3.2 SS carb'ed engine. Fuel pump question

[Gideon911]

My 72 has a 3.2 SS, recently rebuilt by Tyson Schmidt. Car runs great until I'm driving it hard in canyons for more than say 20 minutes or so. The problem is clearly lack of fuel. When the car starts to bog and not run properly, I pulled over and checked fuel pressure. Very low pressure, and hardly any fuel in the glass on the carbs. Tyson recommended a larger pump (Pierburg universal high flow low pressure PB-91160811006).

I ordered the pump and the inlet/outlets are 12mm. My fuel lines are 8mm. New lines and my tank was overhauled under the PO ownership. Filter is located back near the trailing arms. Tyson was going to install this and make fittings. I'm doing it myself to save the cash and JIC this is not the issue..to avoid an on going case of spending more and more to find the issue.

I've got fittings on their way in the mail, but I'm thinking this pump won't allow for more flow as the lines are smaller. Tyson said I should replace the tank with a 76 and newer tank, and re-locate the pump (up under the tank)..I really don't want to do that unless I'm simply not going to get the performance from good fuel flow/pressure this engine needs. Obviously its a big job to swap to larger diameter fuel lines and replace the tank. Kind of a bummer since this car has a newly built engine, recently been completely over hauled, and I can't really enjoy the car. Any recommendations? I haven't played with the pressure adjustment screw at the regulator. I'm going to verify my fuel filter is good while I wait for the fittings to come in the mail.

[Gideon911]

old vs new pump. I spoke to a friend who thought this larger pump was unnecessary without adding the larger fuel lines. He said most carb'ed motors have the size pump I've already got. Should I be able to get 3.5-4.0 psi from a small pump like this?

[snbush67]

Run clear fuel through all the lines and back flush the regulator. See if there is any debris plugging up things. Check the filters and make sure they aren’t plugged. If you can get steady 3psi at idle then increase the rpm’s and watch the pressure and the floats.

[snbush67]

Open the regulator all the way and flow a couple of gallons through it. Check for debris.

[Trackrash]

Just disconnect the fuel line from the carbs and hold it over a can or small bucket. Without running the motor turn on the pump and see how much it flows.

[Gideon911]

will do. Not sure how to back flush the regulator but I'm sure google will help me figure that out. If I have 3psi at idle, what should the psi be as the rpms go up?

Originally Posted by snbush67

Run clear fuel through all the lines and back flush the regulator. See if there is any debris plugging up things. Check the filters and make sure they aren’t plugged. If you can get steady 3psi at idle then increase the rpm’s and watch the pressure and the floats.

[Gideon911]

These cars should just have two filters right? one up near the tank and one in the engine bay? I know there's the little screen at the bottom of the tank. I believe the tank was gone through a few years ago, new lines were installed..but the car did sit at a body shop for 9 months in my ownership. I've read that guys will replace the pump, then find it wasn't the pump..but later ended up needing to replace the pump again since the crud got in their new pump..good advice above. I'll plug the old pump back in and see what flows out/check the filters before hooking up this new pump

[1QuickS]

From my website:

Measuring Fuel Delivery
Fuel pressure and volume are often assumed to be adequate for your carburetor's operational requirements however they are quite important to assure adequate fuel delivery during all power demands of your engine. It is advisable to test for adequate fuel delivery and pressure during peak power demands to verify there is no deficiency of fuel delivery. The following procedure is a complicated but comprehensive method to check adequacy of fuel pressure AND flow during peak engine operational demand:

Estimate the maximum fuel consumption of your engine by multiplying your engine's peak horsepower by 0.10 which results with fuel demand in gallons per hour

Install a flow control valve into the end of the fuel line connected to one of the inlets to the carburetors and downstream from all fuel pressure regulators and filters and close it.

Install a fuel pressure gauge into the fuel line just before the fuel flow control valve. The pressure gauge needs to be a quality item with a peak pressure range of no more than 10 psi.

Install a shut-off valve downstream from the flow control valve and open it.

Connect a fuel line from the shut-off valve and have it drain into a graduated vessel. This vessel should be placed into a larger catch tank to assure spillage of gas will be contained.

This entire setup should be prepared for and the fuel delivery testing conducted out of doors and with a fire extinguisher near at hand.

Connect a battery charger to the battery to simulate power delivery to the fuel pump during normal engine operation.

Activate the ignition switch to start the fuel pump but do not activate the starter for the engine

Adjust the flow control valve to generate a 3.5 psi pressure reading in the fuel line before the flow control valve; this is important since fuel delivery rate under operational pressure will be less than freely flowing fuel

Close the shut-off valve and prepare to collect fuel in the graduated vessel

Open the shut-off valve and collect fuel in the graduated vessel and record the time for the filling process

Repeat the procedure to improve measurement technique and accuracy

Calculate the fuel flow rate and compare with the calculated fuel demand for your engine

Example Calculation For Fuel Flow Rate Requirements:
Assume peak engine horsepower is 210 HP

Fuel consumption is:

210 x 0.1 = 21.0 gallons/hour

Convert gallons to ounces and hours to seconds:

21.0 gallons x 128 ounces/gallon = 2688 ounces

1 hour = 3600 seconds

Convert 2688 ounces/hour to ounces per second:

2661.12/3600 = 0.747 ounces per second

[al lkosmal]

Check to make sure that your gas tank vent line is not plugged......

regards,
al

[Gideon911]

before I get any further..I just did what trackrash recommended. Disconnected from the carbs and ran some fuel through..At first there was reddish brown coming out for a few seconds then started to come through clear. Before I did this, I pulled the filter out and blew threw it..Took a very slight build up of pressure and shot out like a cannon then seem to flow through more easily. My car only has one filter...Should I add another filter? I was under the impression these cars have two filters.

