In my '86 C10, with brand new 4.3l V6, I put a brand new Holley 390 on it. (Brand new motor too). I started with the old Q-Jet (this truck came from factory with non-computer 4bbl q-jet carb), but it had leaky fuel bowl wells, fixed that, ran ok for a little while, then started blowing gas up the top. Scrapped it and put the new Holley on. It starts perfect, runs great, really brings out the best in the motor, and I get 21mpg. My problem is, when I shut it down, it tends to run-on, or it stops immediatly and backfires once out the tailpipe (I have people running for cover in the parking lots). The Q-Jet did not do that, at all, never, so I don't think it's timing or ignition. Timing is set to 2btdc (stock was 0, it's a manual 3sp trans) I can crank the choke forward to stop the run-on thing, and it still starts good cold, but runs really really bad until it warms up, I live in MI, so this isn't good. If I crank the choke back, the fast idle barely kicks off, and it runs on when I shut it off. So I leave it in the middle, fast idle works as it should, and runs good while cold, but runs-on or backfires when I shut it off. Only change I made after putting it on was a 1 size up accell. pump shooter to rid a slight bog on accelleration. Any ideas on what to do here? Do I have some other problem? I hate to give it up, it runs really well, way better than the Q-jet did on this engine. BTW, it's an electric choke.