I recently picked up a 1981 F-350 DRW with a 351M and a 4 speed standard and I am having some issues with it.

The truck had been sitting for some time, but started up after a bit of cranking and a good boost. I managed to drive it yesterday a few kilometres from where it sat to our shop. It seemed to run fine on the drive aside from blown shocks and shot engine and transmission mounts. I pulled it into the shop, did some torching and welding on it, pulled it back out and left for the night. No issues.

Today I wanted to continue working on it so I headed too the shop with a jerry can of fresh fuel and filled about 15L into the auxiliary tank. The truck has a dump box on it which is filled with a pile on junk from the previous owner, so i wanted to head to the dump first thing to empty it out. This is when the issues started.

Thank fully the dump is rather close to the shop. I had driven about 2-3km and all the sudden, as speed, the truck started bucking and stalled out. Even coasting at speed with the truck in gear, the engine was not running. I pulled in over to the side of the road, and tried cranking it a bit, but the battery was too weak to get it going. I walked back the the shop and jumped in my F-150 and headed back to the F-350 on the side of the road. I gave it a boost and the truck fired right up. I started driving again, made it 1km and ran into the same issue. I pulled over onto the side of the road but managed to keep the truck running. It would hesitate and bog when I revved, but it seemed to clear up and I continued along. I made it another .5km or so and the truck died again, right at the turn off into the dump.

I walked back to my F-150 and drove to the parts store. I figured that since the truck had been sitting for so long, pouring in the new fuel stirred up some sediment. I bought some carb cleaner and drove back. I tore into the carb a bit on the side of the road and cleaned it up. It really didn't look as bad as I was expecting. I threw it back together, cranked the truck and it fired right up. I figured that I just needed to clear it out and it was good. I contemplated turning around and just heading back to the shop, but figured I was so close to the dump, I would chance it. I got to the scale, and as I was waiting for my slip, the truck died. I cranked and cranked until the battery was dead with no luck. I got a loader to pull me off to the side, headed back to my F-150 (which was conveniently on the side of the road for me), and back to the shop. I drove back to the dump again with the dually and towed the F-350 back to the shop.

As I pulled into the compound with my dad driving the dually and me in the F-350, I threw it into 2nd and dropped the clutch. The truck bump started and I was able to pull it off to the side. I let it idle and revved it up a bit and it seemed fine. Once it warmed up a bit, it started to break up as I would rev it. I turned it off and thats where I left it.

So, I am thinking that this is either fuel or ignition related (leaning towards fuel) I haven't had a chance to dig into it yet, and I'm still rather un familiar with the truck.

There are two tanks on the truck, the main tank (which I assumed wasn't being used as there was no cap or anything and the gauge showed empty) and an auxiliary tank thats on the frame. The auxiliary tank was the one which I had fuelled up that morning.

There is also a valve located on the floor near the drivers seat with the PTO and dump controls to switch between tanks. Im still not sure exactly what is going on with this.

Apart from that, there is a switch on the dash labelled as auxiliary and main. When switched to auxiliary, I can hear an electric pump running.

My first theory was that sediment had been worked up, plugged up the filter or something on the carb, and caused a lack of fuel. I couldn't find a filer in the system anywhere (in tank?) and I cleaned, the jets, and needle valve in the carb.

Second brief theory that I still haven't ruled out is that the coil is getting hot after running for a bit.

Third theory is that I don't know how to use the main/aux selector valve or switch and ran it on the wrong tank. To me it seems weird that it would run for such distances in between dying..

As of now, my plan is to replace the fuel system. I want to eliminate the main tank and run just the aux on the frame (aux is pretty big tank and this truck won't see any long trips), run new lines to the mech pump on the motor, and to the carb, and a cartridge style filter somewhere in the system. I am wondering if I should add a low pressure electric pump before the mechanical pump to help out? I have never worked with a carbed fuel system aside from dirt bikes.. everything has been fuel injected so this is all new to me. I also would like to replace the coil, plugs, wires, cap, and rotor (just because they are all old and parts are cheap) and rebuild the carb.

Any tips, advice, insight would be appreciated.

 

The next time it quits on you and won't start, get out, take the aircleaner off, climb up in the engine compartment, and look down the carb throat while you pump the throttle by hand. Each time you push back the throttle, you should see two strong streams of gas pour into the engine. If you don't, you have a fuel problem. I would replace the fuel filter and you might end up replacing the fuel pump. Sitting around ruins all the rubber components in the fuel system, and the new fuel they use now attracts water and turns everything into a sticky gum.

If you have fuel, then I would suspect the ignition module.

