Jrider,
Get your drill and a 1/4 in hex driver. Gently remove all the self-tapping screws around panel.
Do with blower off, of course, and very low fire.
Surprise! Creosote out the yingy! Clean all. scrape out square tube leading to blower channel and secondary pipe below. Get you some Liquid Wrench lithium grease and coat anything that moves, to include the self-tapping screws, or whats left of them! Get some red, hi temp permatex and reseal plate. After all crap and old gasket scraped and discarded.
I used shop-vac and paint scrapers, screw driver and anything else you can find to make the clean up job easier. Quite the mess, but it didn't even burn this clean when brand new! I mean after the thorough cleaning.
I did this, along with regular cleaning of entire furnace and it works as advertised now.
Again, wish to thank Martyinmi for his guidance. His cleaning recipe, as listed above. I'm sure you read my other posts here about the bit of fun I had cleaning 'er up. Should have been a complete, blow by blow, listing of all items to look after in the manual. These things require thorough cleaning very 7-10-14 days MAX! Maybe not the airbox that often, but the other items for sure.
Interested on your findings...let us know how goes....