I have a strange issue with my 390 in my '75 F150. Had some issues with the stock holly and switched it out with a edelbrock 1406. Been driving it for a while now and it starts and runs really strong.

Sometimes tho, especially more lately (warmer weather), after ~20 minutes of driving it gets to this point where if you dump the gas pedal, it loses power, doesnt accelerate and it wont recover, but if I release the pedal a bit and ease into again it accelerates fine.

Its like when the vacuum suddenly goes to zero it cant recover. My initial thought is that its some kind of timing issue, but my initial timing is at 8 deg BTDC, and my vac advance seems to operate. at least it moves when vacuum is applied, albeit slowly, and its also slow to return (i dont really know what to expect from these).

I dont think its a fuel delivery issue, but who knows.

Just tryin to figure out where to start. ideas are appreciated.

 

Accelerator pump is the only thing that comes to mind.

 

I dont think its the accelerator pump because when they are not squirting enough gas the motor hesitates for a moment due to a sudden lean condition but then recovers. My motor will not recover until the gas is let up a bit. plus Ive verified that its squirting gas consistently.

could a vac advance diaphram cause this? its action is really slow to respond. If i apply vac to the advance it takes ~2 sec for the arm to move completely and then it takes another ~2 sec for it to return once the vac is released. seems slow to me but i dont have any point of reference.

 

Is this a new carb or used? Maybe you have the advance on the wrong port? Even if the advance was slow, I would expect the motor after the 2sec dizzy response. If you were on the wrong port that did something unusual at wot, (but not while easing into it), that might give the behavior you are experiencing.

 

the carb was used but I rebuilt it and everything is where it should be, also jetted it for my altitude. The dizzy is original as far as I can tell (its a motorcraft, not some aftermarket part). In fact when I got the truck Im pretty sure it had the original plugs and wires!

I originally had the vac advance connected to the timed vac from the carb where it should be, but i switched it to manifold vac from the carb just to see. very little change other than a slight increase of idle after warm up (to be expected because timed and manifold vac should be the same once the throttle is opened) but I have to admit it did seem to take longer to start misbehaving.

perhaps this is a problem with the ignition control box? there was another one floatin around in the cab when i bought the truck, so it appears as tho this may have been a previous problem.

I can think of many things I could just start replacing, but I was just hoping that someone might say, "oh yeah I had that problem once, here what I did..."

I do appreciate the input tho.

 

Ah, ok. Yes, look into the ignition. "easing into it" won't get you the same cylinder fill as instant wot. I now remember a peanut roaster Tercel I had a long time ago with the same symptoms. The ignition was really weak and couldn't light off a full cylinder load, but part throttle was ok (ish). I repaired the ignition and it ran like a race car...or as much of a race car as a 3spd carbureted Tercel could be.

 

perhaps this is a timing issue. the emission label calls for 16* BTDC for initial timing so ill start there but itll have to wait another day or two. Little surprising because my parts truck is the same year, trim (explorer), motor and paint, but that motor is a 2v and calls for 8* BTDC. Anyone know why mine is timed so early? ('75, 390ci, 4v, C6, with factory A/C)



This entire weekend was consumed trying to clean the motor. It was filthy when I bought it and I hadnt had the will to clean it till i had to scrape away some muck to get at the dizzy for adjustment... then it snowballed into replacing the leaking valve cover gaskets. 3 cans of degreaser and a couple worn out scrub brushes and I was able to uncover part of a motor in that dirt pile.

Before


After


Unfortunately, the wiring leading to my coil was just aching to break apart, leading me to a new problem that I outta be able to fix with some solder and new harnesses...



I tried harvesting one from my parts truck, and it had the exact same problem. So i imagine this is common... havent had any luck locating a replacement, anyone know where to get original harnesses?

Plus AC came out and got plugged... might replace the compressor with a sanden if I can get some dollars together. I do live in the southwest and AC is nice, but not having the engine bay crowed with all that crap is nice too.

Anyway... well see how the retime goes once i get the wiring fixed.

 

I can't tell what the plug on the left looks like. LMC may have something you can use? Otherwise numberdummy may be able to help!

LMC's ignition page!
LMC Truck Parts - Page 147

Hope that helps...you might want to call them and check that those are the right pieces!

-Brad

 

My 1977 Ford F150 4 X 4 with 400 is doing the same thing with a 2150, new fuel pump metal lines replaced back to the cab, New carb & spacer plate. Are you having any response from slowly applying pedal pressure? Mine acts like the gas pedal is suppose to stop the vehicle instead of speeding it up. If it gets gas gently it speeds up nicely.

 

Couple of things. Sounds like secondaries are coming in too soon. Vacuum advance canisters (at least some) are adjustable with an allen wrench thru the vacuum port to control how much vacuum it takes to move it. Lastly, new electrical plugs are available at NAPA, but I don't know of a source for a complete harness.

 

My secondaries are mechanical and they come in at about 2/3 to 3/4 throttle. the problem Im having will happen before the secondaries open, so the title is a bit of a misnomer.

If i go WOT, the Auto kickdown drops me into a lower gear and the problem is solved by high rpms. However, if i push the throttle down too quickly after say coming around a turn, but dont let the tranny downshift, then thats when it happens, when the engine is under high load.

Were the original vac advances adjustable? pretty sure mine is stock, everything else seems to be.... including the dirt and grease!

So napa has the female end of the connector (below), but I need the male end...


Might just say screw it and put a universal one on there. I want to finish this already.

 

Problem seems to be solved... engine degreased, new valve cover gaskets (please dont leak!), A/C system removed (hopefully to be replaced with a sanden soon), and new wiring to the dizzy and coil just to change the timing a few degrees. talk about a snowballing project.

So I got the motor timed to 16* BTDC as listed on the emissions label on the valve cover and the problem seems to have been solved.

seems like most of the FEs are timed between 6 and 8. Any ideas why mine is set at 16? I dont think the truck was a cali truck (although it was made in San Jose). I cant see any evidence that it had any emissions junk (i.e. no egr etc.), plus Im pretty sure it was purchased new here in NM, where we still dont have emissions laws (except Abq)

Would factory A/C require this timing?

PS. Im sure this has been said a million times before but to all those replacing wiring in their truck, please use solder and heat shrink! Im tired of finding mysterious wads of electrical tape that are hiding twisted together wires.