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

holdmypocket

holdmypocket

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Joined
May 24, 2020
Member Number
1298
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522
Loc
OC, CA
Getting a popping/backfiring sound through the carb around light-moderate throttle at light load (like a small incline) or accelerating to 40-50 MPH. Seems to be around 2500-ish RPM.

Since I got the Bronco running I've been cruising around the neighborhood up to about 50mph and never had a problem, even ripping around and flooring it. A few days ago I finally took it on the freeway, accelerated fine up to 70, cruised there for about two miles and then got a pop/backfire through the carb as I steadily but lightly accelerated to get past someone. Once I exited, I went from a stop and accelerated to 50mph when it popped again.

Other than this the Bronco runs great.

Details:
1. Rebuilt 302 maybe 2000 miles ago.
2. Holley 4160 completely original. Bought this new in 2019, ran it on my truck for a couple of years, then it sat in a box for a year or so before I ran it on this Bronco a few months ago.
3. Pertronix ignition module and coil. The coil is brand new, the Pertronix came with the engine.
4. Timing was 10, increased it to 12 which made no difference.
5. Found that I had vacuum advanced hooked up to manifold, moved it to ported, made no difference.
6. Put a vacuum pump on the vacuum advance with the cap off and confirmed stuff is moving around under there as vacuum is applied and then removed.
7. Revving the engine in idle with the advance hooked up, it hits 30 degrees at about 2500 RPM, and then continues to increase.
8. The gas in the tank has been in there for over month, I suppose it could be bad gas but I've never experienced this in the past with other Broncos/Trucks that I've had sit for much longer.


Here's a video of it happening (sorry about the orientation, trying to drive :flipoff2:). You after the second pop I let off the gas.
photos.app.goo.gl

New video by Charles Howard

photos.app.goo.gl photos.app.goo.gl

I don't know if this would be bad enough to cause this but this is what the cap and rotor look like, I'll be replacing these:
IMG_3923.jpg

IMG_3922.jpg
 
I haven’t tried anything else besides timing and checking things over.. it really seems like this just started happening all of the sudden.

With that said, I read up on the accelerator pump and found some Holley documentation that says it’s common for the check valve in the accelerator pump circuit to get stuck when sitting for awhile. I’ll look into dislodging that if it is, and then try adjusting the accelerator pump shot.

Might just rebuild the carb either way since it sat for a year or two.
 
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I removed the accelerator pump nozzle and peeked at the check valve in there, moved it around with a pick, actuated the throttle and watched it move up a bit. I suppose it could have been stuck but I don't think so.

Drove it around and same result, but this time once it started popping I decided to smash the throttle through it and it eventually got past it. Found another stretch of road and instead of a light throttle acceleration I gave it a heavy foot and it handled that great.

I do think there's something up with the accelerator pump, and adjusting it could fix the issue by giving it more gas at part throttle, so I'l give that a try but are there any other potential areas of concern? Should I just rebuild the carb to rule out any issues from it sitting?

Video of the accelerator pump nozzle.. doesn't look as smooth as I'd expect but don't really have anything to compare it to.
photos.app.goo.gl

New video by Charles Howard

photos.app.goo.gl photos.app.goo.gl
 
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As the butterflies open, you loose vacuum so the carburetor doesn't pull fuel in like it should. So if the accelerator pump isn't kicking in as early as it should you'll get a stumble symptom like you are running out of fuel.

Your boosters may be slightly plugged too.
 
M92PV4U said:
As the butterflies open, you loose vacuum so the carburetor doesn't pull fuel in like it should. So if the accelerator pump isn't kicking in as early as it should you'll get a stumble symptom like you are running out of fuel.

Your boosters may be slightly plugged too.
Click to expand...
Made a small adjustment to the accelerator pump arm/spring/thing and took it for a ride, couldn't replicate the issue so far. Thanks for pointing me in the right direction once again :beer:
 
While it’s better it’s still there, just doesn’t happen as often. This time I was going up a much steeper hill and giving it cruising throttle to get up it when it happened.

I was watching a Holley video that recommended a smaller accelerator pump nozzle for higher geared, heavier vehicles to extend the pump shot. Since the Bronco has 33’s and 3.50s I think that makes sense so will give that a try.
 
