Spark plugs

Joined
May 5, 2009
Messages
74
Hey there guys. I have a 71 mercruiser 165. I put new plugs in two weeks ago and I took them out today to do a compression test and found that the brand new plugs were black and looked burnt. Can anyone tell me why they looked like this. Thanks
 

danond

Lieutenant Junior Grade
Joined
Jun 11, 2007
Messages
1,118
Re: Spark plugs

When was your last tune-up? Sounds like your carb needs a rebuild or some adjusting.

I'd vote rebuild.
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,514
Re: Spark plugs

You should look at the plugs after a long high speed run with no idle to see how they look. That way you can see if its just the idle circuit or choke that is causimg them to be black.
 

ziggy

Admiral
Joined
Jun 30, 2004
Messages
7,473
Re: Spark plugs

what kinda tune up did you do? cap, rotor, plugs, points, condenser, fuel filter (s), dwell and then time it, idle speed adj.? how did you rebuild the carb? carb totally disassembled and soaked in carb cleaner, scrubbed clean then blowed out all passages with compressed air? all new gaskets, all specs set proper on the carb. per the book?

your description sounds like the cold fouled plugs that are mentioned in the book. from s/m #2, "dry, black deposits indicate rich fuel mixture or weak ign. clogged flame arrestor, flooding carb. sticky choke or weak ign. components all are probable causes. if only one or two plugs in a set are fouled, however, check for sticking valves or bad ign. leads"

how did the compression numbers turn out?
 
Joined
May 5, 2009
Messages
74
Re: Spark plugs

I did a complete tune up just as ziggy described. I cleaned the heck out of that carb. hehe. The numbers didn't turn out bad but they could be better. I still can't get it to idle right without dying yet so I had to do the test with a cold motor. The numbers were

1 - 105
2 - 130
3 - 125
4 - 127
5 - 130
6 - 112
 

danond

Lieutenant Junior Grade
Joined
Jun 11, 2007
Messages
1,118
Re: Spark plugs

Your compression numbers are close enough that your plugs shouldn't be fouling. You're still dealing with a bad fuel mix.
 
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