Riso V800

Question

Running 20lb paper 8.5 x 11 runs great and at 11 X 8.5. When I go to 10 x 14 I keep getting paper jam but its not it runs about 5-10 sheets and shuts off. But if I run 10 x 14 with print on the sheet, it will run all day until it comes to a blank sheet of paper and it says paper jam. If I try to run 11 x 17 its worse yet as it will run 1-2 sheets and then say paper jam. "SOS!!"
I dont know why it runs with ink on the paper but wont run a clean sheet, and why the longer the sheet the worse it gets.
Thanks!

Scott