electric fuel pums and carbs, bad idea?

[Alf]

i have an 85 300zx with a stock 305 chevy, i retained the electric in tank pump and used a mallory regulator set at 6psi. i have a 650 spread bore double pumper holley and i just cannot get this car to run right. could the fuel pump be providing too much fuel? it just seems like any modification i make to the carb it has no effect, the car stumbles and runs rough at low rpm, and then it hits a certain point and it totally smooths out and picks up. it almost seems like its retarted or its loading up, i cant figure it out. any ideas? would a mechanical pump be a better choice for a carb?

[Guest dats240]

It sounds like you may have a blown power valve in the carb which i'm sure would have been caused by that high pressure fuel pump if that is the problem. I had a blown powervalve and the same symptoms where at idle it would back up real bad but once you cleared it out some or got on it, it would be fine. It basically just pours way too much fuel out of the carb, so at low revs its just puddling up. Holley claims they dont blow powervalves anymore, but they do. Also, I would recommend calling their tech-support as they can be very helpful, or just checking out their web sit for technical info, faq's, etc. And i would switch to a low pressure fuel pump designed for use with a carb.

- Good Luck

[Alf]

wouldnt the car idle crappy?

[Turbo Meister]

i have an 85 300zx with a stock 305 chevy, i retained the electric in tank pump and used a mallory regulator set at 6psi. i have a 650 spread bore double pumper holley and i just cannot get this car to run right. could the fuel pump be providing too much fuel?

 

If your regulator is working properly and you truly only have 6 psi to the carb the type of fuel pump is irrelevant.

Is a gauge measuring the fuel pressure?

Is the carb jetting correct?

Is the power valve working and of the correct designation for your engine( does it open too soon or too late)?

Is the float level correct?

Is there a vacuum leak?

Good Luck

Hanns

[Alf]

i have hooked a guage to it and the pressure is steady till it passes 4psi and then it shoots up and down at least 2psi. the jetting is good, the power valves are not really ideal. the float level is correct, and there are no vacuum leaks, i had one earlier but fixed it. i just changed both power valves with no change in performance, it seams rich just off ide at cruise spead cause it smells like gas real bad, but when i pull a plug they look perfect. i also recently advanced the timing and it seemed to smooth out a lot, i think maybe the ring on my dampner has spun giving me false timing info.

[grumpyvette]

I don,t know if the regulator is your main problem but its certainly suspect! check the other things the guys mentioned like the power valve and timing, check your float levels, check for vacuum leaks check the fuel filter, etc. but since I don,t know exactly HOW you have the fuel pressure regulator hooked up, but to function CORRECTLY Ill point out a regulator MUST have a non-restricted fuel return line back to the tank

designed like that pressure remains dead even and constant at the SET pressure level

 

regulators and fuel systems NOT having a free flow,non-restricted bye-pass line and a good regulator act like you described...pressure fluxuates rapidly and pressure is never constant

 

 

FUEL PRESSURE REGULATORS DEADHEAD vs BYE-PASS STYLE

 

 

 

dead head requlators seldom allow a stable fuel system pressure,you NEED a byepass style system with the regulator mounted close to the carb, you can ignore the nitrous part of this diagram if your not running nitrous but the concept of the higher pressure feed bleeding off excess pressure at the regulator so that the carb sees a constant feed pressure is valid.it may help if you understand the differance in concept of how the regulators work

 

PRESSURE IS THE MEASURE OF RESISTANCE TO FLOW

 

THE DEAD HEAD STYLE REGULATOR

works with a spring on a valve that allows the valve to open once the DIFFERANCE IN PRESSURE between the sides of the regulator valves fuel lines has changed

think of it as a door that has 7-10psi on the feed side and you want lets assume 5.5 psi at the carb

