: I got it off of ebay, from a dealer, so no way to speak to the original owner.