Motorcycle Repair: misfire, lean condition, 5th gear

Question
QUESTION: Hello,my 1983 XT200 is missing under load only, starts well, but miss fires under acceleration, carburetor appears good.

ANSWER: I probably need a little more info to answer this accutately, is the bike stock ? Has there been mods made to the intake or exhaust ? Has it sat for an extended period of time ? Does it idle fine without the choke ? Did this problem happen all of a sudden ? Or has it been there always ?
Without knowing these answers I can only guide you a little.
One if the carb is clean and the bike is in good shape there is a chance that this is a lean condition, this can be caused by the air filter or air box being removed or modified, is the pipe stock ? If an aftermarket pipe has been installed this could be the issue. Have the valves been adjusted correctly ? If they are tight you can have this problem. If you are in 4th or 5th gear and at full throttle back the throttle off slightly, does the bike accelerate ? This indicates leanness.... Is the fuel fresh or did it sit ?
If possible let me know more, if what I have given you here fixes the issue then Great !
Thanks.

---------- FOLLOW-UP ----------

QUESTION: Thank-you for your reply,the engine is stock,no modifications made, has sat for an extended period, idles good, problem came on suddenly.Carb has been cleaned, float level checked, tappets checked and adjusted, clean fuel administered, exhaust stock. Back-fires on de-acceleration and misses under acceleration mainly 3rd 4th and balks at fifth,slight missing 1st and second.Compression good.

Answer
The situation here is that, yes, it runs and idles, BUT...the fact that due to gas sitting in the carb bowl, and its connection to the atmosphere through the bowls atmospheric vent the gas  has gone stale and started the process of clogging the jets. Whilst they are not clogged completely, and it does idle, etc..., the jets have a growing coating of goo on the inside of their orifices, effectively making these jets leaner than they would be normally. Clean all the jets, especially the PILOT jet with a small wire, running it in and out to clear the inside of the jet, then clean all of the passages, PILOT mainly, with carb cleaner.

Problem gone.