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Old 02-17-2012, 09:23 PM
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exhaust backfire

I have a 496 procharged motor. I went for a blst off last week and it broke up so I let off. I took the cap off and noticed the carbon ball was worn down to nothing. It is an msd distributor. It ran fine except when I stepped on it so I figured a new cap and rotor was in order. I pulled the plugs they were a little fouled so I had another set and changed them out. The distributor pick up and sprocket(don't know actual name) under the cap were all rusty so I pulled the distributor and cleaned all the parts and put it back together. I installed the plugs wires etc cranked it up and the afr's are exactly the way it was before but I am getting gun shots out the exhaust. It comes out of either side randomly. When it got a little warm and I revved a little it smoothed out. I started it at night to see if there was any crossfiring but did not see any. I got the replacement cap from summit it was their brand not msd. I am not sure if that is the problem but will put the old cap on tomorrow. Firing order etc is all good. Any ideas? Help

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Old 02-17-2012, 10:20 PM
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dont mix up cap and rotors. they should be matched. i put an msd cap on a summit dist. without changing the rotor once. i just wanted a red cap instead of black. the car backfired so bad it sounded like my buddies .50 cal. out of both sides randomly just like ur car. buy the cap and rotor as a set cuz as soon as i changed the rotor to match the backfiring ceased. the summit cap and rotor worked fine together and were brand new. mixed up the parts and whaddya know....pow! try it out. matched cap and rotor. 20 bucks says it fixes the issue.
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Old 02-17-2012, 10:27 PM
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Man I wish it was that simple. I had put on the cap and rotor from summit first backfired like crazy so I figured maybe it was the rotor it did not feel like it sat as nice as the msd but it made no difference. But I think that helps me because it must have something to do with the cap and rotor then.
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Old 02-17-2012, 10:33 PM
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now that i think about it the timing could be retarded a lil due to a flawed cap. if the plastic was molded slightly off from the original it will put the terminals in slightly different positions.maybe the vacuum advance is taking care of this when you give it gas and thats y it quiets down. just using my imagination on that one but its definately not out of the question!
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Old 02-17-2012, 10:36 PM
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nope locked out timing very frustrating a simple cap and rotor change turns nightmare. Thanks though and I did double check with a light
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Old 02-18-2012, 03:54 PM
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Well for those who care I figured it out. I feel so stupid lol. When I took the magnetic pickup out of the msd billet distributor to clean up all the rust I replaced it with the wrong polarity facing the pick up. I found out by looking up the directions on msd magnetic pick up replacement instructions. I am amazed I guess it causes it to fire in between the high points instead of when the high points (of the sprocket). So it must have been firing for a longer time then it usually would igniting the exhaust gases. I am so freakin happy it was something simple, chased my tale for 2 days. I figured I would ad this in case someone else in cyberspace ever has the same issue.
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Old 02-19-2012, 05:43 PM
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dude i'd have never thought of that!
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Old 02-19-2012, 05:48 PM
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I just retraced my steps, pulled the distributor held the magnet in my hand and said well it will fit both ways so maybe one way is the right way. What a relief the car sounded like the timing chain slipped a gear. My neighbor called up and asked what the gun shots were lol. If you want a good laugh I even swapped out the center carbon ball piece and put it into the msd cap thinking it was the summit cap. So the msd is back in with a new carbon ball.
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