I have an 850 Mighty Demon on my big block engine. I bought it back in 2005 I think.
Things I had to do to it:
Send the metering blocks back for exchange because the idle screw holes were drilled improperly. ( they were offset, they didn't line-up which caused the idle adjuster needles to break when you screwed them all the way in )
Change the power valve.
Change the jetting. ( It was ridiculously pig-rich. Came with 86pri and 94sec jets. )
Add notched float and jet extenders on the secondary side.
After all that.... it works great now!