Reading Spark Plugs

Trying to find a miss on my engine from 1,000 RPM's to 2,000 RPMs, no smoke but hesitation under load.

Pulled the plugs to see what I could see. All were grey/brown, and appeared Ok but a few had a black dark brown line down the porcelain, like a pencil line, not the insulator, but the porcelain.

I've searched but found nothing that mentions this, is this common?
It was only on a few.

 

Could be distributor / plug wires / coil / electrical I would say.

Frenchy

 

Yup, definitely electrical or fuel or compression related.

I thought the porcelain part IS the insulator?

 

Would like to see pix of the line you mention to know for sure but the black line on the insulator is most likely a carbon track from the spark shorting to ground and not jumping the gap.

Also the line can be a fracture in the insulator caused by detonation also a short now.

I recomend "86 ing" the plugs and cleaning the inside of the plug boots to make sure they are not fouled and causing the tracking.

 

Yep, start with the plugs.

 

Yes it is but what I was trying to say was that it was the part that fires, not the long part that sticks out of the head...

Looking at the pic below the line is on the white part

 

probably a crack, as mentioned. If plugs look flaky and the engine is running poorly, it's a good idea to replace them early in the troubleshooting process. It's also a good idea to do a compression check while you have them out.

 

By the looks of that plug , put in a new set

 

That pic is not my plug just a pic I found online.
The line I have runs vertical from the top of the white down.

Could a ground problem cause this?

Yes I am replacing the plugs.

 

How many do you have like that, was it dropped during the installation? I worked in one good shop that if a plug was dropped during installation, it was thrown away. We were also fussy about handling packages of spark plugs.

I had one once in a stock car motor that gave it a miss right after a plug change. Since nothing else was done, it helped narrow it down fast. The bad plug stuck out like the proverbial sore thumb.

Bob

 

Be sure the engine block and head are well grounded.

 

I just did, or redid my ground..

 

i had one that mis-fired like you mention..it was cracked..once replaced problem gone..yep it was the one that fliped out of my hand while installing it and hit the floor..( should have known not to use it)

the spark is probably coming out of that crack in the side and jumping to the metal part of the plug..it may not show up when you try it out of the engine..but under combustion pressure it will do some wierd ****