Ok, I have my trucks timing set to 3º BTDC. I am running an Edelbrock 390 Performer intake with an Edelbrock 1407 carb. The truck fired up real easy but it is still back firing through the carb and exhaust. Scared the hell outta my wife when a flame came out the carb! HA It is really bad when I try to drive it around. I'm trying to figure out how the vacuum lines are supposed to be ran and maybe a suggestion for my mixture screws to be set at.

 

3 is WAY too low - is the vacuum advance connected to "ported" or "manifold" vacuum?

In other words, is the vacuum advance getting vacuum at idle, or not?

Try 8 degrees. The higher the advance, the more manifold vacuum is created, so you'll have to lower the idle a bit. Which also involves something else where the "ported vacuum" actually pulls vacuum at idle if you don't have enough advance.

 

I'll try 8º in a day or so, have another project I have to finish first.


The vacuum advance is hooked to a constant vacuum from the carb.

 

For a Ford, and a basically stock engine, the vacuum advance should be connected to the port with the blue cap on it in your picture

What's happening is, you have the vacuum advance connected to manifold vacuum which is very high at idle, but very low under load.

The actual timing is dropping WAY TOO LOW when you hit the gas pedal (probably like 10-15 degrees ATDC). That's why you're getting the backfiring through the carb.

Connect the vacuum advance to the port with the blue cap on it, cap the other port, and retime. Go for around 6-8 degrees BTDC.

You'll have to correct the idle as you do this. And the idle mix is likely to be out of whack too.

 

That does explain a lot. That line does have a high vacuum and when I hook it up after setting the timing I can hear a big difference. I will try the other side this weekend.


I do believe the mix is all sorts of crazy as well. I am going to mess with that once I get the timing set good.


And just FYI, no I didn't do this. I bought the truck and I am trying to get it back to running good again. It was two owners ago that did all this to the truck, the guy I bought it from was going to do a diesel swap so he never touched it the whole time he had it and can' tell me anything about it so I'm going blind.

 

Lots of people get that vacuum advance thing backwards, especially Chevy people

Report back on how it goes...

 

Here's a picture showing the vac ports...

 

Ok, got the lines switched, now plugged into the timed vacuum port. I was messing with the timing, started with 8 and now have it sitting at 10 BTDC. It is running better but I still have no power off the line. It does drive good but when I try to give it gas it bogs out.


I see on this diagram that there is an available port in the back for the brakes. The PO has the brakes hooked to the intake in the back. I wonder if I should switch that.

 

Nah, the power brakes can be connected to the intake, the Edel probably just does that to be compatible with some other applications that don't have a brake-booster hookup on the intake.

Make sure the vacuum advance diaphragm isn't busted. With the distributor cap off, apply vacuum to the vacuum advance and see first if the points plate even moves, and second if it stays pulled and doesn't leak down.

 

Did you set the timing with the vacuum line to the distributer hooked up or disconnected? Timing is set with the line disconnected, I always had good luck setting base timing at 9 degrees.

 

While that's completely true, if you have vacuum at idle on the ported vacuum port, you have other issues

But definitely worth checking.

If you DO have vacuum at idle on the "timed" vacuum port, the throttle plates are open too far.

 

True, could be a idle quality problem, Just trying to help fix one problem at a time without causing confusion ending up with multiable issues fighting each other.
My motto is fix one problem then move on to the next, make sure the ignition system is working and in time before moving to the fuel system.


Just checking, but in the photo number plug wire looks to be off a post from normal but if someone installed the dist a tooth or two off and moved the wires on the cap that's ok and it will run just fine that way.
Some folk get confused on Fords firing order and the fact that number one cylinder is the front plug on the passenger side where GM and Chrysler call the front plug on the drivers side number one.


Also where does the hose connected to the front PCV port go to, it goes around the carb but doesn't look like it connects to a PCV valve in the passengers side valve cover.


Edelbrock's description of the two small vacuum ports would confuse anyone, none of my old non emission vehicles had the dist vacuum connected to the manifold vacuum side. May be a GM thing, I only owned a couple of those back then and not for long.

 

How come when I quote a post with pics it doesn't show them like it does on other forums?

 

Sorry it has taken me so long to get back on here. I been busy doing work around the house, trying to finish putting in flooring right now.

I have the timing set at 10º BTDC right now. Timing was set with the vacuum line disconnected. The dizzy was set correctly with #1 at TDC, when I got the truck it was set up as you described. It idles great but if I hit the throttle hard from idle it will stumble and backfire out the carb. It does drive nice if you are easy on the throttle but it stumbles as soon as you try to give it any kind of throttle.

I am thinking it might be too much carb for the truck since it is a 750cfm. I have a friend that has a smaller Holley he said we can swap on and see if that fixes the problem. Also there is a 1" carb spacer on the motor I am thinking of taking off.

As for the PCV line, that is running down to a port in my transmission. I assume that it is for it to be able to shift.

 

Is there vacuum on the vacuum advance line when it's idling?