Help troubleshooting backfire issue

[srobertsfsj]

I am still having issues with my sons 78 Cherokee. It starts and idles fine but when I put it under load it backfires out of the tailpipe and sometimes thru the carb. It has an Edlebrock 1826 carb. When I replaced the plugs they were black and sooty.

Here is what I have done so far:

New plugs and plug wires
New cap and rotor
New ignition pickup
New coil
New fuel filter
Complete rebuild on carb, including new acclerator pump
Completely went thru the vacuum lines and made sure everything is hooked up properly
No exhaust leaks
Timing checked

Possible issues?

Fuel is old and could be bad
Fuel pump has not been replaced
Wiring is a little hacked, I am only getting 10.5V to the coil, is that the correct voltage?
EGR valve bad?
PCV valve bad?

What else am I missing? What are the most likely causes?

[TrenchFoot]

What's the state of your choke? When warm, the carb still needs to choke and choke pulloff when you hit the throttle and the carb is opened up. Otherwise, the vacuum drops momentarily to zero which stops fuel from being pulled through the jets. The engine will stumble and if it recovers instead of stalling it'll get a slug of fuel that can cause a backfire.

[carnuck]

Compression test? Broken rockers after a motor has sat awhile is fairly common too.

[srobertsfsj]

What's the state of your choke? When warm, the carb still needs to choke and choke pulloff when you hit the throttle and the carb is opened up. Otherwise, the vacuum drops momentarily to zero which stops fuel from being pulled through the jets. The engine will stumble and if it recovers instead of stalling it'll get a slug of fuel that can cause a backfire.

The choke is functioning properly, this is happening when fully warm and the choke is open.

[srobertsfsj]

Compression test? Broken rockers after a motor has sat awhile is fairly common too.

I have not done a compression test, I may do that this weekend. The jeep idles very smoothly so I would think if a piston is dead or a rocker is broken I would get a stumble at idle. The backfiring only happens if I get quick gas or under load.

[TrenchFoot]

The choke is functioning properly, this is happening when fully warm and the choke is open.

Try cranking the choke to the rich side and see if the issue goes away or diminishes. When you hit the throttle and the secondaries open the choke pulloff should partially close the choke even when the engine is at temp. This is to keep vacuum up momentarily to counter the effect of opening the secondaries.

Mine acts like this and when I dial the choke to the rich side throttle performance improves greatly. But then it won't start when warm because it can't breathe. I just got a replacement choke coil for mine yesterday and I'm fiddling with it. Definitely wasn't a silver bullet. My issue may be a tired choke coil and busted pulloff.

[srobertsfsj]

The choke is functioning properly, this is happening when fully warm and the choke is open.

Try cranking the choke to the rich side and see if the issue goes away or diminishes. When you hit the throttle and the secondaries open the choke pulloff should partially close the choke even when the engine is at temp. This is to keep vacuum up momentarily to counter the effect of opening the secondaries.

Mine acts like this and when I dial the choke to the rich side throttle performance improves greatly. But then it won't start when warm because it can't breathe. I just got a replacement choke coil for mine yesterday and I'm fiddling with it. Definitely wasn't a silver bullet. My issue may be a tired choke coil and busted pulloff.

Good points, i will try that tonight.

[carnuck]

Vacuum advance gone bad will do that.