High rpm backfire through exhaust & misfire

Posted By: T/A CARe

High rpm backfire through exhaust & misfire - 05/30/11 09:13 PM

Here's the problem: misfire and backfire through the exhaust above 3,500 rpm, in any gear, "MOST" of them time (not always) typically 1x out of 20 3,500+ rpm run ups will it NOT backfire/miss.

Car's a 340 6 Pack automatic with all 3 Holley carbs running well. Car was on the dyno and is not running lean (before this all started happening).

Here's what I've changed or replaced:
Cap, rotor, plugs
Have checked the wires with a spray bottle
New Orange ECU box (have a Chrysler electronic ignition conversion)
Ballast resistor
tried a friend's known working distributor
Some even suggested the Sun tach be removed from the equation. Done.

Here's another thread before I heard the backfire and changed the distributor: https://board.moparts.org/ubbthreads/show...&PHPSESSID=

Thoughts?

Posted By: RapidRobert

Re: High rpm backfire through exhaust & misfire - 05/30/11 11:55 PM

Quote:

Thoughts?

Any parts/tuning changes right b 4 it started? & it was good b 4 this & started out of the clear blue sky? (1) ECU (2) plug wires (3) disconnect/plug vac adv & see if that helps/hurts/no ch (rotor phasing)

Posted By: T/A CARe

Re: High rpm backfire through exhaust & misfire - 05/31/11 03:41 AM

No. I wasn't doing anything, working on anything, etc. Started it one day and had the problem...

Posted By: None2Slow

Re: High rpm backfire through exhaust & misfire - 05/31/11 04:32 AM

Whats the voltage at the alternator when this is going on? Loose or dirty connections. Bad grounds. Just some other things to check.

Posted By: dennismopar73

Re: High rpm backfire through exhaust & misfire - 05/31/11 05:07 AM

check phasing on the dist cap see where the 'fire ' is actualy hitting on the electrods ?? just a outside thought
may run fine at or just above idle but when the advance stats working it could actualy 'blow' the fire off the electode??

Posted By: 69chargeryeehaa

Re: High rpm backfire through exhaust & misfire - 05/31/11 09:41 PM

i've had that exact same problem on 2 mopars and it ended up being 3 different things....
1. bad ecu/coil.
2. bad reluctor/pickup (make sure you use a brass feeler to set gap).
3. low coil voltage (supply at about 4VDC - which was a bad ballast)...

but the most persistent problem i had was the ecu, that one was exactly how you describe...tried 3 orange boxes and all eventually after a few weeks did it (including just dying after 40mins on hot days)...finally went to a chrome mp box, all gone for 5yrs now, and there really is a big difference between the orange and chrome mp boxes...chrome much much better, better idle, better power...and they work!!

hope that helps!

Posted By: kenworth_goose

Re: High rpm backfire through exhaust & misfire - 05/31/11 11:34 PM

Check your grounds first.