1973 Charger Backfire through tailpipe during cranking

[class115]

Hi all,
My 1973 Charger has a 440 with an automatic transmission.
Its always been hard to start after it sat for a week - taking 10-20 seconds of intermittent cranking to finally start. Once it started, it was good for the next few days. I just thought this was the fuel bowl drying out...annoying but not insurmountable.

Starting last month, the car would backfire while I attempted to start it up, so loud it sounds like a gun and gets my wife out to the garage to check if I'm still alive. But it doesn't start.
if I don't pump the gas it just cranks but no backfire.

There are no old-school Mopar experts around here that can help me. Plenty of Mopar fans, but no Mopar experts.

So, although I know NEW parts don't always equal WORKING parts, I started reading my service manual and replacing parts.
The plugs looked good, but I changed them.
I checked the order of the plug wires - the order was correct.
I changed the rotor and the cap and tried again - BOOM out the tailpipe but it did not start.
I changed the fuel filter and cut open the old one to see if there was any crud. It looked Ok, but the filter is now new.
I checked TDC on the #1 piston, the Piston is up when the rotor is on the number 1 contact on the distributor...and pretty close to 0 on the timing mark.
Replaced the coil.
Charged the battery
Tried again - BOOM out the tailpipe but no start.
Installed a new ballast resistor-BOOM, no start
Installed a new ECU-BOOM...no start

So, I clearly need help or advice. If I can find a local shop that has "seen this type of thing before" I'll take it in.
Until then, my theories have all been wrong.... is there anything I missed?

Very Respectfully,
Victor

[71se3834v]

While everyone is on vacation this weekend, ha, I'll try to get you started.
When there is a backfire my thinking is a valve is open when the plug fires. Not the only reason possible but it's a start. Whenever you're trying to diagnose a problem look at the basics first. You need fuel/air mixture, compression and spark for an engine to work. You've done well basically do a tune up and checking to see if the timing is close. I would do two things next. Perform a compression test and check to see how much slop is in the timing chain by putting the engine at TDC, remove the dist cap and move the crank back and forth looking to see how many degrees you can move it before the rotor starts moving. If I remember right you should only have 6-8 degrees of movement. My car had 13-15* of movement before I replaced the timing chain. It didn't help solve my problem as I had a wiped out cam.
Good luck. Gives a couple of things to look at until others chime in.

[brigond]

I would check the pickup coil under distributor cap. If the gap between the reluctor and pick up coil is off / too wide or Too Close. Use a brass (non magnetic) feeler gauge to gap this distance. .008 i believe.

[brigond]

Gap between one of the reluctor ( prongs ) and the face of the puckup coil.
The pickup coil should not be loose.

[class115]

May 28, 2018 8:53:14 GMT -5 71se3834v said:

While everyone is on vacation this weekend, ha, I'll try to get you started.
When there is a backfire my thinking is a valve is open when the plug fires. Not the only reason possible but it's a start. Whenever you're trying to diagnose a problem look at the basics first. You need fuel/air mixture, compression and spark for an engine to work. You've done well basically do a tune up and checking to see if the timing is close. I would do two things next. Perform a compression test and check to see how much slop is in the timing chain by putting the engine at TDC, remove the dist cap and move the crank back and forth looking to see how many degrees you can move it before the rotor starts moving. If I remember right you should only have 6-8 degrees of movement. My car had 13-15* of movement before I replaced the timing chain. It didn't help solve my problem as I had a wiped out cam.
Good luck. Gives a couple of things to look at until others chime in.

Thank you for helping me.
I ran out and got a compression tester and now I among the proud few who can get a tester up and over the power steering housing and down into the #3 spark plug hole on a big-block in a B-body
My compression test results are
cyl 1=150 psi
cyl 2=155 psi
cyl 3=135 psi
cyl 4=140 psi
cyl 5=140 psi
cyl 6= 140psi
cyl 7= 130psi
cyl 8= 140psi
73% humidity& 73 deg F
I did not try to start it
Cam is a Comp Cams 280H P/N 21-237-4 with about a thousand miles on it, and plenty of zinc (Gibbs High Zinc oil). Stock heads, stock intake, and exhaust manifolds.
going to check for the reluctor gap and crankshaft-to-rotor slop right now
I did not try to start it afterward
Thanks, again
Vic

[Tom]

I keep a variety of "spark checkers" in my tool box.

I would be hooking one up to coil wire to see if any spark is present

[class115]

May 28, 2018 19:07:28 GMT -5 Tom said:

I keep a variety of "spark checkers" in my tool box.

I would be hooking one up to coil wire to see if any spark is present

Will do!! I get one and a helper/watcher

[class115]

May 28, 2018 12:26:14 GMT -5 brigond said:

Gap between one of the reluctor ( prongs ) and the face of the puckup coil.
The pickup coil should not be loose.

Thanks brigond, I can get a .016 inch feeler gauge in there.  I'll set it back to .008.
Nothing is loose but I can rotate the reluctor a tiny bit by hand
very respectfully,
Vic

[Tom]

On that spark checker.....if possible, get one with an adjustable gap. 

Seems to me that I had bad luck using (non adjustable)  spark tester for HEI ignition on older/weaker ignitions. 

[Bob]

My 74 would crank and crank and backfire through the exhaust. Enough one time that it expanded the muffler. Turned out to be an intermediate short in the ignition module. Put in a spare one and never had the problem again. The way I found out it was that was my spare was prettier and when I swapped them the problem went away.

Bob