When was the roof, heating system, etc. last serviced or replaced?

You may be better off rebooting and trying again in March or April when things start warming up (and heating up in the market).

I just Changed my thermocouple and now it gives off a weird smell ( not egg or gas) and the metal shield gets hot during heat up.

Primarily, a sealed envelope of building, 2 years of roof, health & safety items like adequate heating, venting, no visible discoloration (mold), egress from each bedroom, termites, peeling lead paint, drainage issues, secured railings, and as miniscule as trip hazards if they are having a bad day.

What is the typical Cap Ex on a 24 year old duplex house, in Kansas City, MO which is between 1000 sq ft and 2000 sq ft, wherein one AC electric unit is new, the other one is 8 years old, where the heating systems in the 2 units are 8 years old and where the ages of the water heater and roof are undisclosed? 

These are folks that are going to be paying massive heating bills no matter the heat source.  

Keep a fluid timeline, and always have an out, and backups to that out with your properties.

I found in each case, care for things like roof, soffets, exterior decking, interior closet doors, walls, AC units, window rot, long expired heating units, etc, were in generally beat up and run down condition.  

@Robert Carl Curious to hear more details about your experience with the tenants getting more time just by claiming repairs were needed.Typically, a tenant must file for a day in court if you are not repairing items that affect their safety such as heat in the winter, no windows, etc.

I do have solid construction background, the only things I do not do are major electric and  heating and air.