While pulling out from a stop, the engine seems to rev. It's like the bike is slipping out of gear or the clutch is slipping. The bike has 53,000 miles on it now.

It sounds like the issue with your Honda XLR125 (1998) not picking up speed in top gear could be related to a clutch problem. If the clutch is slipping, it could be causing the engine to rev high and prevent the bike from reaching higher speeds. Clutch slipping is often a result of worn clutch plates, which can cause decreased friction and ineffective transfer of power from the engine to the transmission.
Based on your description, it may be necessary to replace the clutch plates to resolve the issue. If you're unsure about the condition of the clutch, I would recommend having a professional mechanic inspect it to diagnose the problem and recommend the appropriate repairs. It's important to address any clutch issues as soon as possible to prevent further damage to the engine and transmissio

Open throttle, in gear engine builds up revs but road speed drops, especially up hill; is a slipping clutch.

Mechanical issue: clutch not disengaging completely...
engine off - put in gear and roll back and forth with clutch released does engine stop the bike from rolling? Now pull clutch lever in and try rolling again is clutch cable adjusted properly?
Electrical issue: bike thinks clutch is still engaged and kills motor when not in neutral
Is the clutch switch properly connected and working? Is it adjusted properly? Use a multimeter to read resistance across switch contacts does it change state when clutch is pulled in?

If engine revs increase but road speed does not increase, your clutch is coming to the end of its life and needs to be replaced

make sure there is 3mm play on the clutch lever , select 5th gear while engine is running and press the rear brake to stop you moving away ! with a little revs try and set off , if clutch is worn it will slip and not stall the engine ! if this happens then replace the clutch plates !

the easy way to find out if the cluch is slipping is to ride the bike in top gear pull in the cluch rev the bike up to high revs and drop the cluch if the engine dont drop in revs very fast and the revs only drop slowly its slipping if this happens check that you have a small amount of slack in the cluck cabel ie: 1/4 of an inch is ok if its to tight it will cause the cluch to slip as the engine warms up

u sound smart n thanks for goin oldschool-1st buy a manual-thats how the pros work--in bike yrs its in its 50's-parts wear out-try ajustin cable n then do clutch itself-that does not help-rebuild is probable

its either low on fluid or the clutches & bands are worn or need adjusting!
DONT REV IT!
get itserviced QUICK SMART! & check for leaks!

I have a 2000 Vulcan Nomad FI & had a very similar problem. In fact, it's a very common problem with these bikes. I fixed it by replacing the Clutch Spring. (As Described here: http://www.gadgetjq.com/clutch_spring_replacement.htm

It cost me $40 and 3 hours of time with an impact wrench.

you say it doesn't slip ,pull hand brake hard, start engine
put it in fifth gear accellerate and let clutch out if it dies quick the clutch is ok if not it needs replaced ou adjusted