I have a 1983 f150 and recently put an ofenhouser DP intake with a 390 holly carb on it.

Now when i drive it i have to have the idle set around 1200 rpm then when i put it into drive it stalls.

I have checked to make sure there are no vacuum leaks.

What could be the problem?

 

Despite the fact you've already checked it does indeed sound like a vacuum leak. I would unplug/cap every single vacuum line to the carb/intake and try it. If it runs right, then hook the lines up one at a time and test drive again until you find the culprit.

Guess two would be that the carb is jetted too lean.

 

Hi swmr, Is the carburetor new or used? Where there any issues before you put the 4bbl intake and carb on there?

I agree that it still sounds like a vacuum leak.

I second what Baron suggested and unplug and cap everything (vacuum advance, brake booster, PCV, etc.) and see how it does. You can drive it without those for a few test runs. Just know your brakes are going to really suck ahead of time (but will still work) and you're going to really have to push hard on them. Do NOT forget that while test driving and you'll be fine. I've had mine on the freeway with the brake booster unplugged, but wouldn't recommend much more than some tests up and down the street if you can avoid it.

For what it's worth, when I had a 390 on mine, for some tests I jetted it down so lean I could barely drive it (stock 51 jet, I had a 47 in it), but it still idled fine.

 

Also check where the dizzy vacuum advance works best either connected to a full vacuum port or a "timed" port. Some (like me) found that after the mods and upgrades the dizzy vac advance worked on full vac rather than the timed port of a stock set-up.

 

Thanks i will try the vacuum stuff tomorrow.

Its a new carb and it ran pretty good before i put the new one on.

 

Okay, just wanted to be sure there weren't any pre-existing issues.
If it was good before, and doing that now, I'd check for vacuum leaks around each of the intake runners (be sure to spray top and bottom of each one), and on all 4 sides of the carb base plate gasket(s). Also, check directly on where the throttle shafts go into the base plate to be sure there isn't a defect and a loose fit on the base plate.
A couple cans of the cheapest carb cleaner works well for checking for leaks.