Never heard of that before. It may be that we are a younger set of drivers with the newer engines and oils. The use of non-detergent oil and leaded gas went the way of gobs of sludge I cleaned out of engines of the early sixties!

These newer engines have better fuel management and should never carbon up that badly in the first place.
Besides, if yor car did that those emissions testers, or those "EPA" signal chiefs, would be all over that like fresh "smoke" over a campfire, reporting you to the state!

As last I remember the threads retreat out of the combustion chamber so there is no "blowing off" the threads.

So the only place the color of carbon soot can get attached to the plug is on the under cut diameter in front of and at the base of the threads.
It has always appeared as clean or cleaner as the face diameter when I removed them. No sign of cleaner specks being uncovered on it.

There is a lot of piston surface area and combustion head surface area to catch a minuscule particle when compared to the suface area of the sealing angles on the two valves. The valves are even smaller on three and four valve engines so how much more area is there really to catch on too, is a guess from this ole' boy!

I have read though, that you should open the throttle all the way so it can get more air in there to compress. Maybe that is the standard specification for some manufactures but I do not see it in all books.

I wonder if it makes that much difference. Because when I start my engine I leave the throttle closed and it gets what air it gets. All those cylinders have to suck as good as they get, IMHO.
I would want to know the compression readings under those conditions, wouldn't you?

Should they not work morning or night or would high noon be better? I want those readings either or both ways. I spin the engine for about three chugs minimum and I have never read how many is good or a standard!
The engine should be cool to remove the spark plugs from a aluminum! That is a standard I read a lot!
My grippers like it better too!

Oh well, came with a nickle and left two cents and someone will probably give me some change back! (:-)

Phil