One sooty plug

Jimbo

Jimbo

1978 260z in yellow
Club Member
I've been driving the datsun a fair bit this week and thought I would pull the plugs and see how the mixtures affected the plugs coulouring and all but one is a sandy colour with a bit of soot around the base which is about right maybe a little rich
But the no4 plug was sooty all over. I've gone through all the usuall checks, first thing I did was compression check and was 165-170 psi which is same as the others. Fuelling can't be it otherwise the no5 & 6 would be the same.
Valve clearance seems ok.
I replaced the plug and the new one went the same way.
I went to swap HT leads with no5 but noticed the lead was a bit loose on the dizzy cap
Do you think this could cause the plug to soot up maybe as a result of misfire/weak spark?
Doesn't seem to misfire and pulls well.



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if it was burning oil i would have thought it would be crustier and wetter than it is maybe like this?
https://www.google.co.uk/search?q=o...UIBygC&biw=1240&bih=655#imgrc=ibpC_mpOUjryMM:

this is more like overfueling or something which seems impossible on one cylinder surely?
looks like below
https://www.google.co.uk/search?q=o...&imgdii=sFT7lIXehGpLdM:&imgrc=tfM0suOlWN47OM:

whats stranger though is it wasn't doing this last year as the plugs were all pretty much the same when i changed them.
 
will have to try running it some more maybe up the motorway and see if it cleans up.
 
Jimbo said:
if it was burning oil i would have thought it would be crustier and wetter than it is maybe like this?
https://www.google.co.uk/search?q=o...UIBygC&biw=1240&bih=655#imgrc=ibpC_mpOUjryMM:

this is more like overfueling or something which seems impossible on one cylinder surely?
looks like below
https://www.google.co.uk/search?q=o...&imgdii=sFT7lIXehGpLdM:&imgrc=tfM0suOlWN47OM:

whats stranger though is it wasn't doing this last year as the plugs were all pretty much the same when i changed them.
Click to expand...

I think on SUs it would be very hard to over fuel only one cylinder, easy on sidedraughts though.
Perhaps you could have an air leak on the other 5 :)
 
True true there's always something going on with these old motors lol.



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Jimbo said:
I went to swap HT leads with no5 but noticed the lead was a bit loose on the dizzy cap
Do you think this could cause the plug to soot up maybe as a result of misfire/weak spark?
Click to expand...

After changing everything else be the lead or the dizzy cap imo, or very last resort the cam on the dizzy but unlikely
 
well i took it for a test run the other day and checked the plug all fine and dandy:thumbs:
must literally have been loose ht lead:rolleyes:
always pays to check the simple things lol
 
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