I have a 77 f150. 400, motorcraft 2150 carb. Now I have decided it has a vacuum leak. Idles really rough and has hardly any power at all (maybe partly cause it's a stock 400?). Anyways, a few things I've noticed:

hot air stove line (i think that's what it is) from the automatic choke to the intake is broke.

The EGR valve appears to be disconnected

And If I spray carb cleaner around the air horn, the idle picks up

Could any of these things be a major issue?

 

Choke Hot air feed no would not impact it.
EGR valve not being hooked up would not cause a cause a vacuum leak. But the EGR valve not sealing would.

And what exactly are you referring to as the air horn ?

 

Throw a vacuum gauge on the motor before you decide it has a vacuum leak. Post up the results of what your vacuum gauge is telling you (inches of mercury) and if the needle is moving/bouncing/tossing on the gauge during the test. Your test with the carb cleaner is a good indication that you've got a vacuum leak, but lets get some real numbers before you start messing with stuff.

You'll need a good baseline to see how much vacuum you're pulling, and how weak it is so I'd say start with the vacuum gauge and move forward from there.

He's referring to the air cleaner as the air horn.

 

If you have the stock EGR plate under the carb it may be eaten up internally.

 

Well in that case the air cleaner is not designed to be nor does it absolutely need to be air tight.

 

The choke housing does have a tiny hole into vacuum, so when the stove tube is busted or disconnected, you do have a vacuum leak. It's a very small one, but it's there, and it's large enough to cause a slightly rough idle. And of course the change in idle when you spray something into it.
But the big difference you're talking about sounds like you might have either another, larger vacuum leak, or multiples.

Might be a good time to get more precise with your spraying to see just exactly where it makes the most difference, and/or take the carb off and re-seal the base gaskets and inspect the spacer/egr plate.

Paul

 

The top of the carburetor, actually.

 

Thar carb is calibrated for that otherwise how would it draw heated air in the choke housing to help heat the choke cap...

Don't worry about that that leak it is supposed to be there.

 

Good point, thanks. It will still show effect when spraying stuff around it though, so there's that. Again, not much. But it's there.
And if spraying a little wide around the air horn Rustyolef, it's easy for some to get into the venturi and act like you found a leak. Figure you know that but wanted to point it out anyway.

Is this truck new to you and has never run well? Or did this just start running badly?
No power is relative. Yes, they were pretty anemic and after 40 years and however many thousands of miles, are likely even more so! But they're quite capable with minimal changes. Let us know more about your setup.

Thanks

Paul

 

It hasn't run very good at all since I've had it. I've replaced the fuel tank (it was NASTY), the fuel pump, and rebuilt the carb just to get it to run at all.

​​​​​​At this point, it looks like the carb/EGR plate might be causing my vacuum problem, maybe.

As for the setup, it's pretty much stock as far as I know, 2bbl 400/c6. It's just super gutless, but I really won't know for sure until I get it dialed in properly.

 

Well, that's why I said lack of power is relative with these things. They really were gutless even when almost new!
My own had about 70k miles on it when I got it if I remember, and could not get out of it's own way. Ran fantastic, but felt like it was pulling those 6500lbs around with maybe 150hp and 150lbft of torque.

In fact, it was so bad that, even with just slightly larger than stock tires and 4.10 gears (or whatever they are in Dana 60's) I used to joke that I could put the rear tires in mud, turn the front wheels hard to turn, floor the gas pedal and not break the inside tire loose.
The problem was though, that it was absolutely true!

So yeah, gutless might just be normal in your case.
The good news is that these engines are real stump-pulling monsters when slightly built up. Slightly warm cam, better intake and exhaust, and you'd be hard pressed to find a sweeter engine running a carburetor.
Just not that way from the factory at least in '79. Not sure how the earlier ones fared. I drove plenty of miles in passenger cars with 400's in them during the mid-seventies, and while none of them could ever be accused of being hot rods, they were quite capable for the vintage.

Paul