It's most likely a TC88 set up, measure the inside width of the driven pulley. If it's 5'8", TC88. If it's 3/4", then it's a 20 series.
No funky spring/ washer set up like the rupps. If it's sticking, pull it apart and de-grease everything and re lube with COMET spray. Very important to lube these correctly for proper operation.
https://www.OldMiniBikes.com/Lube-Spray-11oz.html
I thought you sold this bike?