CIS to Carbs, which fuel pump?

[QueWhy]

I’m hoping to undertake the switch from CIS to carbs in the next couple months.

Should I stick with my high pressure pump(recently replaced) and get the PMO regulator to bring the pressure down to what the carbs need or get an appropriate low pressure fuel pump?

Either way I’ll probably take the opportunity to replace the fuel lines in the tunnel.

[fizeus]

Originally Posted by QueWhy

I’m hoping to undertake the switch from CIS to carbs in the next couple months.

Should I stick with my high pressure pump(recently replaced) and get the PMO regulator to bring the pressure down to what the carbs need or get an appropriate low pressure fuel pump?

Either way I’ll probably take the opportunity to replace the fuel lines in the tunnel.

Yes... A pressure regulator is mandatory or you will fllod your carbs.
In carbed system pressure is not important as the volume of fuel delivered...

[Vin-barrett]

I've been using the CIS pump with my carbs for many years now.
The PMO regulator brings the pressure up, not down. Will not flood the carbs.

[jpnovak]

Originally Posted by Vin-barrett

The PMO regulator brings the pressure up, not down.

Sorry, this is incorrect.

The CIS pump will normally put out about 43psi (3bar) at maximum. The carbs (weber, PMO, Zeniths) all use 3 - 3.5 psi. The PMO regulator will reduce the pressure.

Using the PMO regulator is the most simple method for switching the fuel system to carbs.

The best way (IMO) is to find the early 911 , 3 port, carburetor fuel pump that is internally regulated at 3psi. This will likely require other changes to your fuel system such as fuel pump location and/or fuel line relocation.

[Vin-barrett]

Originally Posted by jpnovak

The CIS pump will normally put out about 43psi (3bar) at maximum. The carbs (weber, PMO, Zeniths) all use 3 - 3.5 psi. The PMO regulator will reduce the pressure.

Using the PMO regulator is the most simple method for switching the fuel system to carbs.

It has baffled me how many on this board believe this to be true.

While a CIS system has high pressure, without the iron fuel head you will only get about 2psi with a factory CIS pump. The PMO regulator brings it up to 3-4 psi

[jpnovak]

semantics. Any pump has almost zero pressure when run wide open without restriction. Referring to system pressure not open flow pressure.

suppose its is best to say the PMO regulator has less end of line restriction than the stock CIS system and therefore has less pressure to the carbs.

[Vin-barrett]

Originally Posted by jpnovak

semantics. Any pump has almost zero pressure when run wide open without restriction. Referring to system pressure not open flow pressure.

Yes that is essentially what you get with a return to the tank on a SC.
The PMO regulator brings pressure up not down.
What exactly did I say that was incorrect?

[Vin-barrett]

Originally Posted by jpnovak

The PMO regulator will reduce the pressure.

incorrect statement here

[juanbenae]

I went to carbs maintaining a couple year old CIS pump and it only lasted a year n a half or so. not sure if it was a coincidence or the high pressure being dead headed with the regulator and return line.

[Vin-barrett]

Ive been using the same CIS pump around 10 years on my SC but that is not a deadhead system having a return. There is no high pressure.

[mgatepi]

I am really confused how a regulator can bring the pressure up? The regulator can in no way increase the pressure.
Please explain.
Thanks

[Vin-barrett]

Originally Posted by mgatepi

I am really confused how a regulator can bring the pressure up? The regulator can in no way increase the pressure.
Please explain.
Thanks

Its just a simple restriction like putting your finger over a garden hose. There is really nothing much to a PMO regulator.

[mgatepi]

Originally Posted by Vin-barrett

Its just a simple restriction like putting your finger over a garden hose. There is really nothing much to a PMO regulator.

So you are saying that if I put my finger over the end of a garden hose and hold back all the water, then move my thumb slightly off to allow some water to escape, that this pressure is somehow greater than the maximum created by the pump?

I believe you are confusing pressure with velocity.

[Vin-barrett]

pressure in the system ( created by pump, size of lines from the tank to carbs, all the parts) is increased with a restriction in the regulator. With the regulator wide open will be about 2lbs or so. Offering the restriction in the PMO regulator or "screwing it in" raises the pressure.

[QueWhy]

Originally Posted by juanbenae

I went to carbs maintaining a couple year old CIS pump and it only lasted a year n a half or so. not sure if it was a coincidence or the high pressure being dead headed with the regulator and return line.

This is my concern. I’d rather do it right from the get go than make a suboptimal solution work.

I guess it also depends on what carbs I go with. I’d like to find a set of Webber 40s that are in good shape for a couple grand vs the full blown PMO kit at $4K+ But this may be one of those “buy once, cry once” instances where I’m better served spending all the money up front.

The other thing I still need to solve for is the wiring. Is there someone that makes a “carb conversion” wiring harness or am I going to have to make the existing CIS setup work?

[Vin-barrett]

I have Webers but I'd much prefer the PMO's. The list of advantages over Webers are a google away. I often wish I just had the float windows like PMOs have.
If I did it again, id spend more upfront on the PMO. No doubt... or these days why not efi...

It's been years but I remember removing all sorts of wiring,

[Quickstep192]

Based on the buy once; cry once model, have you considered going EFI?

[boyt911sc]

Originally Posted by jpnovak

semantics. Any pump has almost zero pressure when run wide open without restriction. Referring to system pressure not open flow pressure.

suppose its is best to say the PMO regulator has less end of line restriction than the stock CIS system and therefore has less pressure to the carbs.

I had the same discussion with Vin several years ago about this same topic. I gave up convincing him about his flawed understanding how a pump works. His logic is flawed and I let it go unchallenged because he gave me something (gift). I feel some disappointment about his failure to grasp and understand his strong conviction about a PMO regulator raising the pressure.

Here is an example: If you connect a typical CIS fuel pump directly to a carb system, the flow rate or pressure would be too high for carb system to work. So people install a device called REGULATOR. The regulator does its job by reducing the existing fuel pressure not increase because it was already too HIGH to begin with.

Tony

[RWebb]

the factory Bosch fuel pump is very high quality and should be retained - you do need the PMO regulator

contact PMO if you want more info

[QueWhy]

Originally Posted by Quickstep192

Based on the buy once; cry once model, have you considered going EFI?

EFI is 100% out. I use a laptop everyday at work, I have no desire to use one in my 911.

Originally Posted by RWebb

the factory Bosch fuel pump is very high quality and should be retained - you do need the PMO regulator

contact PMO if you want more info

My factory Bosch pump **** the bed a couple years ago. I replaced it with a Dansk unit because I wasn’t 100% it was the problem.