Intrepid 1 draft problems

Hello, all,
A few months ago I found an old Intrepid 1 and fixed it up. I had a Jotul 602 in the fireplace and it's a great stove, but I've always liked the look of the VC stoves and the open fireplace feature.
Anyway, the Jotul burned like a forge-- tremendous draft, I started every fire with one match and it cranked! It warmed the room in no time and, though it consumed a lot of wood, it was very simple to control.
Not so the Intrepid... I'm having a lot of trouble getting it to behave.
I did add a 90 degree section to the pipe, but there's otherwise no difference, and before I connected the stove I could feel air being sucked into the chimney, so I know the pipe is drawing just fine.
However, even if I let the stove burn through 2 loads with the doors either cracked or wide open and the damper and draft flap in the rear open, it doesn't roar the way the Jotul did.
I also can't close the doors or the damper without the fire just going out... even though the draft flap is wide open.
I don't know what's causing the problem. I pushed a wire through the rear flap opening to make sure there was no blockage, but still the air tubes are obviously not working.
It's pretty frustrating... any help from more experienced members would really be appreciated.
Thanks,

Andrew

Hi, Homebrewz, thanks for the response-

To install the Jotul, I removed the fireplace damper and replaced it with a plate with a 5" (or metric equiv.) hole in it. I ran a section of pipe straight up from the stove top for about a foot and into two 45 degree elbows that go through the plate. Then there are about 6 or 8 feet of pipe above that, all in an existing square masonry chimney about 20 feet tall. That chimney clears the roof peak by 3 or 4 feet.

That configuration with the Jotul worked really well-- there were no draft problems at all and I easily got burns that had to be throttled down because the stove temps would get too high.

With the Intrepid, I added the 90 and also needed to reduce from 6" to 5" to use the existing pipe-- I forgot to mention that in my original post. I installed the reducer on the top end of the 90.

I tested the air tubes yesterday with some compressed air... they're definitely clear.

The only thing I can think of is that maybe I need to replace all the pipe with 6" instead of reducing... but I can feel a good amount of air being sucked up the pipe from the open damper of the Intrepid (when cold).

I'd really like to get this thing working right, so any and all input is welcome.

Also, the wood is dry and, while not the best I've ever seen, burns well in the Jotul and in the other open fireplace in the house.
Thanks again!

Thanks, Paul, I'll give that a try... I thought the reduction might be a problem. I know that the "slammer" setup isn't optimal, but it worked really well before, with no discernible buildups, and the chimney liners are just ridiculously expensive.
I appreciate the responses and the helpful advice from both you and Homebrewz... thanks again!