If you are testing the dryer with the drum empty, this short cycle time could be normal. I recommend that you fill the dryer with a medium load of wet towels when testing the burner. Make sure that the dryer is fully reassembled.
If the dryer still is not heating properly, then you could have a bad operating thermostat or failed high limit thermostat causing this problem.
The first image below shows the wiring diagram for this type of dryer. The exact wiring for your model may be different depending on your serial number.
The second image shows troubleshooting for the heating problem that you have and a location of the components in the dryer. The third image shows the diagnostic test procedures for this dryer.
Since the burner initially ignites, the operating thermostat and high limit thermostat are apparently okay at room temperature. You can heat up those thermostats with a heat gun or hair dryer. If one of the thermostats "opens" just above room temperature then it will need to be replaced. There is also a thermistor (temperature sensor) that is used in this dryer. If that thermistor is bad, the heat will shut off prematurely. You can check the resistance of that thermistor using a volt/ohm meter. It should measure around 10K ohms (10,000 ohms) at room temperature. If that thermistor is bad it will need to be replaced.
You can order parts from the
website. You may need assistance in ordering the correct parts for your dryer. You will normally need the serial number when ordering parts. If you determine the failed component and need help ordering the correct part, reply with the serial number and a description of the part that you need.
This dryer can be complicated to diagnose and repair. Reply with additional details if you need more help diagnosing this failure. Include the serial number in your next reply.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services