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Discussion Starter #1
Help, I've got an all stock 72 350 4bbl(q-jet), the darn thing has developed what at times seems to be a fatal miss (back fires out the exhaust), runs extremely rough, and sometimes dies. It will fire back up, clean itself out and run semi ok (if I drive easy),
I've gone thru the electricals, carburation,
fuel pump(6psi), vacuum (steady @ 15"), compression (150 psi). Now the interesting note: once while testing the compression on #5 cyl the pressure rapidly self relieved, I have a Sun compression tester with a vent switch and usually have to vent every cyl after checking, I've tried to get it to do it again but no luck, also the spark plug #5 has fouled on 2 occasions (all the others are fine), any thought on bent valve or worn springs, ps. the motor has 96000 orig miles and never been apart. Thanks
 

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Rick,
I had this happen once to me. May not be the same thing but it's worth a try. If you have a Ohm meter try reading the resistance of the #5 wire while shaking it. I had a wire do this once. Took shaking the wire to find the break. Just one place to start. Good luck.

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Mike Hurta
(Aug98)
Gold#25
Clute, Texas
www.chevelles.com/feature/aug98.html
 

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Rick,
What type of distributor are you running..points or HEI? Before I changed over to HEI a few years ago on my 70 Chevelle, I found the condenser under the cap breaking down. The car would be running fine, and then it would start to miss, backfire from the exhaust and die. Then, I could start it back up again and drive some more. After driving myself crazy, I checked the condenser at work on a capacitor tester and found it to be leaking. The "uniset" points seemed more prone to this. After awhile I bit the bullet and changed over to an HEI distributor from Year One. The "old Chevelle" never ran better..and I've owned her for 29 years! Change the condenser with a known good one and see what happens. Good luck!
 

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Almost sounds like two problems. With backfiring out the exhaust, I would definately dig into the ingnition system and do a thorough tuneup. As far as the self relieving cylinder, possible sticky valve or leaking seat, but normally with valve train problems vacuum jumps around and also could get backfiring back through the carb. Do the tuneup first and then go from there.

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70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
55 chevy prostreet
 

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I had a similar problem once that was a real bear to diagnose......it ended up that the coil was intermittantly breaking down. I replaced the coil and the problem was solved.
 
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Discussion Starter #6
The distributor is the original points type, that I just went thru, cap, rotor, points and condenser (yes they are the uni-type, but were Echlin-NAPA hi-perf unit over $40),
Wires are new Autolite,I'm going to put the old points and condenser back in, check the #5 wire, the coil is also a NAPA unit less than a year old, but you never know !!!!!
Thanks
 
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Picked-up an Accel coil and points set, put them in, fired her up, let her run for 1/2 hour, took her for a test cruise/blast, seemed okay, went to my favorite private long secluded straight away, stopped, punched her, pulled hard to 5000 rpm, shifted, tires chirped, pulled hard to 5000 rpm, shifted, going fast by now,and shut her down. I know point type ignitions are not the premo set -up by todays hot-rodders, but I love-em. Thanks for the help, and you all have a HAPPY NEW YEAR !!!!!!!!!!!!
 
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