spark plug fouling question

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May 13, 2008 | 09:08 AM

btorear

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spark plug fouling question

I have a 1974 f 100 with a 302. I bought it last weekend. It ran very rough, backfiring, stalling, engine quitting at stops, etc... changed the plugs and plug wires, distributor cap and rotor when i got home. one plug (for the number 3 cylinder) was fouled with caked up with a lot of black soot. Changed the plugs out and ran it a couple of days. pulled the new plug out from #3 and it's all black with soot too. Any idea what could be causing this? The other plugs look good, it's just this one. I would really appreciate any help anyone can offer, thanks in advance.

May 13, 2008 | 09:57 AM

beartracks

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From: Albuquerque

It could be that there is a problem with the ignition firing that particular plug, i.e. a bad plug wire etc. However the more serious possibility is something like a broken ring or piston. I'd try to do a compression test and resistance test the wires.

May 13, 2008 | 10:02 AM

NOCO77

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From: Northern Colorado

Sounds like #3 cylinder has issues - bad valve seal, guide or crack(s), bent/burned valve, bad/broken piston rings, etc., or some combination of this. I would suspect a fuel delivery issue except you say that the other 7 plugs look ok, so there is a compression problem in that cylinder. When the engine is under a slight load, does it have a miss (studder, stumble,etc.)?

May 13, 2008 | 10:32 AM

btorear

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Yeah, under a slight load it will spudder and cough a little.

May 13, 2008 | 07:59 PM

flcracker9

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You need to test for spark in that cylinder. If that checks ok, run a wet and a dry compression test on all cylinders. This will give you an idea of the condition of your motor. Could be something simple like a valve seal or guide, or could be rings. The test should narrow it down.

May 16, 2008 | 10:17 AM

btorear

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ok. i did the compression tests last night. the cylinders read like this 1=120, 2=120, 3=110, 4=120, 5=140, 6=140, 7=120, 8=125. since #3 was the lowest i did a wet comp test, and it jumped up to almost 150. does this definitely tell me it's the rings, or is there still a possibility it could be something else? also, is there another test i could do to narrow things down? Any help is greatly appreciated.

May 16, 2008 | 11:46 AM

tractoman

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From: Sonoma, CA

I think that is an indicator of bad rings. You could do a leakdown test to confirm this but normally the wet test allows the rings to seal better and raise the compression.

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May 16, 2008 | 02:42 PM

btorear

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Yeah i had the ignition wire off and all the plugs out. What do you mean it's a pretty wide spread? Could that mean something else other than worn rings? You can probably tell, but I'm very much a novice.