Hi I have a 1987 colman gas heater and it seems like the unit will not stay on till the thermosta temp is reached will short cycle ttill temp is reached

Model number?

Do not know it's a gas heater just got the home but I do have a separate outside air unit and I had two thermestats but combined to one this spring. Also I was wandering if I had to shut off anything on the outside air unit so it won't suck any cold air in . The heat seems luke warm

It depends upon if the outside is a packaged unit or just a central air condenser. You might have made a mistake when you combined the thermostats.