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Old 12-15-2002, 07:33 PM
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Post S.B.C. Backfires

I got a 75 C-10 1/2 ton with a 350 2 barrel. I have been driving this truck for a couple years now, I'm not sure what year the motor is but it has been switched to a HEI. Ok so I was out on the highway and it just started to miss so I drove it home.I still cruised home about 20 miles at around 70 and it missed here and there but wasnt real bad. Once you get the idle down specially from a stop or something it backfires really bad through the carb. It idles fairly smooth though. Its a 2 barrel and I replaced it with another carb but still had the same problem. Its a quick fairly loud popping noise it is making. I also put a new cap wires rotor and plugs in it but it didnt help. Any suggestions? I'd really appreciate it since its my daily.

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Old 12-15-2002, 07:42 PM
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Is it in time correctly
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Old 12-15-2002, 07:47 PM
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[quote]Originally posted by femadorob420:
<strong>Is it in time correctly</strong><hr></blockquote>

I have not checked the timing on it, I would find it hard to believe that it would jump time, and if so I would think it would hardly run at all. It runs pretty decent other than the backfire. The dis. is good and tight and has been for a few years. I have had a couple people listen to it run and everyone thinks the timing is ok.
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Old 12-15-2002, 08:45 PM
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I hate to be the bearer of bad news but I had a 350 that once had the exact same problem, I finnally was told to pull the cam and sure enough two of the lobes were totally rounded off. On the good side I did put a new cam and lifters in and it ran fine for many years after that!
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Old 12-16-2002, 02:34 AM
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May be a rocker arm? that has jumped to the side and is opening the wrong valve!This will make it Pop and miss and you may have a flat lobe on the cam, but check your rockers first, run the motor with the covers off.

[ December 16, 2002: Message edited by: roys63 ]</p>
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Old 12-16-2002, 08:20 AM
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Take off the oil filter and turn it upsides down and look for metal particles. This will tell you real fast about cam ware. Oil will be silver in color
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Old 12-16-2002, 08:44 AM
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You might also check for an intake valve going bad. Had a Plymouth small block that would pop through the carb at idle, but not at higher rpms.
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Old 12-16-2002, 11:54 AM
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It is obvious that you need to get your timing light out and check it....don't assume it is right because someone agrees with you that it probably is OK.

Another thing to check is a vacuum leak. A bad one can cause a stack backfire.

If all this checks out, examine the rockers and look for lifter problems.

Finally and last I would suspect a timing chain jump, it does happen.
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Old 12-16-2002, 12:12 PM
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One thing you might do if all the other advise deos'nt work is try doing a compression test. you might have a bad intake valve or it might not be seated all the way. Hope you find out soon. You might want to think about a portable fire extinguisher incase it gets worse.
Mike
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Old 12-16-2002, 12:35 PM
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maybe the timing chain skipped a toothe, of if its the origonal chain, maybe one of the nylon teeth striped??
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Old 12-16-2002, 04:17 PM
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The timing is ok on it. I'm hoping its not the cam, I think its a bad intake valve. Today was my only day off and I only had a little time to work on it. I ended up taking it to a mechanic and I guess I'll get the verdict on it tomorrow.
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Old 12-17-2002, 08:50 AM
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The First thing I would do is replace the cap and rotor just to make sure there were no shorts in there. Iv'e had caps crack and start firing two cyls at the same time.
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Old 12-17-2002, 09:43 AM
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You said you put a new cap and rotor and everything on it for HEI. Did you check the coil and check to see if it is putting out a good current flow? I had a coil go bad in my 350 and it made it backfire and miss, because it wasn't burning the fuel/air mix right or not at all. Just a thought!
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