Tuning the 390...

[Darrman]

Ok, I have been piddling around today tuning the 390, and now I have some questions.
First of all the motor is fresh rebuilt, not even a hours worth of time on it yet, only been revved to 2k to for break-in. I have rebuilt the autolite carb, and had some power valve leak problems at first, but thats fine now.
What degree timing should I set it at, I have it a 8 degrees btc now, runs good, but have mild intermittant popping coming from the exhuast. I am still running open headers, no pipes yet.
I cannot get the damn thing to run any lower than 1k rpm at idle, its a automatic, so it drops to about 800 in gear. However it will stick at 1300 if revved up. I have the fast idle screwed all the way out, nothing else there but when you push the linkage manually it will drop it down somemore, but will not go back after revved up again.
Lastly it is running a bit rich, I have been fiddling around with the mixture screws, but they appear to be doing nothing. I screwed them all the way in and the motor just kept running like I did nothing, whats up with that?
Thanks for any help!

[Dragon]

Carb problem. There is a time that rebuilding does not work. Once the throttle shafts get wore out they leak air then they need to be richened and that leads to higher rpm. It sounds like you are also leaking fuel into the bores like the float is too high or a power valve blew. Stock FE timing 6 runs better in the 10-12 range.

[My427stang]

I dont disagree with Dragon, but the first thing I would do is verify the firing order.

A crossed wire will backfire and require higher idle.

[rubiranch]

I would check for vacuum leaks around the base of the carburetor and spacer if you have one.

[70shortwide]

sounds like maybe your choke is on. is it electric? mine is and i had to mess with it to get it right, it used to stick on a lil. ran it up to about 12-1300. also if you pulled back on the throttle it would idle down. different motrs act different but this sounds like what i was experiencing.

[Dragon]

sounds like maybe your choke is on. is it electric? mine is and i had to mess with it to get it right, it used to stick on a lil. ran it up to about 12-1300. also if you pulled back on the throttle it would idle down. different motrs act different but this sounds like what i was experiencing.

If you have to pull back on the throttle you have bent throttle shafts or linkage problems. Autolites did not come stock with Electric choke and most that were converted are so rare because most people are afraid to wire anything so they leave it to the last resort .