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Old 08-16-2003, 09:06 PM
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My 392 is going from bad to worse

After a short time of running on 8 cylinders, my 392 is now running well on 4! It is all the more confusing, because even though I can remove the plug wires from any of the front 4 plugs and can hear that the cylinder is not firing, it is driveable, and can still lay rubber by just punching the throttle. which tells me that even though cylinders 1&3 and 2&4 are not running well, they must be doing something! I remember once upon a time when changing plugs I dropped one, and being young and foolish did not check the gap before reinstalling it, and of course it was closed and did not fire, and it barely even had enough power to move, with just one cylinder nor firing, so clearly somthing is going on in the front four, but not much. To repeat specs, it has 155 to 160 lbs cylinder pressure on all cylinders, idle vacuum is now 17" which is up from 6" before I replaced carb and intake gaskets. I have tried setting timing by vacuum gauge, and what seems to happen is, I will advance the dist, the vacuum will go up a few inches, as will the idle speed. I back off on the idle screw, and thevacuum goes back to 17". I have done this four or five times, with the same results. I am sure I am in way advanced territory, because the vacuum advance tube had to be bent out of the way because it was hitting the intake manifold, which it never even came close to when timing with a light. I have not tried a timing light on it, because of its position in a 38 Chevy, to time the engine i have to remove part of the radiator support frame, and it isn't any fun. I have about runout of ideas, am wondering if there is something going to hell with the cam or lifters, but if the lobes were worn, so that the intakes were not opening much, wouldn't that by itself decrease cylinder pressure readings? Any suggestions will be appreciated.

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Old 08-16-2003, 09:17 PM
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Basic rules of an engine, as long as you have compression, proper fuel/air mix, and spark, the cylinder will fire. I would fear not having a cylinder fire cause on the exhaust stroke all that fresh air/fuel mix would be pushed into the exhaust and could give you a REAL bad day. So I wouldnt play around by taking the wires off the splugs. Unless you are just doing a compression test on a cylinder at a time.
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Old 08-17-2003, 12:03 AM
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I had the same thing happen to me once with a 455 Pontiac, it dropped about 4 cylinders (it was a restriction in the carburetor) , you mentioned you changed the carb. Have you looked into this?
Also, what about your ignition system? What type of ignition do you have? is it points? never underestimate the treachery of point type ignitions. I would look very hard at your distributor.
Compression is good, so let's guess that your camshaft timing has not slipped out of phase. You can check lobe lift with a dial indicator off of the rocker arms to see if you've got lobes going down. I would not rule anything out at this point, but stick to the basics starting with your ignition system, veryify proper voltage and use a known-good distributor if you have access to one. If you have access to a known good carb, try it--- if these two items dont help, then I would start looking for vacuum leaks somewhere. or a hidden crack creating a vacuum leak. Probably not much help, but use scientific process of elimination to isolate your problem.
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Old 08-17-2003, 12:34 AM
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As for the timing, do you have enough space to rig a mirror without taking anything off. I completed allot of tasks either backwards or without being able to see where my hands were. Have you checked the basics like the distributor cap, something could be shorting certain cylinders. Like NAIRB stated, think about what has been done to the engine since you had complete cylinder burn and review all the procedures up to now. Maybe something small missed.
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