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Popping/backfiring through carb (67 Bronco - 302/C4 with Pertronix/Holley 4160)

Levigarrett76 said:
Does it still pop with vac. advance unhooked?
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This, try this. Might be over advanced at that rpm. Especially for the high gearing and heavy load.

I fought this on my corvette. Kept blaming the carb, but a guy gave me a brand new distributor. He didn't know nothing about it. After some reaserch it is some tricked out high performance thing.

My vette will not run well with the vac advance hooked up. But no vacuum it runs amazing.
 
NOODLES said:
I am sure it went like this. He opened the hood. Looked at the order of the wires to the cap, and since it matched up to the firing order he thinks he needs.... I wonder, if he took the other firing order for the other 302, and just simply moved them around on the cap he could rule out if it is the issue, and probably also identify which 302 he has. They also sell aftermarket cams that will go into a non ho motor, that gives the non HO motor the HO firing order. And this is commonly overlooked by people rebuilding their early 302s. I don't remember him saying what year the bronco is, what year the motor is. Some of these details matter.

But anywho, I would just start asking people online what is wrong with minimal details and ASSume throwing more parts at it will fix the issue. And what is even weirder, is the people with the advice, have not asked these questions, but started throwing out possible fixes.

So how much do you want to sell the shitty running Bronco for OP?
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All that is true but it's probably not a firing order issue since he says it drives fine sometimes.
 
Levigarrett76 said:
Throw a vacuum gauge on it and drive it. About what vac. is it pulled down to when it pops? Might clue you in to power valve change
Does it still pop with vac. advance unhooked?
Have you tried pumping the pedal when it pops to see if it will pick up?
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It cuts out when the popping happens, the only way to get past that point is to let off and then give it more throttle than I was previously (or down shifting).
 
M92PV4U said:
A dead spot while accelerating doesn't sound like a carb issue?
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A dead spot that disappears and comes back randomly does not sound like a carb issue. Carbs do not typically have intermittent issues. It either works or does not. It isn't like a jet is plugging and then unplugging just to plug itself back up again. I think a spark issue would be more likely. resistance changes with heat, moisture, voltage. and can be intermittent..
 
A dead spot would just be flat. A cylinder dropping would buck. It is hard to drop just 1 cylinder with a carburetor. You would have to find some way to redirect air/fuel mixture from the intake runner.
 
NOODLES said:
A dead spot that disappears and comes back randomly does not sound like a carb issue. Carbs do not typically have intermittent issues. It either works or does not. It isn't like a jet is plugging and then unplugging just to plug itself back up again. I think a spark issue would be more likely. resistance changes with heat, moisture, voltage. and can be intermittent..
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It's at the same throttle position and condition each time. Slowly pushing the accelerator it gets to a point then starts popping. He can stab the throttle and not experience the issue.
 
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And here is the real issue. You can't diagnose the truck without the truck in front of you. The dude needs a shop, because apparently, he does not know enough about engines. And also has an issue trying to communicate the actual symptoms.
 
M92PV4U said:
It's at the same throttle position and condition each time. Slowly pushing the accelerator it gets to a point then starts popping. He can stap the throttle and not experience the issue.
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Why doesn't it do it in all gears then?
 
I don't think that this is your issue but, I've had a hole in my fuel line before the mechanical pump. The truck would run fine until you really got on the throttle, at the end of each gear before the auto shifted, it would cut out, it would kill itself around 70 in third because it sucked enough air to quit running. With the rpms low it wouldn't suck air.
 
It would be worst in 3rd no matter what. Dude should contact Holley. And figure out how to tune his carb. He will probably need to have camshaft information for base. He will also need Camshaft info for firing order. I would put a scope on it, and check for spikes in the ignition cycle.
 
M92PV4U said:
I don't think that this is your issue but, I've had a hole in my fuel line before the mechanical pump. The truck would run fine until you really got on the throttle, at the end of each gear before the auto shifted, it would cut out, it would kill itself around 70 in third because it sucked enough air to quit running. With the rpms low it wouldn't suck air.
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I have thought about that as well. But if it bucking.....That would be where I would be looking at an individual cylinder.
 
Ignition module would do it too. It would just cause a random, so drop one cylinder, but not be as precise as the same cylinder on every cycle. Unless of course it is a cam postion/crank position controlled unit with some type of tone ring.
 
NOODLES said:
I have thought about that as well. But if it bucking.....That would be where I would be looking at an individual cylinder.
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This caused the engine to buck/ surge if the truck shifted and rpms dropped it would be okay until the next shift point.
 
Just a hunch, I have seen a bad running small block caused by a bad tach. Unplug the tach and see what happens.
 
I've seen an engine "run" that had the plug wires routed to the old 302 firing order on a 302HO firing order cam.

If the vehicle is driveable in any way it is 100% not the issue.
 
Tiha said:
Did anyone say busted valve spring yet?

It is a busted valve spring.
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Pulled the valve covers this morning, observed all of the springs and saw nothing obvious. Grabbed each one and tried to spin it and they're all solid/remained in place. Not sure if there's more to look at when checking for a broken valve spring.

dave_dj1 said:
fuel filters?
is it the proper coil?
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Current fuel filter has less than 100 miles on it, and the tank was flushed clean, but it wouldn't hurt to grab a new filter anyway so that's being added to the shopping list. Coil is a brand new Pertronix coil.


arse_sidewards said:
I'm still voting distributor.
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I'll turn to this after trying a different power valve. From the looks of it, I want to replace it either way.


Checked the resistance of the plug wires and coil wire, all good.
New power valve is supposed to arrive later today, got the carb rebuild kit in earlier this week. Will try the power valve first.
 
So for curiosities sake, have you pulled plugs just to see what they show?

I know on my bbc 427, it will foul plugs after awhile from idling to much. Which is my own fault since I don't drive the truck really. Just go out and start it occasionally.

I get the popping at idle and part throttle. New plugs clear it up.
 
In the end it just ended up being that the carburetor needed a rebuild. I let a shop with a good reputation in town do it, apparently it was pretty dirty.

I suspect using the carburetor back in 2017-2019 on my F150 and then putting it away wet in a box on a shelf until this year didn't help. It sounds noticeably better, drives better, and has no stumble. I'm supposed to take it back to the shop next week and let him tune it as part of the rebuild.
 
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