Older Heil gas furnace short cycles. Replaced limit switch, T'stst, etc. & still doing it?

I have an older (old) Heil gas furnace and it comes on and runs fine for about 3 minutes and then shuts off for about 2 minutes - then repeats. I've jumped the thermostat, checked the filter and the exhaust pipe/ chimney. I replaced the fan limit switch and the capacitor and it's still doing the same thing. I don't think this uses a flame sensor. I'm a carpenter so I apologize to the HVAC guys for not being able to afford to bring in a tech right now. Thing are just so slow for us right now. Thanks, Mike