Help-Can't find source of backfire.

[dodj]

It doesn't run well at any RPM. It randomly backfires at all RPMs.

I would try re-routing those wires first, then swap out the ecu. What type of ECU do you have? Can you put the original back in to test?
Disconnecting the tach is a good idea as well.

[sprecks]

I would try re-routing those wires first, then swap out the ecu. What type of ECU do you have? Can you put the original back in to test?
Disconnecting the tach is a good idea as well.

I have a new Mopar ECU orange box in there right now. I have another used NAPA ECU that I tried with no luck.  It had a complete Mallory system in it when the car came into my shop.  It has all been removed when I put the new engine wiring harness in it.  I removed the wiring going to the tach and it didn't help.  Good ideas wish they would have worked. Thanks.  I'm really getting frustrated with this thing.  It has to be something simple that I am over looking.

[dodj]

How about the reluctor gap? Checked?

[HP2]

I haven't been able to get the thing to idle below about 1500 rpm without dying.  After running the engine for a couple of minutes I checked the header temps with my point and shoot thermometer.  All of the header pipes vary in temperature.  The right bank had temperatures between 350 and 500 degrees at the head.  The left banks temperatures were considerably lower.  #1 was 220, #3 was 110, #5 was 295 and #7 was 350.  I removed the plug wires from #1 and 3.  Started it up and ran it on 6 cylinders for about a minute.  The backfiring quit.  By the way,  the firing order has been checked and rechecked a hundred times....lol.  1-8-4-3-6-5-7-2 is embedded in my brain.

The idle issue... does it have power brakes? If so, disconnect the hose and plug it at the intake and see if that allows you to drop the idle down to a reasonable range. For that matter, have you hooked up a vacuum gauge to observe the needle action to see if its stable and steady or bouncing around all over the place?

Given the  #1 and 3 temps and popping going away when those are disabled, I tend to say this is where the problem lies. Have you checked cylinder pressure to see where they are at relative to the other holes?

[sprecks]

It's been a long day but I am on the road to whipping this thing.  I started the day off by replacing #1 and 3 plugs again.  The temperature on these two cylinders immediately came up but the random backfiring continued.  I checked the reluctor gap again and it was OK at 0.008".  I took my Eddy carb off of my Duster and put it on.  No change in the way it ran,  still backfiring.  Tried another ECU...still backfiring.  Tried yet another distributor cap...still backfiring.
When the owner brought me the car he told me there had been about 4 or 5 gallons of gas in it for 5 or 6 years so he put 10 gallons of new gas in it to dilute the old gas.  I thought this might be the problem so I dropped the tank and dumped all of the gas.  Blew out the gas lines and emptied the bowls in the carb.  Put 5 gallons of new gas in it and fired it up.  Still backfiring.....damn.

I dug out the old spark plugs that were in the engine when I got it. Took them over to the bead blaster and cleaned them up.   Put these old plugs in it and the backfiring went away....SUCCESS.  The engine is running really good now.  Still having a little trouble getting the idle down to where it should be but that is small stuff compared to the constant backfiring.  I'm sure it has a vacuum leak somewhere so I'll be finding that tomorrow. The car doesn't have power brakes so that's not the issue. I'll find it.  I finally had a successful day.   Thanks everyone for you ideas.

By the way, the new plugs that were causing the backfiring were Champion RJ12YC.  The old plugs that are working are Autolite 85.

[quapman]

...and all my friends mock me for using Autolites!

I really like Champs for plugging the holes when I paint an engine.

[rUNCHARGER]

Champions are my last choice but others have good luck with them. I use Bosch, NGK or Autolite generally.

Sheldon

[HP_Cuda]

I have no issues with RC12YC but I do go and gap them all for .40-.45 when I'm ready to put them in!!!

[dodj]

I really like Champs for plugging the holes when I paint an engine.

[HP_Cuda]

As HP said earlier, I'd put a vacuum guage on it and see where you are at.

It would show you if you have low vacuum and/or a suspected leak.

[BIGSHCLUNK]

I had this situation on a bowtie pickup I had years ago....  Tried everything mentioned here. Never did cure it, maybe time to pull a head? Inspect the cam?? Oh an Champions.... pppphhhhtttt

[sprecks]

As HP said earlier, I'd put a vacuum guage on it and see where you are at.

It would show you if you have low vacuum and/or a suspected leak.

I'll be doing this first thing tomorrow.  I must be getting old.  I haven't even gone to the garage today.  Taking a break.