F350 losing power..... Help!

[Gary Lewis]

I don't know if I mentioned this before, but the truck has plenty of towing power in first and second gear. We totally forgot to remove the wheel chaulks on the trailer and in first gear I towed the trailer right over them. Starting from a stop was not a problem. It feels like higher speeds and gears is where I am losing power.

I've seen a lack of fuel cause those problems, and I've seen a weak spark cause those problems. In both cases the vehicle did better in lower gears.

The reason a weak spark can do it is that taller gears cause lower RPM but the same load. That creates higher cylinder pressure (BMEP) and at high pressure the spark just won't jump across the gap.

But I lean to a fuel system problem. One test I'd be inclined to do is to adjust the opening point of the secondaries. That's easy on that carb and should show in the instruction manual. Try it where they open earlier and open them later and see what happens.

However, I think you are just running out of fuel. Perhaps put a fuel pressure gauge on the system and put a phone under the hood to watch it and Facetime to another phone in the cab? Or, if you have an Apple Watch, put the phone under the hood and see it on the watch. I think you'll find that the pressure drops.

[1986F150Six]

I've seen a lack of fuel cause those problems, and I've seen a weak spark cause those problems. In both cases the vehicle did better in lower gears.

The reason a weak spark can do it is that taller gears cause lower RPM but the same load. That creates higher cylinder pressure (BMEP) and at high pressure the spark just won't jump across the gap.

But I lean to a fuel system problem. One test I'd be inclined to do is to adjust the opening point of the secondaries. That's easy on that carb and should show in the instruction manual. Try it where they open earlier and open them later and see what happens.

However, I think you are just running out of fuel. Perhaps put a fuel pressure gauge on the system and put a phone under the hood to watch it and Facetime to another phone in the cab? Or, if you have an Apple Watch, put the phone under the hood and see it on the watch. I think you'll find that the pressure drops.

Or, the rubber fuel line from the tank could be collapsing or sucking air?

[86-350-460-dually]

I've seen a lack of fuel cause those problems, and I've seen a weak spark cause those problems. In both cases the vehicle did better in lower gears.

The reason a weak spark can do it is that taller gears cause lower RPM but the same load. That creates higher cylinder pressure (BMEP) and at high pressure the spark just won't jump across the gap.

But I lean to a fuel system problem. One test I'd be inclined to do is to adjust the opening point of the secondaries. That's easy on that carb and should show in the instruction manual. Try it where they open earlier and open them later and see what happens.

However, I think you are just running out of fuel. Perhaps put a fuel pressure gauge on the system and put a phone under the hood to watch it and Facetime to another phone in the cab? Or, if you have an Apple Watch, put the phone under the hood and see it on the watch. I think you'll find that the pressure drops.

Thanks Gary! For clarity, do you suspect that the fuel pressure drops or doesn't increase?

I have the wrong ignition coil on the truck now. I do have the stock one on hand from a junk yard, but I just haven't done the swap yet. That wouldn't explain the lack of spark, correct?

[Gary Lewis]

Thanks Gary! For clarity, do you suspect that the fuel pressure drops or doesn't increase?

I have the wrong ignition coil on the truck now. I do have the stock one on hand from a junk yard, but I just haven't done the swap yet. That wouldn't explain the lack of spark, correct?

David - It is possible the fuel lines are causing a problem. But this truck should have the nylon lines rather than rubber. Still, there could be an air leak that would kill the pressure.

Adam - I expect that the fuel pressure is low and then drops. But the reason it drops is that the floats in the carb drop and the needles open up as you open the throttle, so the flow should increase. But if the pump is weak then it can't keep up with the needed flow and the pressure drops.

On the coil, it would depend on what coil you have in. But I doubt that you have a coil so poorly matched to the system that it can't supply a high output voltage. On the other hand, if you have the right coil then put it on and see what happens.

[86-350-460-dually]

David - It is possible the fuel lines are causing a problem. But this truck should have the nylon lines rather than rubber. Still, there could be an air leak that would kill the pressure.

Adam - I expect that the fuel pressure is low and then drops. But the reason it drops is that the floats in the carb drop and the needles open up as you open the throttle, so the flow should increase. But if the pump is weak then it can't keep up with the needed flow and the pressure drops.

On the coil, it would depend on what coil you have in. But I doubt that you have a coil so poorly matched to the system that it can't supply a high output voltage. On the other hand, if you have the right coil then put it on and see what happens.

Thanks Gary. I will have to examine the fuel pressure.

[85lebaront2]

Thanks Gary. I will have to examine the fuel pressure.

