Wouldnt it be a great but rather boring world if we could diagnose a problem with only one symptom and without the problem being in front of us. Even a doctor wont say your leg has been bitten off without taking your BP and Temperature.
Before any attempt at tuning the basics need to be addressed. is the air cleaner clean, is the pipe clean. How about that engine seals and compression? Is the carb float set correctly and is it clean, what about the needle wear and clip? Tested the CDI and coil and is the plug correct, timing okay? is the fuel correctly mixed and the tap and line free flowing? Reeds okay?
When you can answer a confident yes they are all good to all those then its time to think about tuning or your chasing the wrong end of the dog. And just to make it interesting jetting changes with elevation as the air pressure changes, even humidity has an effect.
You have probably seen these charts and they give you a headsup on where to look for carb jetting issues Vs RPM. Remember there is a crossover range where jets share the job. grab a few jets and a box of spark plugs and your angle grinder as its time for plug chops. Flat out underload pull the clutch and press the kill switch at the same time. Remove the plug and check the polaroid it just gave you of what was happening at that time. When you get close with the jetting you can "chop" the thread off the plug and read the porcelain colouring with more accuracy.
Whats happening in the engine at that time? Have a look and the piston and plug will tell you.
