Pop/backfire shifting close to 3k or going up hill

[Abe Jauregui]

This forum has helped me out so much. I've read every post about tuning and possible misfires and still can't get rid of this popping sound from the front passenger side of the exhaust. Here's what I have rebuilt stock 302 with the big spark plugs (only mention this because when I bought spark plugs for a 1970 ford bronco they gave me smaller ones.) I have a holley 600cfm electric choke with this Ready to run distributor. It's a 3 speed column shift. I have checked the firing order (1,5,4,2,6,3,7,8) and the wires are all new and good. Checked them with a timing light. The choke is not sticking and neither is the fast idle speed screw. I have triple checked the idle mixture screws moving them 1/8 of turn using a vacuum gauge connected to full manifold vacuum. The cars runs great and starts up fine, its just under load and shifting 3k and later does it backfire/pop. For timing, I have tried stock 6-8 degrees, advanced it all the way until 16-18 and nothing changes. I have hooked the distributor to timed vacuum on the carb and then switch to full vacuum and nothing cures it. Spark plugs are nice and tight. The curb idle screw isnt even engaged because when its armed up it sits around 750-850 without it. I have checked for a vacuum leak using the carb cleaner method and the idle does not fluctuate. Could it be the regular gas I am using? Should I use premium? I have noticed that it does burn through gas pretty fast. fuel pump is new. Any help would be appreciated. Like I said, it runs like a champ I just don;t like the popping sound going into second and third gear.

 

[JB Fab]

Check for exhaust leaks

 

[Abe Jauregui]

I have using carb cleaner and spraying around the headers. The exhaust is brand new too. If it was a vacuum leak wouldn’t it pop in neutral and opening the throttle ?

 

[BOBS 2 68S]

I chased a backfire-popping on a buddys EB. He had new wire and dis cap. Replaced them with the parts off my EB and problem went away. Bought him more new wires and cap all was good. Even 30+ years ago some new parts were bad.

 

[bulletpruf]

Vacuum leak would be my guess. Does it have a miss at idle? If so, you could use a temp gun to check temp of the exhaust manifold right at the port to see if any are running cooler. That would help you focus on a particular cylinder.

Could also be a lobe going flat on the cam.

Since it appears you have done just about everything else, I'd do a compression check to see if there are any cylinders with a big discrepancy.

 

[Broncobowsher]

Check for exhaust leaks

My first thought as well.

It isn't exhaust leaking out, it is fresh air getting in. Exhaust is full of hot and flammable exhaust gasses that just need some oxygen to burn. Happens when you lift off the throttle. Fuel is flowing, you shut off the air. Much like the accelerator pump takes up the hesitation as air suddenly starts to flow but the fuel takes a moment to get moving. Well you snap off the air flow the fuel still is flowing. You get a rich spike in the exhaust. leak some air in, hot enough to ignite, get a backfire through the exhaust. Even my first Holley EFI had a setting for adding fuel after you lift off the throttle. EFI had the issue that it would stop the injectors and just pump raw air into the exhaust, same as an exhaust leak.

Now if it is backfiring through the carburator, I only had issues when I was completely positive I had the spark plug wires on correct only to find after a month of fighting it that I actually had 2 of them crossed. But I did get a chance to inspect the valve springs and rebuild a perfectly good new carb for no reason.

 

[.94 OR]

Any chance there is an issue with the transition when the secondary's come in on the carb?

 

[Abe Jauregui]

I pulled the plugs and bought new ones. I'm going to install them today. The plugs were a greyish/white burnt color. Like this: http://st.hotrod.com/uploads/sites/21/2006/10/mopp_0612_5_z-reading_spark_plugs-detonation.jpg

 

[1970 Palmer]

My first thought as well.

It isn't exhaust leaking out, it is fresh air getting in. Exhaust is full of hot and flammable exhaust gasses that just need some oxygen to burn. Happens when you lift off the throttle. Fuel is flowing, you shut off the air. Much like the accelerator pump takes up the hesitation as air suddenly starts to flow but the fuel takes a moment to get moving. Well you snap off the air flow the fuel still is flowing. You get a rich spike in the exhaust. leak some air in, hot enough to ignite, get a backfire through the exhaust.

X2, exactly correct.

You need to determine which backfire/popping noise your chasing. CLOSED THROTTLE POPPING, or UNDER LOAD BACK FIRING. The solution will be different. The carb cleaner test will only work if you have an intake vacuum leak. An exhaust air leak will not respond to the carb cleaner spray, and you have a very good chance of starting a flash fire around the exhaust manifolds and plug wires.

 

[Abe Jauregui]

X2, exactly correct.

You need to determine which backfire/popping noise your chasing. CLOSED THROTTLE POPPING, or UNDER LOAD BACK FIRING. The solution will be different. The carb cleaner test will only work if you have an intake vacuum leak. An exhaust air leak will not respond to the carb cleaner spray, and you have a very good chance of starting a flash fire around the exhaust manifolds and plug wires.

I’m pretty sure it’s an under load back fire. Shifting gears and going up hill shifting gears.

 

[Broncobowsher]

Shifting gears is closed thottle. You are on the throttle, load up the engine, get the RPM up.
Lift off the thottle, shift gears, get back on the throttle again. The backfiring during a gear change is the moment the throttle is closed.
The on the throttle part that makes it happen is just getting the loads setup so it will happen when the throttle is closed.

 

[Abe Jauregui]

Seems I linked to the place when showing my distributor. https://www.summitracing.com/parts/SUM-850303-1#overview

 

[Abe Jauregui]

Update, new spark plugged made no difference. I advanced it further until it sounded running powerful and checked the timing and it was around 18 degrees. It helped a little. Should I also run full advance? Did I maybe get a crappy dizzy with my holley

 

[.94 OR]

Likely not related, but I was fighting issues a while back and I was convinced it was my carb, changed carbs and no luck. Installed Fitech EFI and the readout showed my idle was all over the board while the engine rpm didn't change. Changed out to Duraspark and it cleaned it all up. Had I known, I could have saved the purchase of the EFI. Not sure how you would rule out any ignition issues if that was your problem. With that said, if you are experiencing an issue with a single cylinder, it would likely be a poorly adjusted valve or ignition as the air/fuel is blended after the carburetor to most cylinders.

 

[Abe Jauregui]

Could I be running too much carb on a stock 302?
https://www.holley.com/products/fue...tory_refurbished_carburetors/parts/FR-80457SA

 

[Broncobowsher]

No, that 600 will work just fine on a stock 302.

At this point we need a video clip of your driving to get the backfire. Not a 3-second clip of just the noise. But a decent 15-30 second clip of what leads up to the noise and what happens after the backfiring stops. There is a lot of guessing going on here and it all comes back to people not understanding what the problem is.

 

[m_m70]

There is a lot of guessing going on here and it all comes back to people not understanding what the problem is.

agreed!

 

[Abe Jauregui]

Ok this is the best I can do alone with my phone. I can also wait for someone to give it fuel while I record from underneath.

If you look closely, I added exhaust cement repair where the exhaust welds were in hopes of covering a possible leak.

 

[.94 OR]

Kind of sounds like a baffle in the muffler rattling around.

 

[Abe Jauregui]

How big of a exhaust leak would cause a loud pop?