It is decades since I drove my RP lwb box in stock form. It displayed a characteristic which I wonder if others have observed.

I drove 60 miles to my home town every weekend so had plenty of opportunity to observe. The cyls were well worn and oil consumption generous. After a valve grind and decoke car would (eventually) attain a true 50 mph on a true flat road. But performance immediately started to fall off to just 47 or less after about 2000 miles. Spark advance over a huge range affected only the engine noise! But a quick decoke and all was fully restored!
I have never observed the effect on other cars, or later with the later head.
The carbon was not uncommonly thick.
In the days when many makes of cars received regular decokes, the more usual complaint was lack of performance after, due the reduced c.r.
Possibly the carbon restricts breathing, and acts like a sponge and absorbs heat at the peak of mixture ignition.
(In his early days Stirling Moss used to remove the head from his 500 and polish after each hill run!)
Has anyone noticed a similar effect with their Seven? I suppose now it is no longer considered normal to reuse head gaskets, so few cars receive a decoke alone.

Any better explanations?

(Incidentally, with valves of the type sold in the 1950s, of less stamina than XB steel, itself now considered very inferior, by the second or third decoke a valve would usually have started to burn.)

Location: Auckland