Fuel Pressure
#1
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Join Date: Jul 2007
Location: Cynthiana KY
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Fuel Pressure
I had my carb rebuilt by Nickerson earlier in the spring. Just reinstalled last week and took the boat out, everything ran flawless. My question is that after the rebuild, it seems my fuel pressure is a little lower, steady at about 5-6 psi. when if I remember correctly it was running around 7-8 psi. at cruise at the end of last year. No other real changes to the fuel system other than having to add a 45 deg fitting to one side of the regulator due to changing the mounting bracket. Is this normal after a carb rebuilt to see a small drop in pressure? Is 5-6 psi. sufficient? The motor is a stock long block 454 mag with a B&M 174 blower running at 5psi with a Mallory electric pump.
Last edited by lbaldwin22; 08-27-2014 at 08:09 PM. Reason: psi not lbs??
#3
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Take those gauge readings with a grain of salt. When you're dealing with such low pressure, like 6psi for example, the gauges are notoriously innaccurate. I've had several NEW mechanical fuel psi gauges on the same engine. All read different from each other. Electric gauges, forget about it. They are good to watch for a drop in pressure, but the actual gauge reading number can be erroneous.
I believe on the merc hi perf carb engines the max fuel psi in the service manual was 7psi, and minimum 4psi. I've heard of guys who boat in rough water, running 8-9psi, causing the needle and seats to open prematurely from being to close to overpowering the floats and dumping fuel in waves.
It wouldnt hurt to try turning them up a tad, like Mike said. The typical brass float setup shouldnt have a problem with 7-8psi, if thats the true pressure reading.
I believe on the merc hi perf carb engines the max fuel psi in the service manual was 7psi, and minimum 4psi. I've heard of guys who boat in rough water, running 8-9psi, causing the needle and seats to open prematurely from being to close to overpowering the floats and dumping fuel in waves.
It wouldnt hurt to try turning them up a tad, like Mike said. The typical brass float setup shouldnt have a problem with 7-8psi, if thats the true pressure reading.
#7
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I think the most important thing is, keep the fuel pressure around 6-7psi, and run good pumps, and big lines. Mercury used self regulated mechanical pumps on the 600sc, 800sc, and 900sc. That mechanical holley pump, was enough to feed a pair of 1050 dominators on top of a 10-71 intercooled setup. The smalller merc pump used on the 600sc, was enough to feed a pair of holley 700 carbs. I never understood why on oso, there are guys running single carbed 500HP engines, but have aftermarket electric pumps, adjustable regulators, and all kinds of crap. Keep it simple.
#9
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Joe, you coming to Hotboat with the Indiana guys?