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PMO fuel regulator

I am trying to put a set of webers on a 3.0. Stock fuel pump, stock fuel filter (no accumulator), to PMO pressure regulator, back to tank via return.

Can somebody explain how this pressure regulator thing works? It just doesnt make sense to me.

It restricts the return line to maintain your desired supply pressure. What if your pump has too much pressure? How does restricting the return line reduce the supply pressure to the carbs?

If you restrict the return the supply pressure goes up. If it is full open the pressure it still too high. These CIS fuel pumps put out more than the recommended 4 psi. I would think you want the pressure drop BEFORE the supply to the carbs. Am I going crazy?

I checked for return line restrictions. I will check again.

Any ideas?

Thanks,
BK
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Old 11-13-2010, 02:06 PM
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Your not crazy just not an engineer! Say you take the fuel line off the regulator and turn on the fuel pump and let the fuel just flow out (don't do this, just do it in your mind) there will be very little pressure in that hose. Now put your finger on the hose and the pressure goes up. So think of the needle valve in the return line as a finger on the hose. And it's AFTER the carbs so no matter what demand (volume) there is from the carbs, the pressure will always be what you set, like 3.0 PSI.
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Old 11-13-2010, 02:47 PM
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Not a good idea to run a CIS or any other FI style pump on carbs. Carbs like about 3-5 psi. "I" would get a rotary fuel pump, put the regulator on the supply side and "consider" blocking the return....

Last edited by Joe Bob; 11-13-2010 at 02:51 PM..
Old 11-13-2010, 02:49 PM
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running that (cis, sc pump) on my car and the PMO regulator can handle it (3.2 psi). Ideally, you only need a low pressure pump like the Faucet or the Holley.
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Old 11-13-2010, 02:54 PM
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Quote:
Originally Posted by Joe Bob View Post
Not a good idea to run a CIS or any other FI style pump on carbs. Carbs like about 3-5 psi. "I" would get a rotary fuel pump, put the regulator on the supply side and "consider" blocking the return....
- this is the opposite of what Richard Parr told me

the Bosch pump is of very high quality and will run a looooong time - just regulate it down with his pressure regulator
Old 11-13-2010, 03:49 PM
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Quote:
Originally Posted by RWebb View Post
- this is the opposite of what Richard Parr told me

the Bosch pump is of very high quality and will run a looooong time - just regulate it down with his pressure regulator
Me too, and it works perfectly putting out a steady 3PSI from my stock SC pump to my carbs..
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Old 11-13-2010, 03:56 PM
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Yea, I found a restriction in my fuel line. I took the return right to a can and the pressure was good. No visible restriction so I will go to each connection and see.

Are you SC guys using the stock filter?
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Old 11-13-2010, 04:03 PM
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you can, I'm not, have 2 filters just before the carbs.
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Old 11-13-2010, 04:15 PM
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Quote:
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you can, I'm not, have 2 filters just before the carbs.
Same here, in-line between the regulator and the carbs.
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Old 11-13-2010, 04:21 PM
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Racor makes a combo filter & water separator that is real nice - I don't have one on (yet).
Old 11-13-2010, 04:55 PM
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Quote:
Originally Posted by RWebb View Post
Racor makes a combo filter & water separator that is real nice - I don't have one on (yet).
Once my car is actually built I was thinking of using the Racor before the regulator and then the two replaceable ones between the reg and the carbs, think that may be overkill?
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Old 11-13-2010, 05:36 PM
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The PMO regulator has 1 in, 1 to carbs,1 return. I would imagine that the restriction is in the in side, not the return side. If it was on the return side that wouldn't make much sense because the high pressure has already passed the carbs.

The problem with what you are thinking is that you are going to be using an 80 PSI pump and restricting it down to 4 at the rear of the car.
That whole line from tank to regulator is going to be filled with 76 PSI at all times. That is not a good idea.

You can get a 3 o 4 PSI pump cheap. I would recommend it.
I use the holley red pump and I think it only puts out about 7 PSI max. To restrict that down to 3.5 is not as dangerous.
Or hard on the pump.
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Old 11-13-2010, 05:46 PM
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Been using a Holley fuel pressure regulator for years in front of my old Webers with no problems, and I don't have a return line either.
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Old 11-13-2010, 05:47 PM
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a by-pass reg restricts the return line to a set psi reading before the restriction. This psi is tapped to feed the carbs. The rest gets dumped into the return line at minimal psi

a dead head reg only restricts and doesn't dump excess. It raises psi before the reg. Pump output psi rises


one member uses a by-pass set to 10 psi and then that 10 psi to feed a dead head set to 4 psi to feed carbs. Gauges before the filter, after the filter where 10 psi is, and a dead head psi gauge of 4 psi enables full monitoring of pump condition, filter condition, and carbs

CIS Bosch is a great pump for carbs
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Old 11-13-2010, 06:17 PM
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Quote:
Originally Posted by dipso View Post
The problem with what you are thinking is that you are going to be using an 80 PSI pump and restricting it down to 4 at the rear of the car.
That whole line from tank to regulator is going to be filled with 76 PSI at all times. That is not a good idea.
Dipso,

What is the pressure is in the same line when it is connected to a CIS system? Is there a difference?

Quote:
Originally Posted by BK911 View Post
Yea, I found a restriction in my fuel line. I took the return right to a can and the pressure was good. No visible restriction so I will go to each connection and see.

Are you SC guys using the stock filter?
I run, stock filter, then to pressure gauge, then to small filters to each carb.

Shane
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Old 11-13-2010, 06:28 PM
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Check out my saga with the PMO regulator. Had to build a bypass. As others have said it already, it is just not safe to restrict the line to reduce the pressure. And using the PMO regulator alone, my pressure would only reduce down to about 9 pounds, which is way too high for the carbs...

PMO conversion, it's done...almost
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Old 11-13-2010, 07:57 PM
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KISS. Start with a 4 psi pump (Airtex or equivalent). 3.5 psi at the carbs, after the filter. No PR needed. Simple.

Sherwood
Old 11-13-2010, 08:02 PM
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Your so right Sherwood. But I was too lazy to buy another pump...Haha
FWIW, I have had this system in place for two years now and logged over 15k miles without a single issue...
Andy
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'75 911S Silver Anniversary #164(Formerly JTO's)
Rebuilt 2.7 with full ARP, 9:1 CR.
SSI, GHL, ER polybronze bushings, finned oil lines.
Lowered and corner balanced. CIS retired, now PMOs!
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Old 11-13-2010, 08:09 PM
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Im a little confused on where some of you are restricting your fuel lines, i know several of you have had problems with restricting down pressure. but if the return lines are free of restrictions you should have no pressure buildup in the lines or very little maybe 1psi, then you slowly restrict the return to 3psi and the fuel pump is not building anymore pressure then 3psi. so there is not 76 psi of pressure anywhere in the system. if i remove my restrictor from my return line it makes less then 1psi. i would think the cis would last forever under these conditions. so far two years with the PMO system with no problems to date, I understand if the return line gets kinked then the dead head pressure goes to the carbs which is bad.

cheers ed
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Old 11-13-2010, 08:17 PM
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You are correct Ed, pressure has no chance to build up as it is bypassed. The CSI pump is mostly bypassing
Old 11-13-2010, 08:23 PM
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Old 11-13-2010, 08:23 PM
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