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fuel pressure low vs tune-up calculator

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alkyal:
REAR FUEL TANK, about 6ft behind pump. Waterman 400 pump @ 50 percent of crank speed [GOOD flow test less than 7 percent fall off/internal leak between 50 and 100 PSI]. Hilborn 2 3/16 injector with #54 spool. Nozzels K385 s[.024] or 410 s Fuel pressure from tune-up shows approx 140 lbs[@8000] @ 75 deg. and 2500 ft. used 103 perccent VE. There is no filter between tank/pump/#12 line. fuel is pulled approx 18 ins uphill to pump. There is a K filter-new element[not inst d backwards] between the pump and barrel valve. MAX press @ 7000RPM is 78 lbs on your telltale nozzle set-up. My cals show approx 120 lbs @ 7000 RPM. Any Ideas? I suspect the rear mount tank? and the accel factor? NO fuel leaks but have not checked for vac or collapse of -12 line. Thanks Al

Spud Miller:

 Are you running the same main pill that the calculator suggests? If not, the calculated pressure and actual will be way out.

 If the calculator suggests a .070 pill for example and you're really running a .090 to run your best, you actual measured fuel pressure will be much lower.

 Spud

alkyal:
actually running a pill that is.004 Smaller than the tune-up suggested-just to be SAFE. The engine  is 315 cu ins [didnt state that in first post]

Spud Miller:

 I'm not sure why you're seeing that pressure discrepancy then. It should be slightly HIGHER than calculated pressure. Something is not as it seems.

 Is the engine running well? If it's doing well the way it's tuned, then it may be time to start backing into the numbers to try to figure out why the pressure is wrong. Gee, that's probably the point of this post isn't it :)

 If it isn't doing well, then the low pressure is likely the reason...root cause could indeed be a hose flapper, plugged vent, kink, etc.

 Spud


 

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