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Old 04-17-2018, 05:39 AM   #1
int77345
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Default spark plug fouling analysis

hey guys,
about every 2 years give or take cylinder #4 fouls its spark plug.
this is on a scout II carberated 345
any idea what the deposits are? my guess is oil ?
only way to fix this would be an engine rebuild correct ?

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Old 04-17-2018, 06:12 AM   #2
FDChappie
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Default Re: spark plug fouling analysis

How does the compression on that cylinder compare to the others?
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Old 04-17-2018, 08:12 AM   #3
int77345
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Default Re: spark plug fouling analysis

i haven't done a compression check in years, so i really dont know. this is a guess, but when a new spark plug is installed the engine runs very smooth at all load levels . so my guess is compression is fairly even through out all 8 cylinders .

for a 40 year old 500,000 mile engine its held up very well.
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Old 04-17-2018, 08:48 AM   #4
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Default Re: spark plug fouling analysis

Amazing longevity. I'd say you've gotten your money's worth out of it and then some.
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Old 04-17-2018, 03:04 PM   #5
Greg R
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Default Re: spark plug fouling analysis

How many miles? Going 2 years is a bit of a stretch considering in the day they where checked or replaced about every 10-12 thousand miles or once a year.

That color tells me it is some fuel deposits and you're probably running a bit cold or cold and slightly rich. Either get the engine to run warmer or go to a hotter plug or both. Maybe some valve guide leakage going on with that; but with a proper fuel ratio and operating temps your plugs may clean up a bit.
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Old 04-17-2018, 07:14 PM   #6
int77345
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Default Re: spark plug fouling analysis

thanks Greg,
over2 years about 10-15K miles. not a daily driven scout but it gets drove alot.

yes i have thought about leaning the carb jetting another step. im at 5,500 feet. any one at around my altitude with the edle 1406 carburetor?

thermostat is 180

what do yall think maybe position #7 on this chart ?

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Old 04-17-2018, 08:36 PM   #7
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Default Re: spark plug fouling analysis

My Edlebrock manual does recommend 2% for each 1,500 ft of altitude so #7 would put you in the ball park. What plugs are being used now? How do the others look? Are your miles driven short haul stop 'n go or some freeway in there? If it's short haul across town a few miles, some hotter plugs can help with the deposits. If you go too lean your performance can suffer along with some engine ping to annoy you.
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Old 04-18-2018, 05:25 AM   #8
int77345
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Default Re: spark plug fouling analysis

these were champion RJ12YC plugs

the other cylinders look ok, not perfect white tip like a modern car but clean burning looking.

most drives are over 7 miles. across town i know that because that is the nearest places to go. and when going 4 wheeling about 50 miles of interstate.

i replaced these plugs with MDS plugs that i had.
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Old 04-22-2018, 10:07 AM   #9
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Default Re: spark plug fouling analysis

The main buildup of carbon is oil. If the others are not loaded up with carbon and in the off white to tan color range, you are very close on fuel mixture.

Sustained hi load driving and plug temperatures will usually leave even an oil burning cylinders plug clean enough to not foul.
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