Cylinders 3 and 7 not firing - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-21-2003, 04:11 PM
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Cylinders 3 and 7 not firing

sbc 350, mallory ign, msd coil, msd 8.5 wires.

plugs look ok, i switched plug wires around but cannot get 3 and 7 to fire. I took off distributor and visually looks ok.

any suggestions?

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Old 11-21-2003, 04:20 PM
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Is this a rebuilt? I'd check compression next, while it's running, pull the plug wire and check for spark w/ well insulated pliers and screwdriver!
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Old 11-21-2003, 04:41 PM
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yes, rebuilt about 3000 miles ago.

when you say check spark, do i just pull the plug wire off and look up inside for spark?
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Old 11-21-2003, 04:56 PM
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Pull it off, stick a screwdriver up in the clip, and hold the blade 1/4 " from metal, be sure it's a well insulated one so you don't get bit! Is there any noise from under the valve covers? Try switching plugs w/ known good ones also! Can you check compression?
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Old 11-21-2003, 07:24 PM
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thx Wrencher, there's no abnormal noise from valve cover area. I will move the plugs around tomorrow am.

I don't have a compression gauge but I will buy one if needed. BTW, when the engine was rebuilt, no work was done to the edelbrock heads. They looked rather new and the mechanic said I could always do them later as I was on a budget.
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Old 11-21-2003, 09:04 PM
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The easiest way to check spark is to unplug wire from sparkplug, put a new plug in the wire, hook an alligator clipped ground wire to the base metal of the plug and then start the engine. You can't get shocked this way. The problem isn't going to be a cracked dist cap problem since the fire order is 2 cylinders apart,normally 2 together in order. Not a headgasket since they are apart. You don't say if problem just started or has been there since rebuild. I would start by checking fire order(18436572). Then a compression test. If compression below 25% of highest clyinder then do a leakdown test to see where its going (more on that if needs)Also if valves were adjusted too tight will cause low compression and a dead hole. Also make sure no vacuum hoses are off on th manifold.
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Old 11-21-2003, 09:40 PM
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hlweyl,
This is what I teach my students.
Switch spark plugs with a cylinder that is not mismiring.
Switch plug wires with a different cylinder that is not misfiring.
Then restart the engine and figure out which cylinder is misfiring.
If the miss is now in the cylinder that got the plug wire then that is the thing that is causing the miss. If the cylinder that is not firing has moved to the cylinder with the spark plug then that is what is causing the misfire. But if the miss is still in the orignal cylinder then there is something wrong with either the cylinder itself(flat intake lobe on the camshaft or a broken intake push rod) or the magnet for that cylinder inside the distributor has gone bad so as to not fire the coil for that cylinder. I would also check the intake manifold for vacuum leaks. It could be the power brake booster is leaking and that is connected to the # 7 intake runner. The runner for the # three cylinder might be leaking from the vacuum canister. From my experince pulling the plug wire at the spark plug leaves the electicity only one way to get to ground and that is through YOU. If you pull the wire at the cap, instead of giving you the shock of your life, it will try to get to ground through the wire to the spark plug. Then by holding the end near the cap you will be able to see the color of the spark. Blue is what you want to see.

I hope this helps.
Scholman
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Old 11-22-2003, 10:42 AM
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well... switched the plugs around and that was it. Just a couple of bad plugs! DUH

Car is almost running back to normal, little bit of backfiring under hard acceleration. Gonna check timing.

Car smells like its running very rich, which i guess it is fouling the plugs also. Think i'm gonna have the demon carb re-jetted as i don't have a clue how to do it.

Thx for all your input fellas
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