72 Bogs down when accelrating under load

[danh25]

Hey everyone,

I'm having an issue with my '72 Bronco, and I'm hoping to get some troubleshooting help. Here’s the situation:

Problem:

• Engine bogs down under acceleration and won’t get above 30 mph.
• Idles and revs fine in park.

Background:

• Has run great for years but sat for a while and wouldn’t start due to no fuel delivery.
• Found the needle seat was stuck, replaced it after removing the fuel bowl, and got it running.
• Now experiencing bogging under load and dieseling when shutting off (less urgent for now).

Specs and What I’ve Done:

• Engine: 347 Stroker
• Carb: Holley 770 Offroad Avenger
• Timing: ~12 degrees (seemed fine before)
• Replaced fuel filter (but looked solid)
• Seafoam fuel additive
• Fuel pressure around 6 psi at carb
• Acceleration pump appears to be working as it should (solid squirt)
• Tuned idle mixture screws with a vacuum gauge. Pulls 17ish vacuum; idles well. I feel like I am doing this right but no expert.
• Float levels look good
• Checked for vacuum leaks; none found, replaced all caps to ports

I’m stumped and would really appreciate any advice or ideas to get this resolved!

Could I have messed up some sort of mechanical linkage?

 

[phred]

How long did it sit? Bad fuel? Clogged fuel filter, gummed or blocked fuel pickup in the tank? Possibly a restricted fuel line. You may have 6 psi at the carb at idle but not enough volume under load.

 

[danh25]

It sat for about 6 weeks.
Fuel Filter was replaced....

 

[DirtDonk]

Six weeks isn’t that long compared to most. Not great maybe, but shouldn’t be a dealbreaker.
How exactly did you check for vacuum leaks?

 

[DirtDonk]

What air cleaner are you running
Is the choke plate opening up
Are you running a PCV valve?

 

[danh25]

• While trouble shooting I don't have an aircleaner on it. Typically its this low profile K&N -but it has run with that for a few year great.
• Choke Plate does open up on acceleration and as it warms up.
• Yes on PCV - have not monkey-ed with that yet.
• Vaccum leaks - Carb cleaner on ports and lines coming out of carb looking for bubbles/suction. Am I wrong there. Also hooked vacuum gauge up and tuned carb and get to a stable 16ish. Not sure if that helps find links. Just want to put all the info out there.
  
  Not an expert on any of this but have tried to be througough but I could be missing something entirely dumb.

 

[BOBS 2 68S]

Off the wall thought...
is the parking brake sticking or hanging up

 

[danh25]

good idea. Ill certainly check - but as a professional 'I left the parking break on' guy - it doesnt feel 'mechanical' like that.

 

[DirtDonk]

Your checking and writeup is very well thought out. Seems like you checked just about everything.
Your description of the accelerator pump sounds like that’s not the issue, but I was leaning towards something leaning out the mixture. Hence, looking for a vacuum leaks. But it sounds like you got that dialed as well.
The only other type of vacuuming would be a leaky intake manifold on the inside. But I wouldn’t think you’d be able to achieve your vacuum reading if that was the case.

What about timing? How do you have your vacuum Advance hooked up? Do you have a timing light? If so, hook it up and see if the Advance is advancing when under load.
All I got for now

 

[danh25]

I appreciate you so much. Thank you

No vacuum advance in the set up I have. I do have a timing light -but admit anything ignition/timing just blows my mind and makes my head spin.
So unfortunatley I don't know what "see if the Advance is advancing when under load". Im guess as I accelerate I should see it advance from 12 degrees to something higher ?

 

[bfoldy]

What are you using for a fuel pump: mechanical or electric? Do you have any inline fuel filters?

 

[danh25]

Electric - yes to inline fuel filters. They were my first guess and replaced.

 

[m_m70]

I would do a rebuild on the carb just to make sure things are cleaned out..

Ignition?
what do your plugs look like??
Rotor, cap and wires??
You still running points???? Doubt it if your running a stroker

 

[danh25]

Rotor, Cap, Wires are great - newer MSD system. I am sorta not thinking its ignition because it ran so well before but I am not the best ignition guy.

I have not pulled a plug but will.

 

[DirtDonk]

No vacuum advance in the set up I have.

What distributor are you using?

I do have a timing light -but admit anything ignition/timing just blows my mind and makes my head spin.

No problem. But the good news is, it’s one of the easiest tools to learn to use. Unless it’s one of the ancient types like we used to use.
What have you got? We can run you through the simple setup and use.

So unfortunatley I don't know what "see if the Advance is advancing when under load". Im guess as I accelerate I should see it advance from 12 degrees to something higher ?

Correct.
Once the light is connected and working, point the flashing light at the timing pointer and watch the timing marks on the damper ring (probably etched into the damper) move.
At any given speed they should be steady and not oscillating back-and-forth. But they should also climb as you rev the engine up.
Since it is not vacuum operated, it still must have a mechanical advance. Simple springs and weights that advance the timing as the engine speed increases.