Engine Starving for air?
 

I have an issue with my new set up, and I have really not found any other topics about this so here is my issue question!

With my new 509's with the hatch off I gain 200 RPM's and 5 mph when compared to when the hatch is on. If I crack the hatch a small amount I will gain the RPM's back but this is hard to do, because the pressure will push the cover closed. If it is just barely cracked, and the hatch bounces open closed, you will feel the boat accelerate and then decelerate. It's very noticeable so this tells me I'm losing a lot of power when the hatch is sealed off.

I'm really baffled that engine bay is that sealed that I'm seeing that big of a fluctuation. Any suggestions here? Would changing carbs, jetting solve anything, or is this just a simple case that I need to get more air to the engine?

The basic engine specs are 509's, Dart Pro 1 heads ported by JimV, 10:1 cr, dart intake, 741 cam, Hp 950 carbs, and I'm running 34 degrees of timing.

Tuning to lack of air, gives you a good tune for lack of air, so it willbe better than not tuning,but you still have lack of air, therefore you will be down a good amt of power no matter what.

Get yourself some air.

magnehelic gauge is what you need to check pressure under hatch. But it sounds like you have not enough air.

Get a digital kitchen thermometer with a flexible probe and put the probe in the engine compartment, preferably close to the flame arrestor, run the boat and have someone monitor the temp

started this thread based off of the direction the drive depth thread was going a week or so ago

http://www.offshoreonly.com/forums/g...rculation.html

I had an issue with my Formula 255 Liberator with twin 355 Vortecs. When I first got the install dialed in I noticed that neither engine would spin over 4200~4300 RPM...either with 21" or 23" Mirage props. I checked timing, fuel pressure, took the Anti Siphon valve out of the fuel line, replaced the fuel line from the tank to each engine and a new tank vent line & screen.

One day while I was out running with a friend on a smooth day offshore, he went to the back and cracked open one side of my engine hatch while running WOT. Suddenly both engines jumped to 4800 RPM. He let the hatch down and within a few seconds the RPM dropped !!! I slowed to an idle, took the rear seat base out and stowed it forward. This revealed a space between the floor and the bulkhead at the front of the bilge. I cranked up again and was able to push 5000 RPM !!!

My fix was to cut a 2" hole on both sides of the bulkhead beneath the side coaming on each side...problem solved.

I guess the side vents weren't enough to provide the air both engines needed plus the deck hatch set up sealed too well !!

I've got a remote probe for my grill. I'll use that, never thought about using that for my boat. Good suggestion!

Did you put the 2" holes in the bulkhead where the side storage are is? That's great you gained that much, I thought 200 RPM's was a lot but it sounds like you gained 700.

it's not only the lack of air but air temp, remember all closed up air get really nasty in the there, you should run the blowers whenever the engine is running, one thing I did was to take one of my blowers and turn it around, so I have one drawing out(hose below engine) one bringing air in above...I know before somebody looses it that this in not CC approved, but you have to get air in for a engine(s) to breathe. years ago I read a TSB from merc as they found several boat did not have enough vents to support the engines to run at WOT they actually have a formula for this..

The blowers don't have enough air flow to do anything at WOT, Idle and low cruise yes, WOT NO.
I had the same problem with the Scarab. I tried to find a solution but couldn't come up with anything that looked good.
The Daytona has the opposite problem. TOO much airflow - IF the hatch wasn't strapped down it would blow open. The previous owner did that a few times by the looks of the old hatch. I am going to seal the scoop to the flame arrestor.