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SteveJ

Can't maintain fuel pressure

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Background:

When I got the car, it was a stock L26 with flat top carburetors. Three years ago, I changed out the carburetors for round top carburetors. For a while I noticed that after I had been running at freeway speeds, returning to stop-and-go traffic would result in hesitation during acceleration until it seemed like the car cooled down for a while. I enriched carburetors some more, and the problem seemed to go away.

In February, I did an engine swap, putting in an L28 with an N47 Maxima head. Since the head wasn't configured for a mechanical fuel pump, I just ran the stock electric fuel pump. The hesitation returned, even with enriching the carburetors even more.

I installed a fuel pressure gauge and found the pressure to be around 2.5 PSI. I changed out to a small Holley fuel pump (12-147), but the pressure dropped down to 2 PSI.

Saturday I changed out the fuel pump for a bigger Holley fuel pump with a fuel pressure regulator (L:12-802-1). The FPR was installed downstream of the fuel rail. At first, the pressure was at 12 PSI. I opened the small orifice on the exit of the fuel rail. The pressure dropped to 6 PSI. Moving the adjustment on top of the FPR did not raise or lower the fuel pressure. I moved the FPR to in front of the fuel rail. The pressure held at 4 PSI. I thought I was good to go.

Today I drove the car to help a friend with his Z. It did fine on the trip down. It did fine when I took him in a ride in the car. When I was leaving, the old problem returned. I checked the fuel pressure when I got home. It was below 2 PSI. I moved the FPR back to downstream of the fuel rail. Pressure was 2.5 PSI. Again, using the adjustment screw did not change the pressure. 

I am out of ideas.

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That hesitation after the new engine installation might have been due to the carbs not keeping up with fuel requirements due to bigger engine. According to official SU publications from years past, even putting aftermarket exhaust and air cleaners can require carb needle changes. When I made those changes plus a non-stock head, couldn't tune out WOT miss at 4000-4500 rpm until I installed SM needles. The SM's plus red springs from APT immediately cured the top end lean-out, and gave me a low rpm rich condition that fouls my plugs instead, and hurts my gas mileage. Pretty sure I need to mod the SM's to get things right. Other option is mod the N-27's.

Looked up your new pump:http://www.jegs.com/InstallationInstructions/500/510/510-12-802-1.pdf

Your fuel pump is 14 psi, regulator is 4.5 to 9 psi. Don't remember FSM recommended pressure but 3.3 to 4.2 psi is close. I read that regulators with a return line are better and more accurate. Possibly the regulator is bad.

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3 minutes ago, Stanley said:

That hesitation after the new engine installation might have been due to the carbs not keeping up with fuel requirements due to bigger engine. According to official SU publications from years past, even putting aftermarket exhaust and air cleaners can require carb needle changes. When I made those changes plus a non-stock head, couldn't tune out WOT miss at 4000-4500 rpm until I installed SM needles. The SM's plus red springs from APT immediately cured the top end lean-out, and gave me a low rpm rich condition that fouls my plugs instead, and hurts my gas mileage. Pretty sure I need to mod the SM's to get things right. Other option is mod the N-27's.

Looked up your new pump:http://www.jegs.com/InstallationInstructions/500/510/510-12-802-1.pdf

Your fuel pump is 14 psi, regulator is 4.5 to 9 psi. Don't remember FSM recommended pressure but 3.3 to 4.2 psi is close. I read that regulators with a return line are better and more accurate. Possibly the regulator is bad.

I have SM needles in the SUs. What are red springs?

I have opened up the carburetors to 3 turns each. 

You may be on to something with the FPR. I ordered the 12-804 FPR.

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Hard mounting those Holley regulators caused me a royal PITA until I let it float in the mount. After the race I got an Aero regulator and it has not been an issue.

I have gotten several comments from others saying not to hard mount a Holley so I have to go with the flow (no pun intended). One of them was Dave Rebello.