I don't speak fluent Holley, is this a situation where the power valve could be doing fucky thing?
 
posford said:
I don't speak fluent Holley, is this a situation where the power valve could be doing fucky thing?
Click to expand...
I don't think so. The power valve adds fuel to the secondaries this His issue seems to be with the primaries. If it was always running rich and the air/fuel not adjusting or running lean or rich in the secondary circuit I'd check the power valve.
 
Ordered a .28 accelerator pump nozzle which is the next size down from stock which was .31. Doesn't look like they have every single step down/up which is helpful because I was going to come and ask.
 
Stuck the .28 accelerator pump in (down from .31):

1. Popping issue still occurs.
2. Feels like slightly less power.
3. It seems to start popping within the threshold that I think it oughta downshift.

Re: #3
  • Should I adjust the kickdown to shift sooner?
  • Maybe the vacuum modulator should be changed/adjusted?
  • If I do either of those things, am I just masking the problem?
 
I'm also not sold on reducing the accelerator pump nozzle size as being the solution to not getting enough fuel spray. I would make sure that the original nozzle is clear and loosen the nut further off, so the pump comes on quicker.
I think that you can also change the cam that drives it.
 
So, have you thought about why sparked fuel would get to the intake to allow this? Like an intake valve opening after or during combustion? Do 302,and 302 ho have different firing orders? Have you checked this? If you are popping out of the intake, this is the first ting I would check 302. And then try a different dizzy. The carburetor does not stop flowing fuel based on intake valve position. If it goes further, possibly spilt valve guides or tight valves,because what do you have for rockers.
 
When you have mechanical and vacuum advances on a distributor and your rpms are sporadic because of fuel delivery issues, I could see it firing (ignition) with a valve open.
 
Not an HO 302, checked firing order.

I’ll get a fuel pressure gauge.

I’m also thinking my power valve is too small(?). Vac is measuring at 19, so I have a 9.5 power valve coming.
Will back off accel pump nut too.
 
It runs great all other times, I t’s just that 2300-2500 rpm point with light load. If I take a hill with just a little bit more throttle applied, no problem.

Off idle, suddenly smashing the throttle.. all great.
 
holdmypocket said:
Not an HO 302, checked firing order.

I’ll get a fuel pressure gauge.

I’m also thinking my power valve is too small(?). Vac is measuring at 19, so I have a 9.5 power valve coming.
Will back off accel pump nut too.
Click to expand...
How did you check the firing order?
 
So, is it backfiring? I don't think you know what the term means. Do you mean the engine bucks? Or is there flame coming out of the top of the carb. 2 different things.
 
NOODLES said:
So, is it backfiring? I don't think you know what the term means. Do you mean the engine bucks? Or is there flame coming out of the top of the carb. 2 different things.
Click to expand...
I was wrong to describe it as backfiring, that was definitely inaccurate. It’s just a stumble with popping coming from under the hood.

Here’s a video: New video by Charles Howard
 
holdmypocket said:
I was wrong to describe it as backfiring, that was definitely inaccurate. It’s just a stumble with popping coming from under the hood.

Here’s a video: New video by Charles Howard
Click to expand...
I don't watch videos of cars running to try and diagnose. Because that is the wrong way to diagnose a vehicle.
You should look into an oscilloscope if you get the carb figure out and still have the problem. To me, from what you have said before, heaviest loads in middle gears, where there is more gear reduction in the lower gear which can mask a misfire, and when cruising when the load is taken off the engine making it seem your misfire is gone. You could have something as simple as a bad, broken, or fouling plug. You could have a loose valve, bad seat, crack in the head, sloppy valve guide. Incorrect rings or ring installation on the pistons or piston. A bad plug wire, have you checked the resistance of each one? A bad plug wire will have this exact symptom.

I bought a snap on multimeter that has the oscilloscope, and also blue tooth, so you can leave it under the hood while you drive, and forecast the information to your smartphones screen while driving.
 
arse_sidewards said:
Not by popping a valve cover. :flipoff2:
Click to expand...
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?
 
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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