I messed with the adjustment screw at the regulator a little bit as fuel was flowing and didn't see any change in flow..not sure if I should or not but figured it was worth a try. I'm tempted to just plug in the new pump and see if I get a lot more flow. Not sure if that's a great idea since I'd like to return that pump if its not what I need/is not going to help..

Al Ikosmal- I'm going to check the vent line tonight thank you

1quickS- This is great info, but damn that is a time consuming project to set up..comprehensive and complicated haha..I don't know my cars peak power. it hasn't been on a dyno. I know it's advisable to take the thing to a dyno and do some pulls but I'm hoping I can simply swap out my pump, get that 3.5-4 psi and test/adjust on the road. If I'm not able to get it to run right I may give this a shot

[Gideon911]

I'm glad I started down this path before changing out my pump. Time to figure out the best way to clean out my tank and replace my screen. I'll replace the filter and the pump..then start focusing on how much pressure I've got/need.

[H-viken]

Important to keep in mind that the right pressure is not everything, the pump also needs to flow enough volume. Low pressure and high volume is what you need, as Tyson rightfully said

[75 911s]

My question is: If the new pump is high volume, but he's reducing on the input and output from 12mm to 8mm, does high flow even matter at that point if you are just reducing to old lines on either end?

[Locker537]

I'm running that Pierburg pump on a 3.0 liter with Webers.

I have a Mahle filter inline before the pump. There are small inline filters between the PMO FPR and carbs. I am running a return line.

[Gideon911]

Duane has a good question. I'm curious also. Tyson said it's not ideal to go from the 8mm up to 10mm..The inlet and outlet on this pump is actually 12mm..which is obvious why its not ideal..clearly you'd rather have 10 or 12mm lines throughout..but I'm not going to deal with pulling new lines through the tunnel, and the tank supply and return sizes are 10mm and 8mm. He did say he's done added this pump for carb'ed motors before without issues.

I still have the question..is this pump going to create the added suction the smaller pump did not even with these smaller lines. Although it is better to have larger diameter lines throughout, it seems like Tyson believes this pump do the job. He mentioned the tiny pierburg pump that's been in the car is good at pressure but doesn't do a good job at suction. He also mentioned that even with the return line blocked, that little pump can only do 3.5 psi max. My motor needs 4-4.5 psi.

Supply line at the tunnel is 10mm and return is 8mm. All soft lines are 8mm and seem to be all in really good shape. They were replaced in 2018. I checked receipts form the PO last night and found that this tank was pulled in 2018. It was filled with 4 gallons of apple cider vinegar, agitated and left to soak to dissolve rust. There is the sediment at the bottom of the tank but I guess it's not too bad? I think I'll just replace the pick up screen, and possibly see how difficult it will be to get the sediment out.

[Gideon911]

Tyson helped me sort out a plan. Fortunately I can replace the line coming from the tunnel to the pump (it has a reducer from 10-8). Now I'll have 10 all the way to the pump. 8 after the pump to the carbs and back to the tank.

Not sure If I'm going to remove the tank to clean it or leave it in the car.

[Tremelune]

This all seems rather involved...It's carbed! I thought pressure could be near-atmospheric at the engine and it would be fine, no? I can't imagine thin fuel lines starving out a carb if the lines are clear and full of fuel. I'm surprised you're still having issues after blowing out all the filters. I would have been more concerned with the hotter engine boiling the fuel without a return line...

I'm only aware of the one large fuel filter in the engine bay...My guess is others were added, and it's not a bad idea to have one before the pump (especially if you know your tank has some...issues).

I'm putting a big port 3.0 in my '71 at some point, and I'm curious to see how this pans out...

[Gideon911]

Well I did have some sheit in the tank. Draining the tank today. I verified the filter was clear, but I didn’t run the motor after doing that. Since I had some brownish fuel that was making it past the filter I’m going through it all to get it clean before. A few filter is $9 so I’m replacing that while at it. As far as carbs and what pumps are used/needed. Idk, I’m still a noob

[Gideon911]

I think I’ll add another filter during this just in case. Anyone have a good recommendation for an inline filter?

[Locker537]

Originally Posted by Gideon911

I think I’ll add another filter during this just in case. Anyone have a good recommendation for an inline filter?

The cheapies from the auto parts store is fine. You can buy the metal housing ones from a bunch of official places as well if you want.

The clear plastic ones are nice cause you can see through them and keep an eye on things.