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Discussion Starter #1 (Edited)
The car is a 69 Mach 1 w/ 428 CJ motor with Blue thunder intake( recently replace gaskets) original heads modified w/solid lifter valve train and Holly 3310 carb. Been running this motor for about 12,000 miles, VR 1 10/30 oil, and the fouled plug is a recent, last six months, event. May have been developing for a while hard to tell. Most recently has become obvious. One plug, AUTOLITE 45 gapped at 35, only is fouling all the rest are showing normal wear. Six months or so back I noticed a slight miss. Pulled all the plus and number 6 plug looked like something from a horror movie. Carbon build up everywhere surprised it fired at all. Replaced all the plugs and have been checking regularly. Number six only requires cleaning or replacement every 200-300 miles. Condition is oiled and carboned. Ran a leak down test came up good. Leak down at # 5 was also good. Compression tests were 150 cold and 190 hot. Compression tests at other cylinders was comparable. Any thoughts or suggestions ? Interested to hear what you think ?
751848
Thanks, Brian
 

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Discussion Starter #3
Thanks for the reply. The seals were my first guess and tried to check them out. Was able to get the dual coil springs off at #5 and that seal look just fine. Because of the dual springs, tight quarters and angles involved getting the springs off is difficult/impossible. Because #5 looked fine and the others are in place and still soft can only assume they are fine as well ? Brian
 

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I wouldn't assume anything. It could have been improperly installed or came up on #6 and you can't see that looking @ #5 for comparison.
Might also put a dial indicator on the valve stem if you have one available to check for side play once you have the springs off.
 

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Discussion Starter #5
Well you are right of course but... look at the picture. The seals are visible and having done a visual inspection nothing seemed out of order. So just looking for other possibilities and experience's. Seems odd that one plug only has the fouling issue ? Brian
 

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Can you see the stem spring on the seal on #6? I'm on my cell phone and can only zoom in so far on the picture. Could be looks good from the outside, but the inner lip of the seal on the stem is bad or defective. My opinion, at this point you've narrowed down the issue to oil infiltration to cylinder 6. Leak down and compression check rules out rings and valve seats, so there isn't much left.even if the valve guides are worn, the seals should keep oil out of the cylinder but something is wrong on number 6.

Its troubleshooting 101 to me, process of elimination.

Sent from my SM-N960U using Tapatalk
 

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Discussion Starter #7
Trouble shooting is what I am doing. Running out of ideas and yes I have swapped plug wires. Brian
 

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Stem seal or plug wires would also by my guesses. Maybe you have an issue at the distributor? Stem seals can be changed without pulling the heads (pressurized cylinder or using a lot of robe).
 

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I tend to agree with others, it sounds like valve stem seal. But I'll throw this out, back in the day I couldn't run the recommended AC plugs in my 64-1/2 because they were too cold, Champion and Autolite's were fine. Maybe your plugs are a tad cold for your current driving style.
 

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Let's see....

One plug out of 8 is oil fouling.
Compression in that cylinder is consistent with the others.
Leakdown in that cylinder is consistent with the others.
Pretty much only 4 ways oil will get into the cylinder...
1. Past the intake valve guide.
2. Past the intake manifold gasket.
3. Through the PCV valve.
4. Past the oil control ring.

You indicate it's cylinder# 6 (second one back on the left bank) so it's not going to be the PCV valve.
If you could put a borescope down the intake runner for #6 you might be able to see oil infiltration at the intake to head parting line.
Removal of the valve springs on #6 intake will permit you to inspect the valve stem seal and check for guide wear.
That leaves the oil control ring. A problem there isn't going to show up on a compression and leakdown test. Hypothetically speaking, it could also be the scraper ring too. Once you've eliminated the other possibilities there is only pulling the slug that will tell the story.
 

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Discussion Starter #11
Golly I was kinda hoping there was a 5 th possibility ? Thanks for the help. Brian
 

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Per what Woodchuck said (#1, #2), if it was a localized leak of the intake gasket near cylinder #6 or a worn intake valve guide I would think that you would see a flutter on your manifold vacuum at idle.
 

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Discussion Starter #14 (Edited)
OK thanks for that suggestion. Will do a test today. Brian
Vacuum reading taken at power disc brake vacuum line. Reading is in the green although does dive into the red 'late ign timing' when accelerating the throttle. Normal. Brian
751955
 

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...meanwhile, open that ones gap a little more or/and use a higher heat range plug there.

Is the PCV going into an intake runner?
 

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OK thanks for that suggestion. Will do a test today. Brian
Vacuum reading taken at power disc brake vacuum line. Reading is in the green although does dive into the red 'late ign timing' when accelerating the throttle. Normal. Brian
When it's idling (and in the green) is there any needle flutter on the vacuum gauge, say, +/-2psi ? If it is rock steady then I'd think your intake runners/gasket and valve guides would be ok. Flutter with a stock cam would tend to indicate some air (and, therefore, oil) leak during intake stroke (of course, if you have a lumpy cam you'd also have some flutter because of valve ovelap). Note, this is just my speculation. Someone on the thread please correct me if my thinking is wrong here.
 

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Discussion Starter #17
Thanks for the added info. Steady needle and stock-ish cam. Brian
 

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Since it's easy, make sure you check that your PCV valve is not being stuck open. Remove it, shake it and make sure it rattles. Usually, a stuck PCV valve should cause fouling of all plugs equally. And, as 1ofAMillion+ said, if the PCV valve is attached on the intake manifold at/near the runner of #6, it might cause localized fouling even if the PCV valve is ok.

If that checks out, my best guess, as others have suggested, is bad a valve seal or (per Woodchuck) an oil ring problem (BTW: I never knew that a compression check would not catch oil getting past the rings... I learn something new every day ;-)
 

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My first vote is the Intake gasket, you just replaced it so if you over torqued it you coudda squirted the #6 gasket into the oil gallery and now you got issues. SInce compression and leakdown are good, I would look to the gasket you just replaced, timing of the project aligns. Otherwise, any chance you ran your PVC into the #6 intake runner during gasket replacement?. Needs to go to teh base of the carb to distribute to all cylinders. Perhaps swapped it with the brake booster? That's all I got - long shot for sure but the guys have the rest that I can think of covered.
 
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