as the fuel pump fills the line it eventually (fractions of a second )reaches the point where theres a voluum of fuel past the valve with enought pressure to allow BOTH the SPRING and the fuel pressure past the valve to close the valve untill the fuel is reduced to the point that the SPRING and the remaining fuel pressure/voluum beyond the valve can not hold the valve closed and the valve is force open and held open untill, that diffearance in pressure is restored. now lets launch the car hard, the pump that had maintained 8-10 psi to the regulator, 5.5 psi past the valve and the spring in the regulator is now fighting the fuel in the line feeding the regulators enertia, and the sudden drop in pressure as the throttle drops full open in the carb,what the pump sees is the full 8-10 psi or MORE the regulator sees a sudden drop off to near zero and it opens wide, if the fuel pumps able too it tends to flood the fuel bowl for a second then the valve slams shut, untill the pressure drops off as you hit each gear the cycle repeats, the result is a surge in pressure and a rapid drop off in voluum then a rapid flood of fuel that rapidly cycles as you go down the track

if you had a accurate fuel pressure sensor at the carb youll see a rapidly cycling pressure/flow

if some crud gets stuck in the valve it cant close and your carb FLOODS OUT, because it must fully close every few fractions of a second to work correctly

 

the bye-pass regulator functions in a totally differant manor

assuming the same set-up but you replace the regulator with a bye-pass style regulator, the bye-pass regulator works by opening a valve too a much lower pressure path for the fuel to return to the tank,the open fuel return line. anytime the pressure exceeds the 5.5 psi,youve set it to, so the fuel line to the carb can only see a max at that 5.5 psi. now the pumps sitting there potenially supplying at 8-10psi just like before, but it can never exceed 5.5 psi because the bye pass regulator bleeds of any excess the pump supplies. but lets look at your launch, if the pressure drops to 6- 7 psi nothing changes at the carb, if it increases to 10 or 12 psi, nothing changes at the carb,if it drops to to 5.5 psi or less the valve to the bye pass line will close maintaining at least what the pumps suppling, but thats seldom a problem, if the sudden changes in pressure and over pressures that happen when you suddenly change the fuel flow required or the (G)loads on the system that potentially screw things up,the bye-pass regulator style regulator isolates the carb and maintains the desired 5.5 psi FAR MORE COBNSISTANTLY

now lets assume the spring get weak over time or the adjustment gets set at 4 psi in error, with the bye-pass style youll probably never notice ,if you had a accurate fuel pressure sensor at the carb youll see a rock steady pressure/flow

should some crud get stuck in the valve and it cant close NOT MUCH HAPPENDS, because its normally OPEN not closed

if you check youll see MOST EFI systems are BYE-PASS regulated designs also due to control and relieability issues

 

but on the dead head the cycle just gets about 20% more erratic and more frequent in the cycles, further weakening the spring over time

[Alf]

my car was originally fuel injected and had a return line factory. the regulator i am using operates with the car's stock return system. i have the regulator positioned on the shock tower about 2 ft from the carb, i have a pressure inlet, bypass, and outlet, and a brand new wix fuel filter between the regulator and carb. thats why i can't understand why the pressure would fluctuate so much, could it be a faulty regulator? the regulator i am using isn't one of those $30 mr. gasket ones, it is a mallory unit and was promised to regulate the super high pressure from the factory pump.

[COZY Z COLE]

 

I also have a Mallory fuel reg. with an electric pump feeding a 700 cfm DP Holley carb. My fuel pressure guage is always moving between 4 and 6 psi depending on throttle action. If you can, before you start changing any parts, dyno it with a/f readings and this will indicate how your fuel system is operating.......

 

 

LARRY

[Alf]

i would love to do that, but the dyno session would cost more than the entire car is worth. i am going to purchase a air/fuel ratio meeter for my camaro and i will use that on the z too. i am planning on installing a more powerfull engine over the winter and i might decide to convert the car to a mechanical fuel pump system.

[COZY Z COLE]

All right then..... Good Luck....

 

LARRY

[Psykovertible]

i would be wondering what volume of fuel is flowing as well. the pressure is most imnportant obviously because as long as the pressure isnt too high it should not be overpowering the float seat. have heard of too much flow even at correct pressure. remember that it is something i was told so it could be as valid as the tooth fairy.

[Alf]

how would you figure that out? the fuel line is 3/8(or close in metric) i can go from 6 psi to 4.5psi and not even tell the difference.

[RedZ85]

i have an 85 300zx with a stock 305 chevy, i retained the electric in tank pump and used a mallory regulator set at 6psi.

