Roper m/n RGD4649YQ1 takes too long to dry

[AWSHandy]

I replaced the cycling thermostat.  The burner stays on for 25 sec then off for 45 sec.  I observed thru the peephole.  I took apart the exhaust covering and it was clean as well as the blower wheel.  The burner does stay on longer when the exhaust vent tube is off.  The exhaust vent is approx. 20ft and flexible plastic.  Is it the exhaust vent?  Do I need the thermal cut off kit?  The burner turns on no problem so I think the coils are fine.  Any help appreciated.

[16345Ed]

11 hours ago, AWSHandy said:

The burner turns on no problem so I think the coils are fine

Coils tend to fail after heating up so they could be the problem. 
 

But if it heats well with vent disconnected it’s likely your vent Is clogged. 

Edited June 10, 2020 by 16345Ed

[Budget Appliance Repair]

17 hours ago, AWSHandy said:

The exhaust vent is approx. 20ft and flexible plastic

This is a big part of your problem - the plastic vent is not up to code, (plastic vent tube is only for bathroom vents never for dryers anymore).

OK to use the foil flex vent designed for dryers but should never really have more 8 ft max of flexible foil vent - it is only designed to be used from the dryer to the wall pipe.

You should replaced all but 3 or 4 feet of the plastic vent tube with 4" metal pipe and only use a 3 to 4 ft section of flexible foil vent behind dryer to metal piping.