efi to carb swap. fresh air to engine?

[Cdn20valve]

Hey guys, i had idle and drivability problems with my 1996 merc 5.7 efi setup. after years of troubleshooting and throwing parts at her, I finally ditched the efi for a edlebrock carb 1409 and intake manifold. the boat has never ran better.

However I'm concerned about safety. My boat has two fresh air vents on the rear. one pulls down to lower bilge, and the other one is connected to the bigle fan. I noticed older(carb'd) boats had two vents, one on either side of the gunwales at the rear. one usually faces front, the other the rear. which helps force fresh air into bilge, and clear out bilge air while underway.

should i bother adding a couple vents on the gunwales or leave it alone and just run the bilge fan when idling around.

thanks, Lou.

 

[GA_Boater]

Engine compartment venting is the same for carb'ed or FI motors.

The safest thing to maintain a leak-free fuel system.

 

[Scott Danforth]

agreed. bilge venting requirements have nothing to do with motor, motor size, induction system or brand.

it has to do with bilge volume size

 

[Lou C]

Yep a number of carbed boats were set up with the transom vents versus the more traditional gunnel mounts. Keep in mind that the vertical surface of the transom becomes a low pressure area and will naturally pull the vapors out of the bilge with the boat in motion.
Looking at how your boat is configured with the walk thru on the starboard side the only place you could add an intake would be on the port side. Not 100% necessary but if easy access I might have done it. I like one vent facing fwd and at least one other facing aft.

An addition I would recommend for any gas inboard especially carbed boats is a gas vapor detector. I added one about 8 years ago and it’s nice to have. Still I always lift the hatch an inspect before starting.
I’m not surprised you had a good outcome from converting back to a carb. Simpler and cheaper all possible with aftermarket parts; now you’re not beholden to the absurd markup Merc or Volvo puts on their parts. What I like most is you have choices; Edelbrock vs Holley or even a Quadrajet for old school guys like myself.

The pic below is from Chapman’s on bilge ventilation

 

[H20Rat]

Your engine at idle is drawing almost 100 cubic feet/minute of air out of the engine compartment. At full tilt, it will be 500+ CFM. The contribution from the vents ends up being pretty minor compared to the big giant air pump in the bilge.

 

[Lou C]

The main thing is to draw air from the lowest part of the bilge to avoid fumes build up there. The blower does that if properly installed but vents allowing air in Help the exchange of air...