2003 5.0 MPI won't start and then backfires

by Jon
(Houston, TX)

Hi I have a mercruiser 5.0 MPI that suddenly won't start. It's always been very good and starts immediately. It will turn over, and over and over, but never fires up. I can crank on it for a bit and then a "puff" of air, no odor, no color, comes out of the flame arrestor/air filter. I've tried giving it some throttle but same results. I tried switching out the IAC from the other motor which runs fine, but same results. I even put IAC from non starting motor on running motor and it cranked right up so it's not the IAC.

Only once did it "puff" out a smoke from the flame arrestor. It was nearly clear- very light colored almost like a vapor and didn't really smell of anything familiar- as in didn't smell like gas or anything burning.

Not much of a mechanic here so I'll prob have to get someone out but the local shop doesn't come back until Jan 3.

Any ideas?