Fuel pressure adjustment on stock fuel pump

[Zedyone_kenobi]

My 71 lately has been having a hard time revving above 5000 rpm. It starts to spit and sputter and just is generally NOT happy there. I have checked the Vacuum advance, it is perfect,, I have set the timing, it is dead nuts on and stable as a rock Set at 14 Deg BTDC as my DiZZY rebuilder recommended since he changed the curve to the european one. The car starts instantaneously and idles as smooth as you can image. Below 5000 rpm it could be a honda it revs so smoothly. So with timing not the issue (new plugs and wires and cap and rotor and pertronix is getting voltage it should), I moved on the the carbs, as it is acting like it is running very lean. I pulled the plugs which have been in the engine for maybe a year (about 700 miles), and they looked light light brown, but almost surgically clean. Installed new plugs as mentioned and nothing. Since I am running the pertronix flamethrower as well, I gap my plugs at 0.045". I have also tried 0.050" with no change in results.

So I proceded to rebalance my carbs. Not that I thought they needed it. NO adjustment was needed to the air balance so I then decided to close the Air/Fuel and then go out 2 full turns. Car started up and idled like it always did. Smooth as butter. I then went a half turn more, and the idle picked up just a bit.. GOOD. 2.5 turns is about standard. I then went another half turn and nothing really happened. But since I was sure I was running lean, I kept it at 3 turns. Buttoned everything up and took it for a drive. Problem actually got worse.

So then it dawned on me that I should check my fuel pressure. I put a T in one of the carb fuel lines and let the engine idle. According to the manual I should be looking at 3.41 to 4.17 psia at any engine speed. Well, I am at 2.5 psia to 2.9 psia when I rev it.

Next stop is to buy a new fuel pump (as this is the original 41 year old pump) and see what happens. Does anybody sell a purdy OEM one, that is all polished up?

Or can I adjust the fuel pressure on my pump

Edited November 6, 2011 by Zedyone_kenobi

[Mike W]

I am assuming that you are referring to the mechanical fuel pump as I do not believe that the early 240's came with the electric pump near the fuel tank.

If that is the case, I do not believe that there is any adjustment to the mechanical pump that will affect pressure.

For my setup (triple Webers), I actually removed the mechanical pump, replaced my original OEM electric one with a new one from Facet, and installed a fuel pressure regulator from Summit Racing that I have set for 3 PSI. Works perfectly.

Not aware of anyone selling an OEM mechanical pump that has been polished.

Mike.

[240260280z]

You can rebuild the pump. http://www.classiczcars.com/forums/showthread.php?33334-Rebuild-a-240Z-mechanical-fuel-pump

Maybe get a used one and take the best parts from both.

I took one apart years ago and I believe the spring and the check valve are part of the pressure regulation.

Also check to ensure that your fuel system has a small orifice on the return.

[Zedyone_kenobi]

Where would the orifice be?

[240260280z]

Pull the hose off the fuel rail at the return line's connection. I believe the rails fitting has a cap/plug stopped down by a small hole drilled in it.

[Zedyone_kenobi]

Many thanks sir! I will certainly check this out!

[Bonzi Lon]

Blue is correct, I will elaborate: The end of the stock fuel rail return line has a smaller size hole (orifice) than the line itself. This smaller hole makes the pressure in the carbs. DO NOT alter this orifice.

Another thing to look at is take the valve cover off and check how far pump arm is traveling, tightness of the front cam bolt, that holds the cam to operate the fuel pump.

Bonzi Lon

[Zedyone_kenobi]

will do...if any of you guys are running fuel pressure gauges, what are you getting from your stock fuel pump at idle and at say 2000 rpm.

[Duffman]

Stephen, am getting around 3.4 to 4.0 on my mechanical fuel pump at idle, but have seen it drop to 2.1 at times. Am swapping out to a new mechanical fuel pump to see if I can get better consistency.

[Zedyone_kenobi]

Thanks Duffman, I appreciate the information!

[Zedyone_kenobi]

I recieved my new fuel pump in the mail from Zcarsource yesterday. I will be installing it this week to see if my pressure returns to the 4 psia range.

[Zedyone_kenobi]

UPDATE:

I installed my new pump and it gives me about 3 psi at idle, I had to kill it before I woke up my kid, as I fired it up in the garage at 10pm!

So I gained at least a half PSI. That may or may not make a difference, but before I tinker anymore with the timing or carbs, I have to get my fuel pressure up to snuff.

by the way, I am using a rather inexpensive dual vacuum/fuel pressure gauge. Not sure how accurate it is. It is also close to 20 years old..