[spldart]

Drilling the exhaust will not, I REPEAT 'WILL NOT', cause damage to the exhaust valves all the way back there.
If you had no exhaust to 4 or 5 inches of pipe then open air... then yes.
If you had terrible seal from pri pipe to head itself then definitely possible.
But those holes at the end of the pipe will not cause valve damage if you have good seal pipe to head.

Is the 140 hot or cold?
When the bike warms up and starts going tango uniform have you done a leak down then?
If so what psi then?

At this point getting a full gasket set + exhaust seals then doing a tear down inspection may not be a
totally bad idea. But
I can't remember the other thread...
Swapping a used carb on the bike, after a clean of course, might be a cheaper next course of action.