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View Full Version : Mercruiser 43l V6 Fuel Pressure or carburetor



Dan Heading
04-29-2006, 02:34 PM
"I have a Mercruiser 4.3 liter V6 (GM 4.3 Liter V6 engine block), that I did an engine swap on. The donor engine came from a pickup truck with throttle body injection, and as such, while most parts were interchangeable, the fuel pump was not. I purchased an electric fuel pump, and the boat runs perfect at WOT. I'm decent with mechanical things, but I could not find any information in my repair manual about what kind of fuel pressure my engine was supposed to have. The carburetor is of 2 barrel design, and is the original carburetor. When I start the boat, it seems ok, and will run at wide open, but when you decelerate the boat tends to stall at low RPM, and refuses to restart. I was told that I have a fuel pressure issue that can damage the carburetor. I was told to buy a Holley pressure regulator and set it between 5 and 7 PSI. Does this sound reasonable? Is 5 to 7 PSI too much or too little for my application? I know fuel injection systems require considerably more pressure in a lot of applications, but I've never put an electric pump on a carbureted engine before..

Thanks for the time and the info."

jimn
04-29-2006, 04:37 PM
"5-7 psi is fine and about the same as a mechanical pump would give you. That 2 BBL may be a bit light for your "new" motor. After going WOT for about a minute, shut it down and look at the spark plugs to see if you're too lean or not. If the new motor is later than 1994, it probably has Vortec heads and may want a bit more fuel/air than the 2 BBL can give it. A 2 BBL carb is different from a throttle body with 2 injectors in terms of air flow. OTOH, you may just be rich at idle and need to open the throttle when starting it."

fastjeff
04-29-2006, 07:25 PM
I have to disagree. Fuel pressures over 4 psig on a carb bouncing up and down in a boat have been known to flood. (And--let's face it--how accurate is the gage one uses to set it at suck low pressures?) Three to four psig is plenty enough to keep from starving her out.

Jeff

jimn
04-29-2006, 09:00 PM
"With a good float, it shouldn't flood and at high RPM, enough gas is used that the flooding may only be momentary. The mechanical fuel pump that came on the GM motors was supposed to provide about 5-7 psi fuel pressure- that's the number recommended in all of the Mercuiser and Indmar manuals."

fastjeff
04-30-2006, 06:44 AM
"..."With a good float, it shouldn't flood and at high RPM,"

That's not his problem, Jim. It floods AFTER a high speed run, when he's idling back to the dock.

Suggest you adjust the pressure WAY down and see if this helps. Be careful at full throttle since it might lean out on you. If this works otherwise, turn the pressure up just a bit and you'll be okay.


Jeff"

jimn
04-30-2006, 12:31 PM
"It floods at low RPM now but I don't think it would be a problem at 5-7 psi- that's what I was saying. If the new pump is for multi-port GM application, it's the wrong pump. Those are supposed to see 30-40 psi and the new ones are even higher. An older TBI pump would be in the 18-30 psi range and could be regulated down to, for sake of practical application, 3-7psi. Even the Mercruisers from the mid-'90s have electric fuel pump and that's basically just a Chrysler part (cylindrical, with inlet on one end, outlet on the other and can be bought with a return fitting). Indmar has also used this pump on their motors since '94 and may still, but Mastercraft (the ones I'm more familiar with) went to the in-tank pump in '99."