Backfiring help!!!

[damianbryand]

Ok so I purchased a motor from a guy and had it installed a few days ago. Well it fires up perfect but during idle it sounds like its backfiring. It's coming out of the driver side exhaust so I unplugged the spark plug wires one by one and narrowed it to the #7 plug. The motor appeared to be clean when purchased and it sounds great but that backfire sound is killing me. Where and how should I start to resolve the issue?
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[damianbryand]

Sorry. 68 c10 with a 350 small block.
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[TBONE1964]

Make sure that #5 and #7 plug wires are not crossed first of all. make sure the plug gaps are good, then do a compression test. See what you get. You should get somewhere around 140 - 160 psi. Even 120 would be OK. See what it is on number 7 cylinder. If it is low, pull the valve cover and roll the engine over by hand and look for a rocker on #7 that has no or very little movement. If that is the case, pull the the rockers and check the push rods to see if they are bent or broken. If they are OK, then pull the cam. You can use #5 cylinder as your good example as far as how far the rockers should move.

[terry b]

Could be a exhaust valve hung up from sitting. Take the rocker off that valve and rap it with a piece of brass a few times and see if it comes up any.

[damianbryand]

I used ac delco 44 rt plugs or something like that. The guy I bought the truck from says firing order is good. Mentioned something about TS plugs?
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[CC69Rat]

It sounds to me like 5 and 7 are crossed or maybe 7 is arcing through the header? Does it have headers on it? Is there a burned place in the boot that plugs on to the spark plug?

On the drivers side of the truck, the cylinders are numbered 1,3,5,7. ... Starting from the front of the truck and moving toward the firewall. The even are on the pass side. 2,4,6,8. Front to back.

Starting with #1 plug wire. Trace each wire clockwise around the distributor cap. The firing order should be 1,8,4,3,6,5,7,2. 5 and 7 are very easy to get crossed up because they are right behind each other in the firing order plus they are physically opposite. (7 comes after 5 in order but before on the plugs ). Happens quite often really and it would cause the backfire sound you're describing.

Let us know
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[damianbryand]

It does have headers on it. Wouldn't it also backfire on the 5 plug if the wires were crossed?
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[70c10stepside]

Could be cracked dizzy cap or loose cam letting the dizzy rotor button tap the cap.. Long shot, if all that ^^^^^ checks out.

[CC69Rat]

Yes 5 would mis fire too but they are so close it would be difficult to tell from where it actually came.. 5 or 7 .. did you trace each wire?
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