Fuel pressure consistently fluctuating on carburated 914, Fuel pressure Options

[reese5997]

The vac port is for a pressurized system but you will see a slight pressure change with it connected. In a boot condition (turbo) it pressurizes the chamber and increases fuel pressure.

In your condition, the vacuum will reduce pressure at idle slightly. Less than 1 psi or so.

Oh ok, but; so should I leave it as it, or plug the removable port? It currently is not connected to anything.

[rhodyguy]

while you cleaned your tank but didn't install the mesh strainer, the first line of defense for the fuel filter is missing. have you tried a new filter? if it's a stocker, they're cheap...toss and replace.

if you soak an old mesh strainer in lemon juice and rinse, more than likely it will have a somewhat brass finish.

read up on that holly pump. jeez...what a monster

[larryM]

and, as said above by several guys - no carb car needs a return fuel line

but carbs do require a open vented tank since the basic physics of "air-pressure pushes fuel into the pump" applies here - pumps don't "suck"

- that is a problem if we are using any oem FI setup 914 & converting it without attending to differences such as the emissions sealed gascap & vapor return system

carb fuel pumps have in internal bypass that obviates the possibility of over pressurizing the lines and a simple inline psi control handles the low 3 psi necessary for webers, etc since a "std" pump is designed to produce about 6 psi min before it "relieves" internally

p.s. - I got one of those Holley pump & psi valve setups on a car I bought running a weber DCOE - "WAY TOO MUCH PSI" - replaced it with a facet and all was well

that the tiny breather located near the fuel cap was pinched closed;

i think you will find that all of those breather tubes are pinched - and that you will also find that they actually allow breathing in that status - it's there for vapor capture on emission controlled cars - should be totally irrelevant to carbs

i have found that it only breaths when the fuel tank is pressurized enuf to overcome the diaphragm in the fuel cap

but it will allow fuel vapor smells into your trunk & thereby in your cockpit if it is not vented overboard somehow - especially if it is full open and or you have defeated the oem sealed gascap

Ha, so thats why the inside of my car smells like a gas station! I will route it out of the car. thanks!

[brant]

just to be complete
a carb system will run without a return line

but a return line on a carb system is superior in many ways
more power, cooler fuel. Many fuel systems (including the 914 original system) included a return line in part for this reason

did the factory add a return line to the 914/6 which was carb'd?

so it may not be needed
it is definitely a good thing though

I do not believe all carb pumps have an internal bi-pass
I do not believe rotary pumps have a bi-pass
I do not believe the holley pumps have a bi-pass



to the original poster
this is not likely your problem
but I use those holly pumps also
they have an internal gross filter screen built in
again probably not your problem, but take the bottom apart and check the screen as well as blade/rotor condition

[larryM]

yes, the 914-6 oem pump is a rotary with 3 pipes - has a return, but no psi regulator is used between it and the carbs, so the bypass valve is in the pump head

I have a new one on the shelf

theoretically - - one could simply "tee" the return outlet to the inlet pipe - so no return line is needed

just to be complete
a carb system will run without a return line

but a return line on a carb system is superior in many ways
more power, cooler fuel. Many fuel systems (including the 914 original system) included a return line in part for this reason

did the factory add a return line to the 914/6 which was carb'd?

so it may not be needed
it is definitely a good thing though

I do not believe all carb pumps have an internal bi-pass
I do not believe rotary pumps have a bi-pass
I do not believe the holley pumps have a bi-pass



to the original poster
this is not likely your problem
but I use those holly pumps also
they have an internal gross filter screen built in
again probably not your problem, but take the bottom apart and check the screen as well as blade/rotor condition

[brant]

but even the factory recognized the benefits of having a return line and thought it was worth building it that way...

a lot of pumps are build/designed for a specific output and don't need a regulator if that out put is what you want


the holley is designed for around 6.5psi output.... its just not the same output a webber wants.

but that desined output is a different thing than having an internal regulator.

[LotusJoe]

I was experiencing the same exact issue. Replaced a bunch of parts. Finally replaced the gauge and the problem that never existed was cured. The gauge I ended up getting was from Pierce Manifold. www.piercemanifold.com .

Good luck

Joe,

I will look into this site to pick out a new gauge. I am very inexperienced in this area... please forgive my, what may seem like obvious answer, questions. Do you think the one I have is faulty? What did a gauge from PM site do to solve your issue? Thanks

The original gauge I had was an expensive oil filled gauge. It would show pressure sometime, then other times it showed fluctuation from no pressure to extremes regardless of how I adjusted the regulator. I was buying some jets and stuff from PM and decided to buy the gauge they recommended. It has a small spread on the pressure (0 to 15). I installed it and found my pressure was sitting solid at 3 lbs. The needle throbs a bit moving between 3 lbs and 2 3/4 lbs depending on engine RPMs. I think what I'm seeing is the result of the float opening and closing. So, for me it turned out to be a bad gauge. However I'm not running a return line so your issue may be different. If I had it to do over I think I would have spec'd everything for the fuel system from Pierce.

I figured since Pierce Manifold is in the weber business the fuel components they recommend should work, and so far it would seem my assumption was correct.

Hope this helps.
Joe

[LotusJoe]

I have looked up the specs on your regulator and it appears you may not have the right one for your application. I copied this from the instructions pdf on the Aeromotive site.

"The enclosed Aeromotive regulator is designed to be used with high flow (300 gph minimum), low pressure fuel pumps similar to Aeromotive P/N 11202. Performance may be degraded if a similar pump is not used."

http://aeromotiveinc.com/wp-content/upload...09/12/13202.pdf

Your holley pump flows 71 gph at 4 lbs.

(IMG:style_emoticons/default/confused24.gif)

[Elliot Cannon]

That whole set-up seems way too complicated. I use a CB Performance rotary pump 3.5 psi, mounted in the engine compartment, lower than the bottom of the tank, NO regulator, NO return line. I have the screen strainer at the bottom of the fuel tank and a filter after the pump going to Weber 44idf carbs. All the an hardware and steel braided lines look cool but not really necessary.
Cheers, Elliot

[rhodyguy]

well put elliot. another instance where less (fuel delivery system) is more. did you leave the metal canister, a filter, that comes attached to the pump in place?

iirc the instructions that came with the CB unit state that you not mount the pump in an inclosed area, trunks, and to place the pump as low as possible.

k

[slivel]

Pics of the fuel system for my Weber carbed six.

Holley Blue high volume pump
Cartridge fuel filter with shut off valve for easy filter element change
7-15 lb adjustable bypass regulator near pump
1-4 lb adjustable regulator on firewall near carbs with pressure gauge
Bypass line to fuel cell

I ran for years without a bypass line but warming the engine for 10 minutes idling, and sitting in pre-grid waiting to go on track resulted in lengthy idling periods which heated up the fuel pump because it was not moving much fuel at idle. The pump got too hot for my comfort and the fuel also heated up so I changed to a bypass setup. Fuel stays cooler, pump stays cooler and fuel pressure seems to be better regulated.

[reese5997]

I have looked up the specs on your regulator and it appears you may not have the right one for your application. I copied this from the instructions pdf on the Aeromotive site.

"The enclosed Aeromotive regulator is designed to be used with high flow (300 gph minimum), low pressure fuel pumps similar to Aeromotive P/N 11202. Performance may be degraded if a similar pump is not used."

http://aeromotiveinc.com/wp-content/upload...09/12/13202.pdf

Your holley pump flows 71 gph at 4 lbs.

(IMG:style_emoticons/default/confused24.gif)

Interesting! I guess I will switch out pumps for sure. Thanks!!!