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Distributor cap

asappin

Full Member
Joined
Oct 14, 2014
Messages
205
I have been experiencing poor performance and backfiring from the exhaust when I let off the gas or switching gears. I removed the distributor cap and noticed there is residue inside the cap. Almost looks like burn marks on each post. What causes something like this? Would y’all recommend replacing the plug wires also while I’m replcing the cap?
 

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pcf_mark

Bronco Guru
Joined
Jun 11, 2010
Messages
3,574
I am not seeing what you are seeing. I would use that cap. You could have some arcing from the rotor to the cap posts but that is normal. If your rotor is severely out of phase you could be jumping a greater distance. You can google rotor phasing but it is a pain in the butt and will cost you a cap because you need to drill a hole in it.

But if you are getting popping or backfire on decel that is most likely an exhaust and / or intake gasket leak. On decel you get high vacuum - you can get a spike of fuel and you get a super rich mixture that basically passes through the engine because the throttle is closed (no oxygen). But if you have an exhaust leak that rich fuel can mix with some cool outside air and get banging and popping in the exhaust.

OR your distributor moved and is too retarded and the engine is still firing when the exhaust valve opens.
 
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asappin

asappin

Full Member
Joined
Oct 14, 2014
Messages
205
Thank you for the information. The residue I am referring to is on the black part of the cap on the inside of the cap posts. Almost looks like a carbon buildup
 

Rustytruck

Bronco Guru
Joined
Feb 24, 2002
Messages
10,875
It is carbon. Too much carbon build will cause crossfire ignition. spray it down with wd-40 and scrub with a paint brush. then rinse it out with brake cleaner. in the top center is a carbon brush the rides on the rotor transferring the coil power to the rotor. Carbon is conductive. If your getting misfires I would check your plug wires and plugs. Electricity wants to jump the path of least resistance. Its supposed to go go down the wires but its jumping somewhere else. So check the connections of the plug wires and the resistance of the wires too.
 

904Bronco

Contributor
Bronco Guru
Joined
Sep 28, 2004
Messages
5,749
Loc.
San Martin, CA
It was called Carbon tracks, usually from an old or dirty Dis cap.

Hard to see it on yours, clean as directed or replace to eliminate a possible source of your problems.
 

Steve83

Bronco Guru
Joined
Jul 16, 2003
Messages
8,980
Loc.
Memphis, TN, USA, Earth, Milky Way
...burn marks on each post. What causes something like this?
Ozone.

A high-voltage spark passing through air breaks down some of the Oxygen molecules (O2), such that they re-combine as ozone (O3). That's why lightning sometimes has that smell. Ozone is highly corrosive; especially on hot metal, like the microscopic point that the spark hits on the cap terminal. That point oxidizes, creating the corrosion you see, and O2. The oxide is not nearly as conductive as the pure metal, so the next spark hits a different spot, causing the crust to eventually grow across the whole terminal. Brass oxidizes to a blackish crust; Aluminum oxide is a lighter gray. In either case, It can simply be scraped or chipped off with a common pocket screwdriver to expose the clean terminal underneath, which will work perfectly for hundreds of thousands more miles.

To reduce the effects of the ozone on the terminals (and to reduce the condensation of water on the distributor cap's plastic), apply silicone grease where this diagram indicates:


(phone app link)
Would y’all recommend replacing the plug wires...?
Only if they fail this test:


(phone app link)
 
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