I am working on my first 4 bbl set up on my 66 f 100 with a t5 trans and a 300. Clifford manifold is installed and efi manifolds should be here tommorow, however, it is rough at idle and has hesitation under mid load. heavy load it runs like a top same when cruising. has a 48 jet in and mid heavy spring for secondaries. Idles at 750 with idle screw bottomed out. thanks for any help!

 

What elevation are you at?
The stock jetting for the 390 is 51. 48s may be getting a bit lean.


If the EFI manifolds aren't there yet, how are you running the engine?

 

The jets having nothing to do with the idle system. The jets will be the last thing to change and then if only absolutely necessary. Gas level in the primary bowl has quite a bit to do with idle functionality. Make sure the float is set properly and the ignition is functioning properly and go from there. Hesitation, bog, stumbling, etc., is usually rectified with a larger pump squirter and possibly a pump cam change. Do you have water heat hooked up to the intake manifold?, If not, you will end up chasing your tail trying to get the carb dialed in.

 

As already mentioned, you have to set the idle correctly.
Make sure to close the throttle plates completely or to barely expose the transfer slots and limit the rpm to 750 range. Then adjust the idle mixture screws for best idle, always using the idle speed screw to limit the rpm.
Exposed transfer slots themselves will cause hesitation.