 

I agree with Dave. But I'm thinking it is electrical, as in alternator or regulator. The symptoms do fit with either fuel or ignition, so it is a coin toss.

 

Ok I'll pull the air cleaner and look into the carb. How does that work? Is there a diaphragm inside that carb that creates a vacuum to draw fuel when the throttle is actuated and the engine is not running?

Forgive my ignorance, is the mechanical fuel pump the only one in the system or would there be a low pressure in tank pump?

I will look into the electrical situation too. The alternator looks fairly new. I should be able to get the truck to run long enough to measure voltage at the coil. Do I want to see 12v at the coil or is there a resistor somewhere in line?

 

There is a resistor. So check battery voltage with it running. As for the stream of gas there is a pump, called the accelerator pump, in the carb. And an 81 should have only the mechanical pump on the engine.

 

If I were the redo the fuel system (new lines, filter, eliminated selector valve..) would there be any reason to add and inline electrical pump, or is the mechanical pump all that's required? How do I prime that pump once I replace the lines? Just a lot of cranking?

 

I had this same problem with my F-350. It turns out that rubber fuel lines don't last as long. Mine would run fine when cold but after a bit of running it would do exactly as your is. Turns out that the fuel lines were cracked at the tank selector valve.

What was happening is when the truck had been sitting for a while the hose would seal itself up but after fuel is ran through it it would get wet and expand thus allow the pump to lose vacuum allow air in. After I replaced the hoses there was no problem any more with this issue. Check all your hoses carefully because if it has sat for a while I'll bet that there are quite a few that have dry rot.

 

I too am having fuel issues, and maybe my question and solution will be helpful to the originator.

One day I was out driving and when I accelerated the truck would act like it was flooding the carb, and choking out. If I backed off of the fuel, it ran ok, but had no power. After a couple of these events I was on the way home and the truck cut out and would not start back up. I suspected fuel. Pulled the feed line to the filter and sure enough no fuel.

Since it's old I decided to replace the fuel pump and fuel sending unit from the saddle tank (the previous owner took the back tank out of play). Prior to this work I replaced the Holley 850 DP with a new Edlebrock 600 single pump, replaced the fuel filter, and have been running on that for 5-6 weeks.

Completed all the work yesterday, filled the fuel tank, and cranked it over for several attempts, 20-30 second attempts, and nothing. Aside from a fuse or the fuel selector body (the one under the truck along the chassis, I am at a loss. I don't suspect a fuse because of the way the fuel was cutting out over time. A fuse would be an on off relationship.

So if you have any suggestions for help, they would be greatly appreciated.

Randy

 

Jeremy is right. It may well be the lines - in both cases. The rubber used in the 80's is almost always bad by now and allows air to get in, which kills the pumps's ability to pull fuel.

 

The original poster needs to do the carb test first to determine what the problem is(fuel or ignition).

Looking down the carb is not to determine if the accel pump is working or not(obviously it is if it runs ok at times) it's to determine if you have fuel or not.

 

Ps. I blew air through the lines to insure there was no obstruction, whichafter have a day to think about it I'm wondering if I have developed a wiringissue that was intermittent. Tomorrow I’m going to take a multi-meter and seeif I have power back to the fuel sending unit.

 

I did have the fuel line at the carb pulled yesterday and pumping into a bottle to verify fuel. There was fuel, but not as much as I would have expected. I didnt have a chance to get to the shop and work on the truck, but I will try to tomorrow if I can track down some fuel line. I would like to find some solid line and run all new from the tank to the carb, just for sake of mind. I will obviously have some rubber line in there too (chassis to motor). I would also like to run the auxiliary tank as my primary.

I'll keep you guys up to date.

 

Well, if you had fuel then you might be wasting your time messing with the fuel lines. I would try the ignition module, it's very common for it to fail when it gets warm.

 

Never hurts to check the lines as old as they are, but yes, Dave, you could be right about the ignition module.

 

The symptoms of a weak spark or a weak fuel system are pretty much the same - bucking and mis-firing under load. So this could be either. I've had both happen on these trucks.

And low fuel volume can easily be from a bad pump or leaking lines. The fuel line is the cheapest to fix as there is a short piece of rubber line from the tank to the rear hard line; from the hard line to the switching valve; from the valve to the front hard line, and from that hard line to the pump. So if you buy 3' of line you'll have some left over. And, by the way, bypassing the valve is easy and gives a very good place to add a fuel filter.

A weak spark can easily be a bad module as Dave said, or dropping battery voltage from a bad alternator or regulator. Or, it can even be a bad coil or plug wires.