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.

the carb should not make any difference because the regulator controls the pressure.i would shoot for 8 lbs fuel pressure.

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.

Thanks for the help guys, I was kinda thinking it should be a touch higher to be on the safe side.

Have you put a wideband o2 sensor on the engine since getting the carbs reworked?

When dealing with pressure changes, or any change really, its best to think of it in percent change. To go from 7 to 5 psi is about an 18% reduction in fuel flow.

Yes, but will only matter, IF the flow cannot keep up with the demand. If lets say the fuel lines were capable of flowing 150GPH at 5psi, and the carb needles are only capable of 130GPH at 5psi, adding pressure wont help. What you dont want to happen, is let the fuel pressure get low enough, where it cannot overcome the float bowls and keep the needle and seat closed.

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. :cartman:

What are your thoughts on Nickersons work?

Because I have s#itty GenV blocks coupled with the s#itty Merc seapump/fuel pump system that went bad, leaned engine out and kaboom! 3K please:)

Joe, you coming to Hotboat with the Indiana guys?

Really wanted to come up, but after loto and traverse city in one month need to get some house chores done :-(