What I have is a Holley 390 on the 300I6 in a 78 F250 4x4. It has the Offy DP with headers. I just got the carb back from holley so it should be all good to tune. There are two main problems I have having. I have some ideas but I would just like some other opinions.
The first problem I have is that I can't get it to idle down past 1000. Both primary and secondary blades are closed all the way. The idle speed screw is out as far as it goes. The only thing that will get it to go any lower is the idle mixture screws but that makes it way too lean. I have a vacuum gauge and a cheapo air/fuel gauge set up in my cab to give me a ballpart idea of whats going on. When I look down the barrles I don't see any fuel dripping in the venturies. I'm nut sure whats keeping the idle so high. I can get it to idle at a steady 18 on the vac gauge but the air/fuel reads real lean.
The other issue i'm having is a real bad stumble at take off and low RPM's. The accelerator pump squirts right away like it should. Right now it has an orange cam in it and i'm assuming the stock squirter. Also, when i'm cruising its running so lean the A/F gauge doesn't even pick up any signal. I'm thinking the main jets are just too small and maybe the powervalve. Everything inside is stock (unless holley changed something up on me). 51 jets and 6.5 powervalve. How does trying 53 jets and 8.5 powervalve sound? I've done some researching on here and it sounds like this would be a good place to start. The fast idle has got me stumped though.

One other question. I've been reading and it sounds like adding some head to the manifold/carb would help some. Which would be better, heated carb spacer or a heated plate on the bottom of the manifold? And where would be a good place to get them?

Thanks in advance. Looking forward to seeing how this thing runs when it's all tuned

 

Heya DM300,
I've had quite a bit of frustrations tuning the 390 for the 300 myself. Hopefully I can help get you in the right direction.

First off, before anything, have you disconnected the throttle cable from the carburetor while trying to set the idle? Last thing you want is to be screwing with carburetor settings over and over again just to find out that the throttle cable's too tight and is pulling on the throttle linkage.

In all honesty though, I think you have a big vacuum leak. If the plates are fully closed, and you're running that lean with the stock jetting (on mine, I'm down to a jet size of 47 before it started to lean out), you're getting a lot of air from somewhere else. Can you turn the idle mixture screws all the way in and kill the engine?

I'd make sure there aren't any intake/vacuum leaks before tuning further.

 

I concur. Sounds like a vacuum leak to me.
Binding throttle cable is plausable, but the rest of the info conflicts with it being as likely.

 

Before you do anymore tweaking on the carb, get heat to the manifold. Without it you will be chasing your tail trying to adjust the carb.

 

heated spacer or plate on the bottom of the intake? where can i look for one or do i need to fabricate my own?

I will check again for vac leakes but didn't find anything major last time I checked. Just a little bit around the secondary shaft. I'll check again though

 

The heated spacer is much easier to come by, since you can just get them on Ebay. The plate on the bottom is something you'll have to fabricate yourself.

That being said, no heat won't cause the problems you're describing with not being able to get the idle down, and the super lean symptoms. I've been driving my other Bronco (also a 300 w/4bbl) without any carb heat in the 25° weather here, and the only issue I'm having is a little bogging off the line.

I'm still thinking it's sucking air somewhere, or your idle mixture is way out of whack.

 

which carbs had the reverse mixture screws? there probably is a vac leak somewhere. just gotta find it.

does anyone have a picture of a bottom plate for manifold heat that they made?

 

Unless there are others, I think it's the carbs designed for emissions that have the reverse mixture screws.

Here's a picture of mine:

 

Ooh, here's mine:



You do have your vacuum advance hooked to the ported vacuum on the carb, right? If it's getting manifold vacuum it will pull your idle way high.

And recheck the carb linkages to ensure they're not binding or otherwise preventing the plates from closing. Happened to me on an Edelbrock carb, liked to went crazy trying to get the idle down until I bent the secondary linkage about .01" and freed up the primary shaft/linkage...

 

Those just plain ole steel or are they aluminum or stainless? Those elbows just anywhere on the plate? You just make your own gasket? I might just go with a heated spacer if I can find one. It took me forever to get my intake to stop leaking so I don't feel like taking it off to get it fitted up.

Ported vac? All this time I was thinking full manifold vac!! I thought I read that somewhere.

The shafts and linkages aren't binding. Both are closing all the way. The only thing I found was that the secondary shaft on the firewall side was leaking just a little bit. Thats the only little vac leak I could find, but it wasn't much.

Anyway, I got my idle set ok now. I guess the idle mixture screws were just all goofed up. They are pretty sensitive. Switching to ported vac advance will proably scew me all up now. That wouldn't affect my lean condition at acceleration and cruise would it?
When its sitting in the shop it will run good. When I rev it, it will go to the rich side on my A/F gauge, but nice as smooth. No stumble at all. As soon as I pull out and its under load its way lean and it stumbles and stutters. Is this saying i need a power valve that comes on sooner? Say an 8.5 and see how it is after that?

 

power valve is the first step in tune'n the 390 pretty much as per the holley web site and all i had to do plus the heated spacer and have been run'n just fine way under 32* F here


heat to the intake is a must with this kind of setup

 

is an 8.5 a good place to start with a 3/4 ton 4x4 or is a 9.5 more like it? It's been cold here too. In the teens for a few days at the begining of the week.

Can anyone give me a few more details on their setup for the heated manifold? Do I have to take mine off to fit it up? I don't feel like wrestling with that again.

 

Me, I just took the gasket that goes between the exhaust and intake manifold to a welder along with the two elbows. We used that gasket as a template for the base plate, the rest was just visualizing where the hoses would run. So no removing the intake here.

 

What gasket are you refering to? I have the offy DP intake and headers. They don't connect. Is it just a plate that gets bolted to in bottom of the intake with an inlet and outlet? Is there still a gasket I can get to use as a pattern?

 

You can make a pattern with a light piece of cardboard from the stock intake or exhaust manifold. Otherwise you might have to buy a gasket. They usually come with a set. I don't know if one can be obtained by itself.