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Discussion Starter #1
OK, to make a long story short, I have a 1976 Ford F100 short bed that I having a problem with. At first, it seemed simple, but is now becoming aggravating. Just wondering if anyone has an idea. The truck was my dads, and he warned me of the situation...he just never really looked at it much, as he basically used it just for "jobs" around the yard. It has not really been driven for 4 years, and had a 2000 sticker on the plates.

The engine is a 1967 390 out of a Fairlane GT. It has been converted from pointed ignition to electronic ignition. It is also carbureted with a Holley 750. Although only having about 10k miles on the rebuilt engine, it sat for several years under a blanket on an engine stand.

Problem...
Truck will run approximately 6 miles at highway speeds before engine dies. Idling, it would run 17-17.5 minutes before stalling- you could almost set your watch by it.

Upon looking at it, I am thinking either a fuel issue, or possibly a problem with the ignition module (common). Tried another ignition module, to no avail.

I then replaced the inline fuel filter with a new transparent one so I could monitor fuel level, and replaced the rubber fuel lines from the fuel pump to the carburetor. After about 17 minutes, it without warning stalled.

I noticed the accelerator pump under the front float bowl was leaking raw fuel onto the intake. I removed the carburetor, and while taking the accelerator pump off to try and fix the gasket, old sweet smelling fuel ran out of the vent. I then pulled all spark plugs just to see how they looked. Dad says they are not very old at all, but they were all fouled; really wet and dark sandy looking texture. Looks pig rich. The plugs are Bosch platinums with a taperd type center electrode. The electrodes were fouled with deposits enough to where the tapered electrodes were almost rounded off. A brass brush cleaned them up nicely.

I then removed the sight window on the side of the carb, and the float level was low. I adjusted the float until fuel just started to trickle out of the hole. After about 17.5 minutes, it stalled. :(

I then found two more filters...there is a thimble shaped screen type filter in the banjo bolt on the carb, right at the fuel fitting. Filter was not clogged. I also found out that the mechanical fuel pump has a fuel filter as well. It is a paper filter, and looks like the filter media in an oil filter. There was rusty looking sediment in the bottom of the canister, so I replaced the whole fuel pump rather than just the filter to alleviate any possibility of a bad fuel pump.

I then removed the fuel line going to the fuel tank at the fuel pump. I blew compressed air, about 30 PSI, into the line. I could hear air entering the fuel tank, and when I removed the air nozzle, about 4 oz. of gas shot back out all over the ground before I could hook it back up. Just air I would think could indicate a clogged fuel pick-up in the tank.

It was Miller time when I surpassed the 18 minute range, but just before I was going to shut it off after about 24 minutes, it stalled. It did however start right back up momentarily before stalling again.

The only thing I can think of that I have not tried is maybe the carburetor is loading up after awhile under load (i.e., needle and seat, leaking power valve?). We have a new Holley 600 still in the box that I can try, but I hate to introduce the carb to fuel if I do not really need to. I mean it obviously has air, has spark, timing is right, so the only thing left is fuel...or so it seems.

What at first seemed as a simple fix, is now haunting me. I drive it to work at times (1.1 miles each way) just to move the damn thing. It has never left me walking, but I just gotta get there quick.

Any ideas?

Greg
 

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Well first unless you are realy spinning it high, a 750cfm is way big for a 390 in a truck. Also these carbs are famous for blown powervalves.

First thing I would do, is take a can of gas, run a hose from it directly to the fuel pump. See how it works. I have seen the tank filters plug up wile running, then clear as soon as the fuel flow stops. Also the fuel lines could have some pin hole leaks. Not enough to leak fuel, but enough to suck air. Thant normaly only couses proplems at high speeds, or loads though.

Next what brand elect ing does it have? Is it a Ford duraspark? Is the unit in a place where it can get heat, or mayb a bad wire?

How about the coil? Maybe try a known good one to see if that fixes it.

When it dies, do you have spark?

These types of problems are the worse to find. Good luck with it.
 

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Discussion Starter #3
The 750 being too big is a good point, with the fact that it was last used at slow speeds on my dads property. And I do not go over 40 MPH or so on my to and from work. It needs to hit the highway to clear out a little carbon, as I have always called it.

As for a truck driven "normally" (slow speeds and highway speeds), the 750 should be about right. I could be wrong, but I think a 750 was OE. My dad also has a 2x4bbl. factory aluminum intake for two 390 CFM carbs, which is 780 CFM overall.

With it running longer at idle speed compared to 60 MPH, it seems as if it would be a starvation problem...but I can smell gas when it stalls. And yes, after it stalls, there is definately adequate spark.

