The direct injected 2-strokes weigh more than the old carb motors. The confusion you may be experiencing is that most 20v had the lighter carb motors from birth. When you step up from there to the 4-stroke it is a huge difference in weight. Example
I have a 200 carb Merc 2-stroke that is fairly light at 389 lbs.
The smaller Verado 4-cyl 200 4-stroke is 510 lbs. The 225 6cyl is 635 lbs.
The Merc 200 Opti 2- stroke is 497 lbs.
Right now the bracket on my 20' cc is riding pretty high so I don't think I am getting any bouyancy from the bracket. I don't have a self bailer so I can't help you answer the bailing/not bailing riddle.
I might be happy with the performance of the 150 4-stroke but my wallet has decided to go with a rebuilt 250 EFI in the next few weeks at 475 lbs. I would be happy to share some images of how she does with the extra 90 lbs. Weight would be very close to the Yamaha 150 4-stroke.
