Fuel pressure problem..I'm at a loss.

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Ok fill us in on what components you currently are using. Fuel pump, carb, regulator etc. it seems you’ve covered a lot of bases already and all of us are picking at it. Tell us about the fuel system front to back.
 
Also, accurate fuel pressure readings are important. Some of the cheaper liquid filled gauges change drastically with engine compartment heat, depending on where they are mounted.
 
TT5.9mag said:
Ok fill us in on what components you currently are using. Fuel pump, carb, regulator etc. it seems you’ve covered a lot of bases already and all of us are picking at it. Tell us about the fuel system front to back.
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It has a stock tank with 3/8 fuel line and, 1/4 return line, a Carter CRT6903 mechanical pump from the pump it has all -6 an steel braided lines to the Holley HLY-12-803 regulator then, -6 an steel braided line through an Earls filter to a Holley 850 double pumper. I really feel the regulator is the wrong one for this set up. The 1/4 return line has never been hooked under the hood and, this regulator does not support a return line. I'm hoping with the new regulator that is set up for a return line it will be able to vent some vapors from the heat under the hood and, keep the flow of fuel cooler to cut down on vapors.
 
what if you took that non bypass regulator and made it one: T off the input before the regulator, straight the carb and then regulate the pressure back to the return line? wide open (screw all the way in ) would reduce the pressure to the carb as it will be in full bypass back to the tank but on a smaller line so youll still have some pressure. Where are you measuring pressure? Jeez, run the darn thing with the cap off once and let us know. were gettign bogged down in "vapor lock" when 1 test can confirm this is or is not a tank venting issue. '69 Corvette fuel filters have a return spud on them, they have been used to cure vapor lock in the lines when they are forced to pass by a hot exhaust and start boiling the fuel in the line
g%2Fwww.corvetteforum.com-vbulletin%2F664x691%2Fgt_8110_4aba07dbba0c6f0ad28ffb52e26533269805c093.jpg
 
plymouth67 said:
It has a stock tank with 3/8 fuel line and, 1/4 return line, a Carter CRT6903 mechanical pump from the pump it has all -6 an steel braided lines to the Holley HLY-12-803 regulator then, -6 an steel braided line through an Earls filter to a Holley 850 double pumper. I really feel the regulator is the wrong one for this set up. The 1/4 return line has never been hooked under the hood and, this regulator does not support a return line. I'm hoping with the new regulator that is set up for a return line it will be able to vent some vapors from the heat under the hood and, keep the flow of fuel cooler to cut down on vapors.
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Couple thoughts. I hate dead head regulators. They never seem to work perfectly and are constantly suspect. I would do as @pishta says and run it without the cap as a test. Then, get rid of the regulator completely and run the mechanical pump straight to the carb. That pump is rated at 7psi max so your 850dp needles and seats should be able to handle it if adjusted correctly. I’d like to see a pick of what Earls filter you’re using. Most of those have very little filtering surface area and are known to be problematic at higher hp levels. Also please let us know where you are measuring the fuel pressure and what you’re using. Remember pressure is resistance to flow so when the carb is dry and the pump is filling it, theoretically you’ll have zero pressure. Unless the pump is way oversized. Although you should never be able to run the bowls dry with a properly sized pump. Lastly 3/8” line IMO is just barely adequate for 600hp. I'd consider using the 3/8” for a return and stepping up the supply to 1/2” once the rest of the system is dialed.
 
TT5.9mag said:
Couple thoughts. I hate dead head regulators. They never seem to work perfectly and are constantly suspect. I would do as @pishta says and run it without the cap as a test. Then, get rid of the regulator completely and run the mechanical pump straight to the carb. That pump is rated at 7psi max so your 850dp needles and seats should be able to handle it if adjusted correctly. I’d like to see a pick of what Earls filter you’re using. Most of those have very little filtering surface area and are known to be problematic at higher hp levels. Also please let us know where you are measuring the fuel pressure and what you’re using. Remember pressure is resistance to flow so when the carb is dry and the pump is filling it, theoretically you’ll have zero pressure. Unless the pump is way oversized. Although you should never be able to run the bowls dry with a properly sized pump. Lastly 3/8” line IMO is just barely adequate for 600hp. I'd consider using the 3/8” for a return and stepping up the supply to 1/2” once the rest of the system is dialed.
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I'm measuring psi at carburetor with a non fluid filled gauge in the rail. It had a vented gas cap on it before and still had this issue. It also had this problem before the regulator was installed. Without the regulator this pump will put out 9.5 psi at the carburetor that was the point of the regulator.
 
