Plug colour.

Steve C

Member
Just been checking my plugs and I am not sure what to do as i have never seen any in this condition.

Car is 3500 v8 low compression (9.45 cr) , starts and runs ok, idles ok at 600 rpm, no pinking etc, runs on unleaded with no additives.

Attached photos of one plug at different angles to try and show condition, but all 8 are very similar

Half of plug looks rich? the other half very weak?

Any advice would be great, many thanks Steve.
 

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Hi Steve,

Just to note, 9.45 : 1 CR, do you mean 9.25:1? In either case, that CR is not low compression. Down into the 8s becomes LC.
Hmmm, not seen a colour combination on the one plug like that before, but that is not to say it is a bad thing. Your background information does not suggest a problem. Which cylinders do these plugs represent? Don't forget, the appearance of a plug is largely influenced by what the engine was doing prior to you switching it off. Was it idling for a time?

Ron.
 
Hi Ron, thanks for the reply, yes you are right i did mean 9.25:1cr (1974).

The photos are of a single plug from different angles, but as mentioned above, all 8 plugs are very similar, the car had been driven as normal prior to being parked over night in the garage, and plugs removed in morning, this is my daily driver and is used on a very regular basis.

Ive had 5 classic cars before getting the rover but never seen plugs like this.

Regards Steve
 
I would read the white as showing a lean mixture. How many miles on these plugs - doesnt look like much. The black part looks a bit like shadow of the electrode - not so? Have you done the SU piston lift test - lift a piston ~ 1/16" at idle, fully hot - revs rise and stay up, rich; revs drop, stumble, die, lean; revs rise then down again, good mixture. Are the heads original, or SD1 or later ? Later heads need long reach plugs.
 
The black on the electrode is a dry sooty coating, just as if the mixture is running rich, and the white as if running lean, very confusing!!

The plugs have been in for about 500 miles, the car is used every day, about 20 miles per day, ( perhaps to many short journeys?)

The mixture at the carbs is good (I think), car idles ok, the car (auto) will reverse on idle 40 metres down my drive without cutting out , when i first got the car it would cut out on selecting reverse, which i thought was a good indicator of running to lean.

When i fitted these plugs, I changed them like for like, not sure about the heads? how how can you tell the difference?

Regards Steve.
 
Hi Steve,

What brand and specification are your plugs? Are they Champion L92YC or an NGK model?
If you were to fit a long reach plug into an early head (prior to 1977), then the plug will protrude a considerable about into the combustion chamber resulting in the piston hitting it with your engine then stopping. Short reach plus in a later head would likely result in the engine not running at all well.

Ron.
 
Plugs are NGK BP5HS. I have about a 100 mile trip to do later today, i will give it a good run and check plugs again, it may be my regular short runs that is causing the problem.

Regards Steve.
 
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