Clean everything!!! its already 21 years old!
Check eveything before you do some damage to it and it will be hell finding parts now days.
from what you are saying it sounds like one cylinder is not working! Could be fuel problem or electric ignition.
To check for ignition just take out the plug while still connected to the plug cap and kick start it. Normaly you will see a spark.
But what it sounds like is a fuel problem, if the cold plug is dry then there is a problem in that side of the carburator, you are not getting fuel in that cylinder. Everything should be ok once you clean the carburator if it is a fuel problem!
Be gental you are not going to find parts easily so dont brake anything while taking out the carb to clean. Open up the bows and clean up the jets.
If the cold plug is wet and dark then you have a fouled plug which is no good anymore. get a new one and see if it works. if it fouls again then you have to adjust your carburator.
The gas tank is mostly rusted from the age, clean it up and fit a fuel filter.
But from my experience its best to get the tank derusted by acid or something, because no fuel filter can 100% filter all the rust while your fuel is 100% contaminated with rust.
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