T20 holds back, what can be wrong?

[RJRamso]

My father has been having some light problems with his T20. I'm gone for work, so I haven't been able to help him or try the bike but this is how he says it acts.
It starts up fine and idles ok. When he drives it and it gets warm, then it seems like it holds back. The crank is just rebuilt with new bearings and oilseals. The ignition and carbs are adjusted according to the book. The points are good.
He had some problems with one cylinder that fell out (not fiering), it came once in awhile. The condencer had previously been changed from previous owner. The wire had been cut and put together, so he took it up and soldered it and then it worked fine.
Any ideas anyone?
Thanks

[H2RICK]

Check the plugs. They will tell you, if not all, at least some of the story.
You may have a lean condition caused by carb setting/jetting/cleanliness......
or it may be a spark problem....not enough spark advance because of incorrect timing. Lastly you want to make sure the injection oil pump is working and there's no air in the system.
If it's the lean fuel condition, you better get it fixed FAST or else your engine
will end up with the dreaded "ventilated piston" syndrome. This is a "bad thing", as we say in the biz.

[Suzsmokeyallan]

All i can add to Ricks input there is to check the pipes/baffles to see how clear they are. Any blockage will cause lack of power at higher rpms/speeds