Ok, a couple of items, first, does your truck have a mechanical fuel pump or the electric in-tank pumps? In either case, if the "socks" (fuel strainers) in the tanks are dirty they will restrict the flow of fuel. Do you know what gear you have in it? How heavy is the 5th wheel? I towed a 10K one with Darth and I will say acceleration in 3rd wasn't anything to brag about, never had a problem losing power even towing up 7 mountains grade in PA, was down in 2nd at 37 mph (FWIW, guy with a Chevy pulling an identical 5th wheel was down in 1st at 22 mph on the same grade).

Do you still have the stock exhaust on, or has it been changed, the original was dual 2.5" pipes to the muffler, then what looks like 3" out and opens up to around 4" with two screens in the end.

I would start with a fuel volume test, it should be a pretty good stream, I can't find the exact spec for it, pressure is 5-7 psi though.

[Notch5oh]

Ok, a couple of items, first, does your truck have a mechanical fuel pump or the electric in-tank pumps? In either case, if the "socks" (fuel strainers) in the tanks are dirty they will restrict the flow of fuel. Do you know what gear you have in it? How heavy is the 5th wheel? I towed a 10K one with Darth and I will say acceleration in 3rd wasn't anything to brag about, never had a problem losing power even towing up 7 mountains grade in PA, was down in 2nd at 37 mph (FWIW, guy with a Chevy pulling an identical 5th wheel was down in 1st at 22 mph on the same grade).

Do you still have the stock exhaust on, or has it been changed, the original was dual 2.5" pipes to the muffler, then what looks like 3" out and opens up to around 4" with two screens in the end.

I would start with a fuel volume test, it should be a pretty good stream, I can't find the exact spec for it, pressure is 5-7 psi though.

Sounds like fuel starvation. Could be a clogged filter. If the truck has one.

Cool rig though!

[86-350-460-dually]

Ok, a couple of items, first, does your truck have a mechanical fuel pump or the electric in-tank pumps? In either case, if the "socks" (fuel strainers) in the tanks are dirty they will restrict the flow of fuel. Do you know what gear you have in it? How heavy is the 5th wheel? I towed a 10K one with Darth and I will say acceleration in 3rd wasn't anything to brag about, never had a problem losing power even towing up 7 mountains grade in PA, was down in 2nd at 37 mph (FWIW, guy with a Chevy pulling an identical 5th wheel was down in 1st at 22 mph on the same grade).

Do you still have the stock exhaust on, or has it been changed, the original was dual 2.5" pipes to the muffler, then what looks like 3" out and opens up to around 4" with two screens in the end.

I would start with a fuel volume test, it should be a pretty good stream, I can't find the exact spec for it, pressure is 5-7 psi though.

Lebaront,

It has the in tank electrical fuel pumps. It has 4.56 gears. That trailer in the photo was 14k lbs, and the trailer I will be towing in 2 weeks is 12k lbs. Regardless, I was losing power going up the same hills without the trailer though. I couldn't sustain 55mph empty. I probably dropped to 35 mph on the freeway going full throttle.

I have 2.5" pipes going to the muffler and then one 2.5" or 3" pipe coming out. Other than that, I have no clue if it's stock or not.

[grumpin]

Grumpin,

Do you have a good way or checking for a leak? I did find a major leak in December, but that was just an uncapped spout.

You can use brake cleaner or starting fluid, but starting fluid can give false indications if it's near the carb inlet.

They will give you a rise in RPM.

Also you can use a cigar! Google Scott Kilmer, I believe is his name.

What about your clutch? Know the history? As in last time it was replaced.

[ArdWrknTrk]

I have the same carburetor on my 460.

With 4.56 gears you shouldn't have a problem pulling that trailer, let alone just getting the truck going at 55 mph.

That's what a rig like this was made for.

You say "mountains". Is altitude a factor where you live?

Carburetors don't compensate well for thin air, but rich is not the same as running lean.

There is no catalytic converter on your truck.

You should check for fuel volume at the carburetor (after the vapor separator T, if it's still there)

Bill can better explain how to jumper the fuel pump circuit to enable this test.

But I think your friend is on to something.

Given your symptoms the secondary throttle plates may well be blocked from opening.

Let me ask. If the AIR pumps and emissions have been removed, is there still an EGR spacer under the carb?

Check for clearance by removing the carb and looking to see if the base gasket is clear of the rear pair of venturis and then seeing if the intake ports do not obstruct that same gasket in any way.

I know I was unable to find a spacer that worked with this carb and had to resort to a die grinder to open up the rear ports for the butterflies to clear.

If the fuel volume test goes well this is the next thing I would check.

Just my 2c.

Good luck with your trip and your new tiny home.