Deceleration Backfire

Hey all,
My 75 with a 304 would backfire when shifting or deceleration, when warm. I have re-torqued the header bolts and used special thick gaskets on them. The one thing I did to stop it was to adjust the fuel mixture screws on the 2100 to make it more rich a quarter turn at a time then a test drive. Several questions:
1) Can a free flowing exhaust (fender well headers, glass packs) cause this?
2) Can a possible over-torqued carb cause this even though there wasn't a fast idle from a vacuum leak?
3) Also, if I understand it correctly, the backfire is caused by unburned fuel in the exhaust so why would increasing the air fuel mixture stop it?

Thanks

 

The backfire is usually caused by a leak in the exhaust system allowing unburned fuel to ignite.
There is usually unburned fuel in the exhaust during deceleration, especially with a carbureted engine. That's why catalytic converters came into use.
I struggled with this same issue until I found the correct deformable gasket to fit between the manifolds and pipes.
Remember, those carb mixture screws are JUST for idle. Those have nothing to do with main jet mixture.
You might try smaller (leaner) main jets.
I don't think the carb bolt torque would cause this, nor would the free flow exhaust.
Keep looking for exhaust leaks.

 

Thanks for the reply. You said, "Remember, those carb mixture screws are JUST for idle.". That's what baffles me, that by increasing the idle mixture the problem goes away. I will keep looking for leaks.

 

Exhaust valve? Could run a compression test on the cylinders.

 

' Cuz adding more air ' Leans' it out . Your air / fuel mixture is 2 isolated functions , with a carb .......Fuel from the carb , Air from the induction system ( IE : air filter & housing ) . Have you noticed that your Jeep is using a lot more Fuel ? Has it been running Hotter ? Timing & distributor slop have been known to cause backfires also.
After my 3 cents here, ...... , what SuperDave & PeteL have suggested are good test too. Me thinks it's more about step by step procedural diagnosis . The TSM can help a ton here......That's how I learn't
Good luck .......Hope you get to the bottom of it soon.

 

Timing or pre-ignition and carbon deposits can also cause this condition. In '75, I believe there was a temp controlled switch that went from manifold vacuum to ported depending on the engine temperature.

 

Check the carb for a deceleration dashpot, mitigates the sudden vacuum at throttle shutdown.

 

Thanks for all the tips! I have readjusted the idle mixture back to what is should be and it does still rumble and pop on deceleration. Now I will replace the header gaskets and check for vacuum leaks. There is no dashpot but it looks like it might be an interesting experiment.

 

I don’t suppose that you still have a smog pump (belt-diven air pump) on your engine? These pump fresh air into the exhaust ports via a tube “manifold. If you have this present, it can close backfiring as you describe.
-Donny

 

No, luckily

 

Have you put a timing light on it yet ?

 

Not recently. When I put the DUI distributor in a year and a half ago I set it for 10 degrees. Ibdid checkbit a few months ago. I will have more time this weekend and I can recheck it. I will also check the plugs too.

 

Ok....I didn't realize you have a new distributor in . There is minimal wear or ' slop ' then . Carry on & Good luck to you.
BTW, check your HEI electrical connections.