A friend had a near mint 1978 Honda XL 125 that had an electrical issue - no high beam or turn signals. Turned out to be a bad battery. I also took the opportunity to do a complete tune-up and adjustment service. It was considerably out of spec in many areas, but I eventually corrected all. the bike started on the first kick and seemed to run well.
The owner reported that although the lighting system works great now, the bike will not go faster than 40mph and has become hard to start. I took the bike back and with him present, went over the entire tune-up procedure - doing it per the Honda shop manual. It still ran poorly.
but here's the kicker - the owner said he had it tuned by a Honda dealer a few years back and the same thing happened - after tune up , the bike would not go over 40! After another trip to the dealer, they gave up and said they could not fix it. Owner said he had an auto mechanic look at it, he adjusted "something" and the bike would go 60 + mph again. We tried to reach the guy for comment but never could.
After much research, trial and experiment, I finally reverted all tune up specs to the " as found" specs (which were considerable off)
guess what fixed the issue? Instead of setting the valve clearance at the factory spec of .002, opening then up to .006 cured the overly rich issue. It really makes no sense, but there is a little more to the story. Apparently at some point the head was pulled off engine to replace some worn out valves and do other engine work. My theory is, some part was incorrect spec or some procedure was botched to cause the valve clearance spec to change. Different length valves, non-standard cam, incorrect valve timing, etc... Who knows!
Morale of the story - just because a bike looks near mint, do not assume it has not been molested!