Bad distributor cap = backfire (pics)?

[mbeers6]

Hello again fellas, tried searching this subforum, didn't find anything.

So my cobra has begun to backfire out the exhaust under acceleration starting around 3,000 rpms. It runs fine at RPM's below that. It didn't do this before and I've tried jetting changes on my eddy 600cfm with no luck.

I took off the cap and I can't tell if this is the problem or not. The area around the contacts look burnt, but the cap/rotor only have 4k miles on them. The prior owner stated he had a problem with the cap and they (meaning Summit) replaced it free. I wonder if it is a defect.

The distributor is a Petronix flame thrower Vaccum Advance on my 351w.

Your thoughts? New cap/rotor is like $40, so I don't want to replace it if I don't have too. Thanks!!

[redmt]

Look under the flexible contact in the center of the rotor. Flip it over and look for tell tale carbon tracking. By the appearance of the cap, it appears that it is firing early/late based on the spark print on one side of the cap contact. Possibly shaft to gear loosening? What ever it is should be fairly easy to find.

[Calbullet]

looks like the rotor is sitting high on the shaft. almost like the rotor is hitting the cap right at the electrodes. on the first pic, is the center electrode cracked?

[mbeers6]

Originally Posted by Calbullet

looks like the rotor is sitting high on the shaft. almost like the rotor is hitting the cap right at the electrodes. on the first pic, is the center electrode cracked?

Yes, the plastic around the center electrode is cracked. I'm looking at the cap now and the plastic around some of the side electrodes is completely missing!

I checked and the rotor was screwed down tight, I wonder if I should try shaving some plastic off the bottom of the rotor to make it sit lower inside the cap?

I don't a receipt which voids the warranty. I'm going to call petronix tomorrow and see what they say.

[PANAVIA]

the other thing to check is if the distributor rotor is being pushed upward by an excessively decked block or a distributor shaft that is too long for the assembly. -- it would only take a few thousandths to kick this out of spec.

[Rick Parker]

It looks like the tip of the rotor might be contacting the Brass towers within the cap. The resulting grindings or "DUST" may be allowing cross firing within the cap. Replace them both, whether they warranty them or not.

[Gaz64]

Too much endfloat or mismatch of parts.

As others have said the ground up brass dust is your crossfiring.

[RICK LAKE]

Looking at the spark fire area, IMO it's too high, should be in the center of the cap terminals. End play of the distributor itself my be too loose and have the shaft slide up and down. Crack in the center electriode housing. The rotor burnt mark is not centered which tells me that you could have the wrong rotor for your setup, it happens. Is it possible that there is a problem with the petronics? check with some of the threads on here about problems. Rick L. Ps that rotor is also an emmissions rotor reason for "E" on the plate and gives a wider gap and has a wider end for the terminals. Look for an Accell cap and rotor or sorenson aftermarket.

[Zoom This]

Yeh, like the guys said. Looks like a bad match of the rotor and dist. cap all around. I will safely assume your plug wires are solid core type? I had this identical misfiring problem on my first new car, a 1970 Plymouth Hemi-Cuda. I installed a Capacitive Discharge Ignition box to the ignition system and the motor would backfire under heavy load at higher rpms. Found out months later that the factory plug wires were carbon core and were cross firing!! Took me almost a year of fussing with the problem before discovering this.

[Jerry Clayton]

How are your spark plug wires routed to the plugs? If they are grouped parallel you can get cross firing especially with a electronic type ignition

[madmaxx]

Firing on the cap terminals should be at the BOTTOM of the terminals not top or middle, arc travel efficiently via sharp edges. Look at the MSD sight for a visual display.