My boat has twin 350 mag's efi/mpi with the stock flame arrestors there was a little sooting ,,,,well I thought I would try a K&N , put them on Friday and went to the lake today, and when I crank it up it looks like black dust floating on the water and the transom was really black when I pulled the boat out . Do you think that the ecm's need programing because it is getting more air????? any help or advise in greatly appreciated

Sounds like the K&Ns are restricting the air and causing a richer mixture. Try it without any flame arresters and see what happens. If it runs clean without any flame arresters than you will know that the K&Ns are causing more air restriction than the stockers. Not supposed to, I know, but maybe they are.?

What octane gas are you running????? You should run the LOWEST recommended octane for your engines--87? It will burn more completely with less black residue and give the best performance.

The K&N's I bought for my engines were way too small even thought they were the ones recommended by them. Lost 400 rpm. Like Macdad said, take them off and try again.