Good go, old man! That was some innovative sleuthing. Carbon knock occurs in cars, too, and sounds like a bad bearing. Detonation can also occur from carbon build-up. We call it 'pinging'. I like to prevent carbon build-up rather than treat it, so I use some additives in my fuel. So far, so good.

Good job!

I performed this operation on my Jetta over the summer. It's amazing what a little carbon build-up can cause.

Decarbonizing is said to be the best way to restore lost power; especially on an L-head.

"I like to prevent carbon build-up rather than treat it, so I use some additives in my fuel."

Like Saxman1 says, a little something like SeaFoam in the gas will do it. You may not have that but something similar, beats removing head, etc.

Walt Conner

Additives that claim to decoke engines seemed to disappear from the shelves here soon after unleaded arrived. Lots of things to clean carbs and injectors. (I used some injector cleaner from Comma, a local firm, in the spring and its effect on my Rover was remarkable.)
What is Seafoam like? Any clues to its composition?

'beats removing head, etc'

You got that right!

Do they have boat/marine supply houses there? Just look for fuel additives for carbon prevention/removal, fuel stabilization and 'top-oil'. I bet you can find it online in the UK. They might not have 'Seafoam' but they will have the equivalent. Also, you might want to verify the mixture on that engine to make sure it's optimal - if there's a mixture adjustment screw. It might be running a little rich or the choke may be partially on.

In my younger days, I had a British Vauxhall that would run all day with the key off and not shut off on a hot day unless I put it in high gear and dumped the clutch. I cranked up the idle and dribbled diesel fuel into the carburetor. I dumped the last few ounces in flooding the engine and let it sit overnight. It really smoked but the problem was solved although it burned oil after that.

Just happened to think that 'Fuel Injection Cleaner' dissolves carbon, so that should work to clean a head. I've seen mechanics pour something in the carb to clean carbon out of cars, resulting in a huge cloud of white smoke, but never did know what it was. Supposed to have eliminated carbon knock and pinging.

I had a Vauxhall Chevette from 1977 to 1991. It took the family all over the place, including Spain. 4 cylinders and 1256 cc. It used to ping if you put your foot down at any speed less than about 1800 rpm. It was only a light sound, like tinkling a teaspoon (read coffee spoon in USA!) against a cup. Not like the rather more alarming sound from the Briggs. It never stopped the Chevette, either! I just might try some injector cleaner next time the Rover is due for it, but does anybody think it might hurt the diaphragm? I don't know what is in these mysterious bottles. The diaphragm is a weakish point in the Classic design.
In some ways I preferred the old design my Father had. The carb jet sucked direct from the tank. I don't see it as a disadvantage if the thing runs a little rich when the tank is full and weak if its nearly empty.
One could fill it full for a cool day in the fall and half full in high summer when the air is hot and thinner. How do feel about checking the barometer and humidity levels before you fill the tank and mow the lawn?!!
I reckon from the look of the plug insulator (grey) and the exhaust valve (white specks on the head and a pretty sound sealing edge) the mixture is pretty well spot on.

Yes, found the same problem on an old Toro with a Tecumseh motor. Popping the head off a flathead is simple, so I added it to the annual ritual of setting points, plug and general cleanup.

seafoamsales.com

A GREAT PRODUCT developed by an Evinrude/OMC engineer
expressly to remove carbon from 2 & 4-cycle engines.
IT WORKS and works well !!!

T-Man

I live on the East Coast, and can tell you that Seafoam is found in all Marine supply houses and boating/fishing supply stores down by the sea. I am forced to conclude from this that it sells there because it works. It can be used in the gas and/or oil.