Edited by gnosez

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So could you clarify that some for someone who is electrically inclined instead of mechanically inclined? I'm not sure what you mean by "let it float in the mount". 

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Various springs, that go in the domes to restrict rise of the pistons, are available for the British SU's. Some will work with Hitachi's. The red springs are intermediate, considerably stiffer than the previous ones in my carbs, which were probably the stock ones. They have blue, red, yellow, and green, in order of stiffness. According to the "Just SU's" book, the needles and springs can be selected by measurement and testing to provide the best performance. Stiffer springs are said to reduce gas mileage, except that very stiff springs increase gas mileage and reduce performance. I sort of understand it, but not well enough to give a clear, in-depth analysis of what springs are best (and why) for a particular application. My car goes better when I stomp on the gas with the red springs. APT has needles and springs.

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Holley provides an "L" bracket that the regulator can be secured to. A line to or from the carbs and the tank along with perhaps a pressure gauge means that if you don't mount the regulator to the "L" bracket it will be held by fuel lines.

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Thanks guys. I'm tracking you now.

I was wondering about whether or not the mounting bracket was critical.

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Something doesn't make sense. I had a heavy moded 2.8 that was running 45 triples and went to stock SUs, stock fuel lines and after market generic fuel pump with out ever having a lean issue. 

For kicks can you run a single gallon of fuel safely in the engine compartment to isolate the fuel system entirely? Start with the closest and work out from there. 

I don't recall you mentioning tank condition from purchase?

edit: my fuel rail was not stock. 

My240z7.jpg

Edited by JSM

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29 minutes ago, JSM said:

Something doesn't make sense. I had a heavy moded 2.8 that was running 45 triples and went to stock SUs, stock fuel lines and after market generic fuel pump with out ever having a lean issue. 

For kicks can run a single gallon of fuel safely in the engine compartment to isolate the fuel system entirely? Start with the closest and work out from there. 

Given how the fuel system is configured in my car, I would have trouble doing that. However, you have given me an idea. If I detect low fuel pressure, I could bypass the FPR. If fuel pressure remains low, it could indicate an issue like garbage in the fuel tank.

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I could also change out the fuel filter and cut up the old one to see what might be lurking in the fuel tank.

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Okay, so I spent some time with the car tonight. The first thing I did was install an autometer fluid-filled gauge in place of the Mr. Gasket fuel pressure gauge. The face of the Mr. Gasket gauge was fogged up from me getting some gasoline on it. Really?

After the gauge was in place, I started the car. The gauge read 9 PSI. I shut the car down and installed the 1-4 PSI FPR. After creating some new curse words trying to get the lock nut loose on it, I started the car again. Pressure was low, around 2 PSI, and it was easily corrected by adjusting the FPR. I set it at 3 and called it a night.

Now, I have to consider the possibility that there is some rust or other junk floating around in the tank. Until I have the chance to drop the tank, I'm going to put some rare earth magnets on the outside of the tank in hopes they will hold the rust away from the pickup.

Thank you, all, for your suggestions.

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I have one of the clear Mr gasket filters that folks don't recommend coming right off the fuel line before the mechanical pump. Supposedly my tank was cleaned and I'm shocked at the amount of crap in that filter. 

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I have a BUNCH of fuel filters on the way. They should arrive tomorrow or Saturday. I might still take off the filter that's on the car now and cut it open.

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I am quickly beginning to think there's much more going on than just a fuel pressure issue. Today I drove the car. It started up and ran great. I stopped for a few minutes to buy some Harbor Freight supplies, and the car ran like crap. I continued on to my destination where the car sat for about 3 hours. Starting out from there, the car ran great, but showed some hesitation after having some stop-and-go traffic for a couple of miles.

When I got home, I pointed a fan at the carburetors and let it run for a few minutes. I took it for a drive, and there was some hesitation. I went back home and sprayed the ignition module (ZX distributor) and coil with a can of air held upside down. The car ran worse, so it wasn't an overheated ignition. I drove back home and put the fan on the carburetors again. About an hour later, I checked, and the intake was nice and cool. Going for another drive, the engine pulled great, with no hesitation. 

