Yes, the throttle stop screw. That isn't right to have the plates closed so far. Do you have a PCV system? Open up the plates a little, re-adjust the mixture screws leaner, you want an idle speed of 600 or so.

 

A=Throttle Stop Screw
B=Mixture Screws

 

Albuq F1, If I am understand PCV systems I think the answer is yes. I have a hose running from a plastic fitting on the intake manifold to the base of the carb. Is there more to it than that?

Thanks for the help,
John

 

Can you take a picture of it, at the intake manifold? There should be a PCV valve in the hole in the manifold, can you pull it out and take a pic?

 

Sounds to me like it may be an ignition issue

 

Sorry for the delay. This is the photo pf the pcv pulled out

 

After researching your issues, My conclusion is you have the wrong carb. Holley says that the 390 with mechanical secondaries is for race only applications not street. That and not adjusting it properly (trying to set the idle speed with the idle mix screws getting them out of sync as you indicated you did for example) only compounds the problems. With the mechanical secondaries the tuning is very complex requiring changing jets, accelerator pump volume and rates and more and after all that it's still likely to stumble and/or have flat spots on the street. My suggestion is to sell that carb to someone looking to race in a class that requires that size carb, such as some bull ring NASCAR classes, and add whatever $$ necessary to buy a vacuum secondary version or better yet a much more easily tuned Edelbrock 1403 or 1404. My guess is you bought that carb off some racer who was selling it cheap. Otherwise you are just playing a losing game, akin to trying to solve a rubiks cube blindfolded.

 

I have heard that the Edelbrock is easier to tune and I have considered it and may still go that way. I like the Rubik's cube analogy. It does feel that way and I was never more than passable at that puzzle. The carb was on the truck when I bought it so I hope I have not misunderstood or misstated what I have. I think this is what it is:
https://www.holley.com/products/fuel...y/parts/0-8007
When I go on forums I see allot of people using this carb for street applications since it is close to a dual 2bbl set up. I did speak with Holley tech support. They didn't feel it was a wrong choice of carb but also did not have many suggestions. Finally, I recently found several threads on Ford Barn that describe similar situations though they all have different solutions ranging from adjusting the advance curve to jet sizes to the accelerator pump cam.
Holley 390 cfm 4 barrel stumble off idle - The Ford Barn
Meanwhile I bought a few new jets and a 7.5 power valve (Holley's suggestion). I keep reminding myself that it ran reasonably well before the old electric fuel pump went out.

John

 

You might want to check that the PCV valve is the correct one. They are calibrated based upon engine size. I don't know what the correct valve is. Maybe Ross or someone else does. I think that you could easily check this by plugging the line and seeing if your problem goes away.

If all that is correct IMHO it still leads toward the distributor. I couldn't get my Bubba distributor to be set right. I went back to the stock distributor with new a Pertronix unit and a new vacuum advance. That corrected my hesitation/sag.

 

That appears to be a PCV for a 5.0/302 Ford. It flows way too much. I had that one for a long time and switched to one for a Toyota, Fram FV306. For one thing it is made much more accurately and meters more accurately than the Ford style. It has a somewhat smaller diameter, so you need to wrap it with tape or get a different rubber grommet with a smaller hole.

With that PCV, I can see why you have the throttle plates closed down so much. It is essentially "leaking" air into the engine below the throttle plates, and the mixture screws need to be opened up much more than they should be to compensate.

 

The model you indicated has vacuum secondaries. See the instruction sheet pdf.
It's easy to determine if it has vacuum or mechanical secondaries, if you look at the pictures in the instruction sheet, you'll see the vacuum servo, it is quite obvious. Unfortunately for every holley carb in use there's a 1/2 dozen "tuning experts" making wholesale changes to them without valid reasons, experience or proper equipment, so with a used one you can never be sure what changes have been made in the past (even "rebuilders" screw them up). I'd suggest buying the book recommended by Holley: Holley Carburetors published by HP books. I would try to return the carb to the way it came out of the box, especially balancing the idle mixture screw settings. They should never be adjusted separately, or used to set the idle speed, but if adjusted (using a vacuum gauge) both should be adjusted an equal amount.

 

If I were in the position you're in right now, I would do as suggested. Block the PVC line to where you no longer have that huge air input to the manifold and then try to tune the carb. Once you get the engine running right, you can figure out your PCV system or go back to a road draft. JMHO, others will vary. I write this having had problems years ago trying to get a PCV system working on an early FE series. I do wish you luck with it.l

 

PROGRESS!

I took the carb off and cleaned it out. There was some powdery substance in the bowls and lots of carbon on the bottom of the throttle plate. I put new 51 jets in, (holley says that's what it came with) the old jets had 512 stamped on them. I replaced the power valve with a 7.5(also Holley's recommendation). I checked the accelerator pump diaphragm and it looked good (I replaced that about a year ago.) I dry adjusted the bowls and made a slight adjustment to the accelerator pump lever. I plugged the PCV at the carb. Set the idle to 600(though it still wanders). And adjusted the mixture screws. They ended at 2 turns out but the variation in vacuum pressure was minor. The vacuum varied slightly (based on idle speed) from 16" to 18". I took it out on the road and it was better but it was a little jerky like I was pumping the gas quickly. I do not yet have the new PCV but I am investigating where I can find one. I am looking for the one Ross mentioned.

 

Glad to hear you're making some progress.

A Holley "512" jet is a 51 main jet that was manufactured and tested to be within a very tight tolerance range.

 

Interesting I'll have to save those.