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Weird spark plug condition # 4 cylinder

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The old grey mare has been pulled out of the barn to once again discuss spark plugs.

Not too long ago, Curt Ingraham posted on his clogged plugs but I ran into an unusual condition yesterday while assisting John McWilliams with his 74tii.

When I pulled his old Bosch W7DC plugs, # 1,2 & 3 looked good except for the large gap. # 4 was all clogged up with crap. Not that it really matters, but # 1,2 and 3 Bosch plugs were "Made in India" and # 4 was a German plug.

The cylinder head and valvetrain looked really good and the valve clearances were pretty close to spec.

John says his car runs really well and the oil is changed at least once a year.

What could be causing the # 4 plug to gum up like this?




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stop worrying - get on with driving -

replace all the plugs with SAME new ones-

change the oil and filter, replace the gas filter -

drain the gas tank with the drain screw if present

on that tank under, behind the right rear wheel

area - and drive the dam thing for 200 miles.

NOW remove the plugs and lets talk. Untill then

drive the dam thing - daily.

(possible causes for your one krapped-up plug:

valve guide, valve seal, injector, pump check valve,

piston ring trouble, scored up cylinder wall from

past broken spark plug bits tearing up the cylinder,

........just drive the dam thing after you perform

every one of the above pre-200 mile roadtest)

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Wait C.D. - me worry? But I'm an Engineer who's paid to worry.

John sent me a follow-up e-mail indicating his trip home was uneventful and the car ran great. Nothing like an "Italian Tune-up".

New plugs are Bosch WR7DC gapped @ 0.028"

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I vaguely remember something about #4 cylinder plug being easier to foul on tii's.



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'69 has the same spark plug condition--# 4 cylinder carbons up much more quickly than the other three cylinders, even though engine compression is pretty much the same on all four cylinders.

The explanation I've been given is that on high mileage engines, that are gonna burn oil anyway, the cylinder furthest from the radiator runs the hottest and therefore has slightly larger clearances than the others.

I noticed the same thing on my Renault engine--the rearmost cylinder was the one that needed the most spark plug attention when the mileage got high.

My solution is to pull #4 plug and give it a good cleaning when I first detect a misfire. You can also install one or two range hotter plug in that cylinder to help burn the carbon off so it won't foul so quickly and/or install a high energy ignition system.



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