The Spectra currently has a fuel system I am suspect of since it's not running a regulator and the fuel pressure seems to be a little low for a blower engine.
It's a loop type of set-up that pumps fuel through a -8 line to a fitting that splits it to the carbs and the return line that is -6. The return line loops right back in front of the fuel pump. When I tested the boat the gauge was saying 9 PSI the next time I tested it the gauge was going right to 15 PSI. So the sender is bad and will be replaced but I just tested the fuel pump with a manual gauge and it's reading 5-5.5 PSI on the trailer (without the engine running). So it looks like the pump may have been modified some how (different spring?). What do you think rebuild the pump back to 12 PSI and run a regulator?
Last edited by 78Southwind; 03-24-2016 at 06:04 PM.
If the engine isn't running, the regulator is probably not getting 12 volts and would have low fuel pressure.
The generator will probably put 14 volt to the pump, but at least 12-14V.
Hook a battery charger to the battery or run the motor and try again.
It might be OK.
Yeah, the test was with the battery charger connected to the battery (13.65 volts). But I was wondering if that last .35 volts would make that much of a difference? When I get the new sending unit back I am going to test it again (engine running).
What is a good fuel pressure setting for a 572 with a 10-71 blower running about 5.5 to 6.5 of boost (about 1,000 HP). I was thinking 6.5 PSI of fuel pressure but I was reading maybe even higher when it's over driven.
Last edited by 78Southwind; 03-24-2016 at 07:05 PM.
My system is similar to yours except I use a bi pass regulator and my pump is a Clay Smith mechanical.
That's what I am use to, my other boat has a Clay Smith mechanical and it's been running strong for over 12 years. I don't really want to make any changes if I don't have to but on the other hand I want it right. What pressure do you set your pressure regulator at?
Last edited by 78Southwind; 03-24-2016 at 07:24 PM.
6.0 To 7.0 psi is a good place to be. The important and maybe the most important pressure is when it's running wide open, and if it's falling below 4.5 and 5.0, your fuel system is not keeping up and the carbs. will run low on fuel.
Your idle pressure will generally be slightly higher than your running pressure.
I can't say the 4.5 - 5.0 psi is an absolute low number, but if it's falling lower, there is a problem.
As far as the need to increase fuel pressure for higher blower pressures, I would say no unless you're running a blow thru system. For a roots system, it's still no.
The carburetor just needs to maintain a certain level of fuel, and unless the carburetor is pressurized itself, let the fuel system do its job for you.
Others may have different opinions, if so I'd like to know also.