 

are you using a stock 300ZX in tank EFI fuel pump to a regulator holding it at 6 PSI to the carb? Or am i reading that incorrectly? If you are using a stock EFI fuel pump, that could be your problem. EFI fuel pumps are not compatible with carburators and installing a conventional regulator will quickely damage the pump because it must be used with a bypass regulator to allow a continuous flow of fuel to cool the pumps electric motor. You may have or are in the process of damaging your 300ZX pump.

[Alf]

that is correct, i am using the stock efi pump. my regulator has a return line, shouldnt that work

[RedZ85]

My main concern is the difference in fuel pressures. On a Carbed engine, you have a fuel pump putting out about 10-14 psi which goes to a regulator and comes out a constant 7 psi (or whatever)

Then you have the EFI fuel pumps which are designed to output a much higher pressure and get regulated to around 50 psi.

Now you take both worlds and mix them up. So a 50+ psi EFI fuel pump is getting restricted to a carb rated 7 psi. I dont believe that those pumps are designed to do that, even if you have a return line for constant flow.

 

I believe that an 85 300zx has a fuel pressure of about 45 PSI, so the regulator is responsible for maintaining that pressure. That means that the fuel pump has to be producing more than that. If you have a fuel pump designed to work with a regulated fuel pressure of about 45 PSI and the restrict it down to 7 PSI...i think thats gonna cause problems. Has it always done the fluxuations? Or did it develop over time and is getting worse?

[spdsk8r]

This is from memory. I believe my first edition JTR manual has something on this. If my memory is correct, there was something about running a carb'ed engine with the stock EFI fuel pump. The manual said to use the stock EFI regulator to bring the pressure down to @40psi and then use an aftermarket regulator to bring it down to @6psi from there. The stock regulator has the return line back to the tank.

[kcelectronics]

If the fuel pressure is correct at about 4~6 psi then you don’t have a problem with it, carbies have a fuel bowl full of fuel so unless the pressure is in excess of this figure you have a different problem. If the pump was over powering the needle and seat it would cause the engine to stop, as eventually the fuel would start to pour out of the carbs vent holes (you would defiantly see this) and the engine would flood. The symptom you describe sounds like the power valve could be too low! What manifold vacuum does your engine have at idle/low throttle setting? I had a 302 that had about 12†of vac at idle and I had a 13†power valve so it was open even at idle I changed it 3 times for the same value, before I worked it out and installed a 8.5 and the problem was solved?

 

 

 

Was the carbie new? Has it been run and then stored for several months as this can cause the Gaskets to shrink and uncover some of the internal passages which send vacuum to the small chamber that the "Power valve" sits in! which would also cause it to be open at Idle Just a thought.

 

 

 

Colin:twisted:

[Alf]

the carb i newly rebuilt. and the psi has always fluctuated. have any of you guys ever changed the pump on a z31, i think im just gonna go mechanical.

[kcelectronics]

Don’t wast money till you are sure of the problem, I am amazed at how much money is thrown at a problem with out it being fixed, I have read many posts where a simple problem has caused grief and has had many expensive parts changed without a positive result. There was a guy who changed his plug leads, distributor cap, distributor, carbie, fuel pump, and battery? because his engine had a misfire after changing his spark plugs! Turns out that he had cracked the porcelain insulator when fitting the plug and convinced himself that it couldn’t possibly be the problem as they were brand new!! So my advice is to find the problem unless you want to go this route!

 

Again I say if the pump was the problem then you would see the fuel being pumped out of the vent tubes on your Holly and the engine will cut out.

 

A low speed miss fire could be running to rich or to lean, but if you say that it “Smooths out and picks up†you should really should look at the carb, it may be an ignition problem but I doubt it as they usually show up under load which you don’t seem to have.

 

Long-range diagnosis is difficult so if you could supply more info about this problem it might help us to pin point your problem.

 

What manifold vacuum is your engine running? Who rebuilt the carb? What value Power Valve is fitted? as the stdandard ones are quite high 14â€~15â€?

 

Colin:twisted:

[Alf]

i rebuilt the carb and i have a lot of experience with holleys. the engines pulls about 18 inches of mercury and i have a 10.5 pv in the front and a 8.5 pv in the rear.