I feel that there is a big-time heat soak issue going on here. By the way, the car had ceramic coated headers and 4 screw SUs.

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Not sure. I just buy ethanol free gas. While I did experience vapor lock with the flat tops & stock exhaust manifold on one summer day, I didn't have these issues before the swap. I had this issue to some degree with the L26. The L28 is a higher compression motor, but I don't know why it would run hotter than the L26 with respect to the exhaust. I'm using a 160 degree thermostat. 

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 The various hydro carbons that make up gasoline have boiling points from 100` f - 400` f. The formula for gas is not precise and the quantity of components can vary from refinery to refinery. If the problem is new to a known working engine, Cliff may be right. The question now is how to cure it. A tankful of gas from a different station might be an easy fix or eliminate gas as the problem.

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8 minutes ago, Captain Obvious said:

You could also blast your carbs with the liquid from the upside down can of Perri-Air and see if that helps.

No, President Skroob took my last can.

Perri Air.jpg

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I can't help with your primary problem, but a heads up on the magnet. Rust is not magnetic.

Can we assume now that you have focused on heat in the engine compartment that you have solved your fuel pressure problems and it still reads 3 psig during the heat soaks.

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11 hours ago, SteveJ said:

No, President Skroob took my last can.

Haha!! It's in his desk drawer with all mine as well!  LOL

Good luck with the problem hunt. On that front, when the issue arises, do you think you're running rich or lean? Rich could be from percolating the fuel in the bowls and forcing additional fuel past the needle, and lean could be from boiling the fuel in the rail and pushing vapor pockets into float bowls expecting liquid. Any guesses as to which direction you are going? How do the plugs look?

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On ‎04‎/‎08‎/‎2017 at 6:35 PM, SteveJ said:

I am quickly beginning to think there's much more going on than just a fuel pressure issue. Today I drove the car. It started up and ran great. I stopped for a few minutes to buy some Harbor Freight supplies, and the car ran like crap. I continued on to my destination where the car sat for about 3 hours. Starting out from there, the car ran great, but showed some hesitation after having some stop-and-go traffic for a couple of miles.

When I got home, I pointed a fan at the carburetors and let it run for a few minutes. I took it for a drive, and there was some hesitation. I went back home and sprayed the ignition module (ZX distributor) and coil with a can of air held upside down. The car ran worse, so it wasn't an overheated ignition. I drove back home and put the fan on the carburetors again. About an hour later, I checked, and the intake was nice and cool. Going for another drive, the engine pulled great, with no hesitation. 

I feel that there is a big-time heat soak issue going on here. By the way, the car had ceramic coated headers and 4 screw SUs.

THis is exactly the same problem i have!! Heat Soak after getting stucked in traffic for several minutes. Cant seem to find the solution!! I even read some crazy ideas of driving with the engine lid off!!

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Okay, so I think I am at a good point with this issue. I fabricated a new heat shield and tested it today. Temperatures between the carburetors was around 260 with the old heat shield. Today, after I tried to induce some heat soak, temperatures were about 120.

 I started with a 24x12 1/16 thick sheet of aluminum from McMaster Carr. I used a brake to give the sheet a slight bend where I wanted it and marked off where I thought the PCV would pass by and the holes for the mounting points to the carbs.

I used various cutting tools, grinding tools, and a drill to get my preliminary design. On the first dry fit, I saw I needed to remove about 1.5 inches from the bottom. After that, I had to clear more space for the PCV tube, widen the mounting holes and grind down a couple of corners where the mounting tabs were hitting the carburetor insulators.

When I was happy with shape, I applied a sheet of insulation (adhesive backed), trimmed it to fit, and secured it with bolts and fender washers. I mounted a couple of bolts for the throttle return springs to mount, and then I bolted the heat shield to the carbs, bending the shield a little to keep it off the header.

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