Ever had a carb that was too lean?

I would rather put up with a little rich and four stroking than a lean bog. It will burn down on you pretty quick.

There's always the possibility to go lean with the stock jet after a considerably less restrictive exhaust is used along with some porting and a free flowing air filter, all these things will add to the amount of air that can go thru the engine so the fuel must be compensated accordingly. If someone goes out and buys a performance pipe and air filter this is going to make the air fuel ratio go from a rich condition to either where it needs to be or even to a lean position. With this added amount of air that can get into the cylinder comes added cylinder pressure... this is what makes the power, but with this added cylinder pressure if your plug gap is on the wide side it's possible the engine could start to miss under acceleration, and especially once the engine reaches it's power band. This can be mistaken for 4 stroking causing the rider to think they need to lean out their jetting instead of leaving it alone or richening it up to where it needs to be which could end up blowing the engine before the rider or tuner realizes the real issue. If you got an engine that ran great, then you added some upgrades like a high compression head, a better pipe, or free flowing air filter, or any combination of these, including doing some port work, then it starts to cut out at a certain rpm range, and may or may not clear up when it gets closer to its top speed, check the plug and try closing the gap a few thousandths or replace the plug with a new one that's been gapped .002" less than the one that's currently imstalled and see if the "4 stroking " clears up or clears up for the most part.
What's happening is that as the cylinder pressure increases it becomes harder and harder for the available voltage to jump the gap so as the engine aproaches it's power band during acceleration it can eventually reach a point where the spark will begin to miss intermittently due to the higher cylinder pressures as the engine gets closer to its peak power.
Have you ever wondered how a higher performance ignition can reduce or eliminate 4 stroking? This is why. It's the bigger coil that can produce a considerably higher voltage and jump the gap regardless of the cylinder pressure, it has nothing to do with the air/fuel mixture, but it could be why your high performance China doll is 4 stroking right where it should be making power. The ignition system didn't fix the 4 stroking, but it did stop the miss so the engine could rev out cleanly under load and thus the power goes up because the plug can fire every time instead of every other or every third time. The high performance ignition didn't add power, but it does allow the engine to make full power when it couldn't before.
Just one more thing to consider when your bike is 4 stroking relentlessly and it doesn't matter which way you go with your jetting.

It can be amazing what just a longer pipe can do for these things... no expansion chamber, same restriction as before, but it runs better as well as stronger. The longer pipe will still have a scavange effect where at a certain length the first pulse can start enough momentum so it can pull out the following exhaust pulses. This length can be tuned to work at a certain rpm range but typically a properly tuned straight pipe would be longer than the bike it's on. One can still benefit by adding length, it won't be as big of a difference like an expansion chamber makes, but it will be noticeable