Short fan cycle caused from fan limit or thermal cpl

[Greggfxi]

Hello
I have 1996 HQCO (HEIL ) 75,000 btu furnace NUGJO75DF02 Home sf 1350 ranch with 1350 partly finished basement.
My fan limit was set at 100-130-200, kept having short fan cycles, moved lower limit to 50 and now it works fine.
- what causes short cycles, bad or out of adjustment fan limit or dirty or bad thermal couple ?
Thanks for any respones

[how]

Fan limit controls like yours will degrade with time and the need for such an adjustment is not uncommon. It 's usually metal fatigue and wear of the bymetal probe that sits in the heat exchanger and is connected to your fan/limit control. Short cycles can be caused by anything that affects the fan/burner or safety controls.
Thermocouples will either allow your gas controls to work or they will completely shut off your pilot flame. No short cycle..just works or it shuts the gas off.

[Freon]

Unless you have a very cold house, the 50 degree low limit (fan-off) setting, under normal operation, would usually mean your fan would always be on since with a house temperature greater than 50, you'll never reach the lower limit. The fact it's working is a good indication the control is failing, as how so clearly described.