Brandon,

Most current engines in this size class are ABC/ABN in construction.
They feel like they have better compression and sealing, especially if compared to a "dry" ringed engine.

Your K&B, unless it is the 6.5 RE, is probably a ringed engine. There was an ABC version too, though.

ABC engines feel more compressed, partly because of the tapered cylinder bore, that is tighter at the top than at the bottom of the piston travel.

It is also possible the engine is worn out...

It may be ill advised to buy a used engine, without seeing it run, unless the seller can be trusted.