Run the 1/4 return off the other end of the rail and take the regulator out of the equation. The 1/4 return will drop the static pressure of the 3/8's 9.5 psi and there will be no chance of vapor lock in the line as it will always be vented back to tank. The calculations for pressure loss are:
where Q=CdAoYρ(1−β4)2ΔP, then ΔP=21ρ(1−β4)(CdAoYQ)2 so its just faster to measure it straight into the return line than do these calculations.
Calculation of Flow through Nozzles and Orifices | Neutrium
 
"steel braided lines" rubber hose? checked for collapsing hoses or folded over inner layer hose at the insert
 
plymouth67 said:
I have checked the seporator and it's free and clear and, hooked up correctly.
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I dont believe a mechanical pump will supply enough fuel for your engine @ WOT. I ran an old Holley electric pump on my 512 other than the noise I never had any fuel issues. Never even ran a regulator and/or return line.
What type of fuel pressure gauge do you have and where is it located?
What size fuel lines?
 
512Stroker said:
I dont believe a mechanical pump will supply enough fuel for your engine @ WOT. I ran an old Holley electric pump on my 512 other than the noise I never had any fuel issues. Never even ran a regulator and/or return line.
What type of fuel pressure gauge do you have and where is it located?
What size fuel lines?
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The gauge is non fluid filled gauge in the rail right before the carb. The line to the pump is 3/8 and from the pump forward is all braided -6 an lines.
 
69FBCuda said:
"steel braided lines" rubber hose? checked for collapsing hoses or folded over inner layer hose at the insert
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I have had them all off and inspected them, they all look good and they are also new.
 
512Stroker said:
I dont believe a mechanical pump will supply enough fuel for your engine @ WOT. I ran an old Holley electric pump on my 512 other than the noise I never had any fuel issues. Never even ran a regulator and/or return line.
What type of fuel pressure gauge do you have and where is it located?
What size fuel lines?
Click to expand...
Its fine under WOT, its about an 1/8 down the road after you get out of the throttle when it has a problem.
 
plymouth67 said:
The gauge is non fluid filled gauge in the rail right before the carb. The line to the pump is 3/8 and from the pump forward is all braided -6 an lines.
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Unfortunately you cant trust the accuracy of that gauge. They all go to zero under the hood from the heat. If you remote the gauge out from under the hood you will get a different story.
 
512Stroker said:
Sounds like you are draining the lines and it just takes 1/8 to empty the carb dry.
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It will run WOT for as long as I want it to after I get out of the throttle it will run just fine for about an 1/8 of a mile before it looses pressure and dies. It also seems heat related as if the motor is cool it doesn't seem to do it.
 
plymouth67 said:
It will run WOT for as long as I want it to after I get out of the throttle it will run just fine for about an 1/8 of a mile before it looses pressure and dies. It also seems heat related as if the motor is cool it doesn't seem to do it.
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I ran 3/8 aluminum hard fuel lines from the tank thru the fuel pump all the way up the firewall and into the back of a dual hard line feed to a 950 cfm Holley. Ran a 1/2 thick thermal spacer under the carb - no issues.
I still think you cant do this with a mechanical pump.
Prove me wrong, I hope you can.
 
512Stroker said:
I ran 3/8 aluminum hard fuel lines from the tank thru the fuel pump all the way up the firewall and into the back of a dual hard line feed to a 950 cfm Holley. Ran a 1/2 thick thermal spacer under the carb - no issues.
I still think you cant do this with a mechanical pump.
Prove me wrong, I hope you can.
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You may be right. My new regulator will be here Wednesday so I will try it before i go with electric pump. I also have a 1/2 im phenolic spacer under the carb.
 
512Stroker said:
I dont believe a mechanical pump will supply enough fuel for your engine @ WOT. I ran an old Holley electric pump on my 512 other than the noise I never had any fuel issues. Never even ran a regulator and/or return line.
What type of fuel pressure gauge do you have and where is it located?
What size fuel lines?
Click to expand...
It is possible, With the right pump. On the engine masters show Freiburger just made over 1000 hp on a BBC with a tunnel ram and two 4s naturally aspirated, all fed by a mechanical pump.
 
@512Stroker Im in the group that says a turret style mechanical can feed a 512. Let me back it up with this:

fuelrate.gif


This data is for a 325 HP 340 but even if you double the horsepower to a very hot 512's potential 650HP all 3 besides the Holley 80 are still capable of feeding that per that graph..and he's losing the fuel pressure at a few seconds out of WOT. Wish we could see if the pump rod is seizing up after a WOT run, then any mechanical pump you run is going to go to zero. Maybe worth a shot to remove the pump rod and check it for galling or run a crocus cloth over it to knock down any high spots? I think the OP may need to get a remote gauge and monitor when the pressure really starts to drop. The pumps are proportional to engine RPM to a point but by design, they top out at the prescribed PSI or close to it under no load conditions.
 
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