I have a 1983 f150 with a 300 in it. I just installed a new DP offenhauser intake with a holly 390 carb. Started it right up and it is missing every 3-4 seconds. When in park it idles around 1500 but when i shift it into gear it wants to cut off, and when i actually get it going it runs great until i try to accelerate then it has about a 2 second lag then finally decides to kick in.

Any ideas into what could be causing this problem?

 

Check for vacuum leaks first. Hesitation is caused by a lean condition 90% of the time. You might try a larger accelerator pump cam to give it a bigger shot. Idle speed of 1,500 rpm is way too high, check the choke to ensure it's opening and the fast idle linkage isn't hanging.

 

x2

The poor idle and the missing makes me think you have some vac leaks.

Easiest way to check for them is to get a vacuum gauge and a couple cans of carb cleaner. Hook the vacuum gauge to direct manifold vacuum and then spray the carb cleaner all around your gasket surfaces. Especially target the six runners for the intake manifold, the carb-to-manifold gasket (or gaskets if you have a spacer), and the throttle shafts on the carb. When you see the vacuum drop / hear the RPMs change, you know you found a leak. Keep in mind there may be more than one and all need to be addressed for a solid running engine.

Here's a recent video I took of a throttle shaft leak on my Holley I took a few weeks ago as an example.

 

I checked for vacuum leeks and couldn't find any. The choke is opening completely and the fast idle is also disengaging. Any other ideas?

 

Did you try a larger accelerator pump squirter?

If you want a decent price, get them from:

WELCOME TO ALLSTATE CARBURETOR & FUEL INJECTION

Under the Parts Category on the left, they're under "Pump Nozzles".
You get a better deal if you get the "tuning kits" that come with a selection.

 

what size nozzle should i get?

 

It's different for every engine, which is why there's a wide range.
The stock is usually 25 and is probably what your carb has. It's printed on the nozzle, you can see the number stamped on it if you peek under the choke plate. It sits above and between the primary barrels.

The general consensus is between 31 and 37 for our engines. 25 is just too small.

But, the size you need depends on a number of factors, such as your gearing, auto or manual trans, etc. The exact same engine and carb would need a different squirter depending on those factors. Generally, a lower geared truck needs a higher number because the engine increases in speed faster and sucks in more fuel. Things like that.

Because of that, the only way you can find out which is the right one is to experiment and try them. Start with the lowest and work your way up. Drive each nozzle for a few days to a week to really get a feel for how your engine responds to it. Then, go on to the next one. If it bogs more when you go up in size, you know it's getting too much fuel. If it bogs less, you know you're getting closer. Narrow it down to whichever bogs the least, or not at all.

After you find the closest one, get a accelerator cam kit and fine tune it even more by trying each cam. The cams change the rate of fuel the squirter puts out. Some give it all right at the beginning, some delay it slowly over the full range, etc.

But, to reiterate, the only way to find out is to experiment. There is NO WAY anyone can just tell you which one is the one for your carb. That's why it's good to get the tuning kits and the cam kit, so you just have all of them on hand.

 

It should idle @ 550-650 rpm, so if it's idling at 1,500 something is not right. Is the idle screw backed out all the way? Can you push the throttle plates closed when it's at idle - i.e. is there enough return spring? Secondary plates are closing tight?

Is the vacuum advance hooked to the ported vacuum side of the carb? What is the base timing?

 

What happens when you turn one or both idle screws in all the way? If it keeps running, generally, there's a vacuum leak.