Got the vent cleared. I was really surprised how much junk could be in there. When I was running just the drain snake up the stack and reached the clog, it was coiling up and making a u turn and springing back out the duct. Since I could not climb on the roof due to a tile roof, I came up with a solution of buying a simple 10 foot RV garden hose and ran the drain snake thru the hose and then up the stack. This kept the snake straight and was able to grab the lint and pull it down.
Worked beautifully. Now with it cleared, I was able to run a 10 ft long vent dryer brush up and get the rest along the duct sides. A cheap means of doing it. Now every 4 to 6 months, I'll run the long bottle brush type made for cleaning these stacks to keep it clear.
Thanks for the help.

Ihave a whirlpool duet same thing checked fuse and thermister ignites flame comes on for 20 seconds then off 3 or 4 mins same thing tried it with hose off no change.flame is smaller than usuaull.

I am not fully acquainted with this model but I have seen this problem from time to time on other models. An item you should check is the temp thermostat. Replace it and evaluate. This thermostat is on the burner tube and is a 2 connector type device. It is easy to access and not very expensive.