I will try replacing the short fuel line from the frame rail to the fuel pump. It is hard plumbed from the tank up to right under the steering box- from there is a rubber fuel line about 18" long or so. I will replace that and try running it from a gas can while I have the line off. That I guess will eliminate a clogged pick-up in the tank if it still stalls. Also, I tried running it with the gas cap off, thinking the vent might be clogged, creating a vacuum- thus starving the engine of fuel. It still stalled.

Also, as for the power valve, I have heard of them going bad, but have never experienced it- so I am not really sure what it would do or how it would affect it. Especially with the leaky acc. pump, the carb definately needs a rebuild.

Thanks for the suggestions, as I am open for all of 'em. LOL

Greg
 

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Originally posted by Fusion1970


As for a truck driven "normally" (slow speeds and highway speeds), the 750 should be about right. I could be wrong, but I think a 750 was OE. My dad also has a 2x4bbl. factory aluminum intake for two 390 CFM carbs, which is 780 CFM overall.


Greg
well first I want to say that the carb size is probably not your present running problem. This is just kinda for future info.

Well it would be doable, but realy a bit big. That motor set up was designed for a HP car. A lighter vehacle, that would probably be run hard, and at high rpms on the track, or street. For a hever truck, that will need more "luging" power, that 600 would be AWSOME. Over carbing is one of the bigest mastakes out there. With a bigger carb, the fuel just does not atamize the feul well at slow speeds. With out the higher volosty of of air through the ventury, the fuel "drops out" and puddles. You would be AMAZED how much lowend you would pick up with a smaller carb. Somewere out on the web is charts to find the right CFM for that motor. cant find it this morning though. I would bet you find out that that 750 would realy only be spec'ed for running in the 6 grand range. I dont know about you, but I would not want to keep an old 390 that high.:D Just as an example of what you realy need, I was running a 351 in my 70 mustang at the track. When I first put it in, it was a stock 90k motor. With only a cam change,[a stock 1984 mustang GT cam] and removing the choke plate, I was running high 15's, low 14's with a stock Ford two bbl carb. Unless your running a TON of cam, or REAL high rev's, you just dont need much.

When you do find the problem, I would hate to see you put a ton of money into the cab, and find its realy not right for your use. Sorry to go off on a tangent :D :D :D Keep digging, I am sure the problem will jump right out to you soon.
 

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Hi
My friend had a similar problem a few years back. After going the fuel pump route plus changing filters we found that the fuel was contaminated. As the truck was running junk would be sucked against the sock filter in the tank. After a few miles it would plug the filter. After sitting a while the junk would float away from the sock and it would run for a while before the pump would suck the junk against the sock filter. We had to pull the tank and clean it and use fresh fuel. it took us a long time to find the problem. Aslo find out haw the tank is supposed to be vented. Some use a vented cap and others are vented elsewere.
If I read right it sounds like your tank is not vented and that would also cause this problem.
Rodster
 

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Rodster mentioned a good point. I had a similar problem with the Sea Ray a few years back and looked at all the items you have mentioned. Come to find out that debry that had built up in the fuel tank was stopping up the fuel pickup screens. I drained and cleaned the tank out and the problem went away.
 

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I would pull the plug out of the floatbowl right after it stalls and see if it's still full of fuel. If it's not, then you have a fuel delivery problem. A fuel pressure gauge and a vacuum gauge would be very helpful. If you have those, put the pressure gauge in the line going to the carb and the vacuum gauge before the pump. You want 5-7psi to the carb and probably no more than 3 inHg before the pump. Too much vacuum before the pump indicates a blockage somewhere. I would guess if it will die at idle and still has spark, it has to be running out of gas. Probably a blockage like a clogged sock.
 

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Discussion Starter #9
Yea, I have thought of the tank think being clogged. After I try the short piece of fuel line and the new carb (new carb might not fix it, but it will definately run alot better, especially without a leaky accelerator pump), I am prepared to drain and remove the tank.:mad:

Good point, too, aguysmiley...I have not removed the sight plug right after it stalls. I do not have a fuel pressure guage, but do have a vacuum guage. I will be busy tomorrow (Sunday) but plan to mess with it Monday.

BTW, my dad tried a new coil as well as a new ignition module before I got it. I am totally convinced it is a fuel issue, and before long, every component in the fuel system will be checked or new.

I will keep y'all posted. Thanks again for the replies.

Greg
 

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Greg,

:ditto: To the opinions that point to the fuel tank.
When I was in high school we would sneak into the
teachers parking lot and put unrolled condoms in the
gas tank of a bad teacher. They soften and balloon in
the tank, then cover the inlet after a few minutes of driving.
With the car off for several minutes, they float up and
allow the car to restart. The problem would drive them
crazy.

PS….I also agree with Paul on the 600 being a better choice
than the 750 for the 390 in a truck application.
 
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