Engine losing power going uphill

[velocityboy]

The truck, 1978 F250, 460, is hitting a wall at about 45-50mph when going up over the local mountain here. It's like a giant flat spot in the throttle where the engine is running but just continues to bog and slow as the grade gets steeper. Used to be she could fly over at 60+ without even really trying. This problem seems to come out of the blue, and only seems to be present when going uphill. Heavy throttle in city or on flat highway yields no hesitation.
Just did a tune-up with new plugs, wires, distributor cap and rotor, ign coil. Didn't solve the problem.
Re-wired questionable wiring for the ignition coil and distributor. Didn't solve the problem.
Checked ignition timing. Base is 10 deg adv, and goes up to approx 36 advance. Seems normal right?
Accelerator pump on carb is working.
Fuel filter is easy to blow through so I'm not thinking it's that...although I haven't checked the microfilter up near the carb. It's not that old though (3 yrs, installed when installed the Demon 625).

Would old"ish" gas cause this prob? The truck is usually used to some extent over the course of a couple weeks, but just prior to it having this problem it had sat for about 4-6 weeks. And the gas in it was probably closer to 2-3 months old. After it hesitated over the mountain I topped off the tank with fresh gas but the majority is still the old stuff. Any recommendations on fuel additives I could try if this is the problem?

 

[Fellro]

With everything else largely attended to, fuel supply or lack of timing advance is about all that is left. If it were newer a plugging cat could also be suspect however if memory serves you shouldn't have one from the factory. Another possible is the timing chain/sprockets. Trying to remember if Ford also used the nylon cased aluminum cam gear to keep chain noise down. The nylon would break up and allow for plenty of slop. Had a Chevy 400 so bad the chain could be removed without taking the sprockets off. It had jumped time at idle.

 

[velocityboy]

Thanks for the reply!
Ya no cats on this one fortunately.
I just ran out and did a quick check on the timing chain. Rotated the engine to TDC and then watched the dist rotor as i slowly rotated the engine backward. Only got abt 2-3 degrees before the rotor started moving, so there doesn't seem to be a lot of slop in the chain thank god.
While I was doing that though I had plenty of time to look at the fuel hose going to the carb. It has to take a turn to run up to the carb fuel intake, and it looks very slightly collapsed/kinked so I may pull that and replace it with proper steel line in case that kink is starving the carb.
Any other ideas though I'm all ears.

 

[Fellro]

Weak fuel pump is my thought. Possible to be something upstream from the pump as well, I don't think there would be a screen in the tank on those. Check the rubber lines underneath as well, pinholes can make for frustrating issues.

 

[velocityboy]

I redid the entire fuel system about 5 yrs ago. The truck used to be a farm truck and ran on propane most of its life. I left it like that for most of the 15 yrs I've owned it, but with the only propane supplier left in town a fair drive away I got sick of making the drive so got the gasoline system going too to make it duel fuel. About three years ago I ditched the propane system altogether and went to straight gasoline.
In all of that process I dropped the tanks and cleaned them, put in new pickup/sending units (and yes they do have those little cylindrical screen filters on the pickup tubes), replaced the fuel caps, replaced all the rubber fuel lines, put a new fuel pump and new carb.

I have a JEGS fuel pressure gauge plumbed into the line up near the carb... and I've been a bit suspicious of the pump given that the pressure readings *usually* read 6psi, but I've seen it vary as low as 2psi recently. I chalked that up to the carb bowl being full and not allowing more flow, but I might be wrong to assume that. So you might be onto something there. That pump might be a little wonky.

Plan now is to get rid of that slight kink in the line up near the carb and if that doesn't solve it, I'll be eyeballing the fuel pump. Probably won't happen till the weekend.

Thanks for the insight!

 

[Fellro]

If it is dropping to 2 psi then you do have an issue - if the bowl is full you should definitely be reading full pressure, not reduced. The gauge is before the needle, not after. Realistically you are reading the resistance to flow in terms of pressure. If the bowl was empty, the flow would be max, making lower pressure as it is technically "leaking" into the bowl, where when the needle is closed, you have maximum resistance to flow.

 

[fatherdoug]

There is/was a rubber fuel line behind the front fuel tank that connects two metal lines, but it sounds like you replaced that line?

 

[velocityboy]

If it is dropping to 2 psi then you do have an issue - if the bowl is full you should definitely be reading full pressure, not reduced. The gauge is before the needle, not after. Realistically you are reading the resistance to flow in terms of pressure. If the bowl was empty, the flow would be max, making lower pressure as it is technically "leaking" into the bowl, where when the needle is closed, you have maximum resistance to flow.

Ok, ya thanks for clarifying. I guess my brain was thinking flow, not pressure. I think part of me was not wanting to accept the fact that a 5 yr old pump was going bad already. I recall the one I installed being a Delphi brand. Motorcraft doesn't seem to be an option anymore. At least not on RockAuto where I normally shop. If you know of a brand you've been happy with by all means I'm willing to consider it.

There is/was a rubber fuel line behind the front fuel tank that connects two metal lines, but it sounds like you replaced that line?

Ya I replaced all the rubber from fuel tanks forward. But I suppose it's all worth re-visiting at this point. With ignition largely being taken out of the equation as the likely cause, It must be a fuel thing so I'll find it as I work my way back toward the tanks.

The problem at this point is finding the time to work on it, with the holidays (and multiple family birthdays) taking up my free time.

 

[Red ASE mechanics]

The truck, 1978 F250, 460, is hitting a wall at about 45-50mph when going up over the local mountain here. It's like a giant flat spot in the throttle where the engine is running but just continues to bog and slow as the grade gets steeper. Used to be she could fly over at 60+ without even really trying. This problem seems to come out of the blue, and only seems to be present when going uphill. Heavy throttle in city or on flat highway yields no hesitation.
Just did a tune-up with new plugs, wires, distributor cap and rotor, ign coil. Didn't solve the problem.
Re-wired questionable wiring for the ignition coil and distributor. Didn't solve the problem.
Checked ignition timing. Base is 10 deg adv, and goes up to approx 36 advance. Seems normal right?
Accelerator pump on carb is working.
Fuel filter is easy to blow through so I'm not thinking it's that...although I haven't checked the microfilter up near the carb. It's not that old though (3 yrs, installed when installed the Demon 625).

Would old"ish" gas cause this prob? The truck is usually used to some extent over the course of a couple weeks, but just prior to it having this problem it had sat for about 4-6 weeks. And the gas in it was probably closer to 2-3 months old. After it hesitated over the mountain I topped off the tank with fresh gas but the majority is still the old stuff. Any recommendations on fuel additives I could try if this is the problem?

Is it 490 CID V-8? Is it aspirated carburetor or EFI - multiple fuel injectors? Try take down gas tank and vacuum clean the empty fuel tank and one front tank is much better power than both front and rear tanks in the super duty truck standard cab. The color blue gas line between two tanks can be cut with hack saw and 3/8" bolt and stainless clamp to plug it.

 

[Fellro]

No trucks had factory injection in 1978, 460's didn't until into the later 80's or so. This will also still have steel lines, not plastic as that was also in the 80's.