A few months ago i commented about the pinging on the overrun on my RT1B.
I said about lowering the compression with the fitment of a thicker copper gasket .
I bought one from a manufacturer in Arizona after messaging to and fro for a few days.
The thickest one available to be cut from sheet without making it susceptible to splitting.
I fitted it as directed to the letter.
Also,i raised the needle by two notches on the carb.
The result, lots smoother running,you can start it without MX boots on and starts first kick every time hot or cold.
The thing is,there's no noticeable performance loss,in fact it's more responsive. The pinging seems to have subsided and the horrible overrun is loads less.
Still room for further improvement though. I have some ideas to try over winter.
Just so you know it works.

A question and a thought: First, what do you mean by overrun? My thought is why couldn't you have just added a second gasket at the base of the cylinder instead of looking for a thicker head gasket? Just a thought...

What main jet do you have?

Now there’s something that’s got my interest… any pictures for comparison?
Sneezles61

if overrun means engine going batshit crazy then you probably have an airleak. I had one once and it melted a hole in my piston and blew piston rings out my tail pipe!

High R's and backing off the throttle.
Makes it lean and knocks is common.

cliff

"Pinging" and/or surging on decel is a complex issue...  closing throttle causes a rich condition and poor scavenging of exhaust gasses.  Engine will misfire and often in my experience the "pinging" is what is colloquially termed "pipe bang"...  unburned fuel is ignited and explodes in the pipe. 

(Years ago I came across a video with a camera pointed at the exhaust flange and with every "bang" on decelleration you could see a puff of smoke and the pipe bounce away from the engine slightly.) 

Engines with a better seal at the head pipe...  like some of the later Yamaha's with a solid flange holding pipe to head...  seem less likely to do this...  so fresh air getting drawn into exhaust from leaky connection at the head pipe could contribute to "pipe bang" in my experience.

There are also some theories that the ultra rich mixtures from misfires and the poor scavenging can result in very high compression pressures (fuel doesn't compress so a few cc's of extra fuel will raise compression pressure). With non-uniform conditions in the chamber a "pocket" of not-too-rich mixture preignites...  this type of "pinging" is usually accompanied by a brief surge of acceleration along with the "ping" in my experience. 

The engines larger than 250 seem more far more likely to have the "surge-ping" which is probably preignition.  I'm not sure anyone knows for sure.

Thank you for taking the time to pass along what your finding….
I’ve just got used to pulling in the clutch… but that’s road riding…
Sneezles61

Great write up MarkT!!!

Rig up some kind of cutout to stop the spark on de-celeration. Nothing to bang then. Naa, maybe not. How bout a compression ignition system like Honda had for a short while. How the hell did that work.