New investor here in need of advice... I came across a deal on two rental homes that each value for ~$145k and the asking price is $110k on each. The builder completed the homes in '08 and rented them out instead of selling when the market turned south. Both have been lived in by the same renters since new. There rent is currently $650 because it has not gone up in a decade. The homes are ~1230 sq ft, 3 bed 2 bath with a carport. I live in a rural area outside Chattanooga, TN. The current owner is looking to free up his mortgage on them and focus on building other homes which is why he is selling below market value. 

My questions is... how should I go about raising the rent without losing quality tenants? Thanks in advance for any help!