Fuel pressure consistently fluctuating on carburated 914, Fuel pressure Options

[reese5997]

Hello,

I have a carburated 73' 914 with fuel pressure issues. I have tried about (5) different fuel pumps from a Faucet with and without a regulator closed circuit to Holly with a regulator open circuit; and have tried 2 different regulators. My current set up for the fuel pump is in the picture. I have an aftermarket return line on the regulator that feeds back into the top of the fuel tank.

The issue that I am having is that my fuel pressure will not remain constant at the small window of 2.5-3.0 required by Raby ACT for his engine I bought. I have found that the performance is degraded when operating outside of this window.

One major issue I found while trouble shooting was that the tiny breather located near the fuel cap was pinched closed; it is now open. Other than that, I cannot find any other issues for why my pressure fluctuates from 2.0 to sometimes 5.0 after setting the pressure to 2.7 then driving for a while and/or when I start it up the next day, after resetting to 2.7, it will be higher or lower. I just about have to adjust it every time I get in the car and in the middle of a drive. Is there a solution for this? java script:emoticon(':confused:',%20'smid_24')

I will post a picture of the engine compartment and return line tomorrow morning during the day light to give a visual of my set up.

Thanks,
-Reese (IMG:style_emoticons/default/confused24.gif)

[Mike Bellis]

Have you tried a bypass regulator and return line? What brand regulator are you using? You get what you pay for with regulators...

[reese5997]

Have you tried a bypass regulator and return line? What brand regulator are you using? You get what you pay for with regulators...

Here is a pic of my current set up, it is a $178.00 Aeromotive #13202 A2000 Carbureted Bypass Fuel Pressure Regulator. Part number A27-13202; says it can be set between 2 and 20 PSI.

[reese5997]

Single steel braided return line from regulator to fuel tank.

[damesandhotrods]

Fuel injection is pressure sensitive, carbs are volume sensitive. I haven’t encountered a return with carbs before, which doesn’t mean it’s wrong but I’d suspect that the fuel bowls may not be staying full fuel because too much gas is being diverted.

[rhodyguy]

possible fuel flow restriction prior to the pump. ever cleaned the the strainer located on the tank outlet? imho all the fancy regulators and pumps are overkill, not to mention the high end braided lines and fittings. in my experience a relatively inexpensive internally self regulated rotary fp is all you need. maybe a quality inline gauge to confirm the self reg pump output.

reese, i'm trying to figure out where the pump is installed. is that the rear trunk or front?

k

[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

[ThePaintedMan]

possible fuel flow restriction prior to the pump. ever cleaned the the strainer located on the tank outlet? imho all the fancy regulators and pumps are overkill, not to mention the high end braided lines and fittings. in my experience a relatively inexpensive internally self regulated rotary fp is all you need. maybe a quality inline gauge to confirm the self reg pump output.

reese, i'm trying to figure out where the pump is installed. is that the rear trunk or front?

k

Listen to Kevin on this one. Ask me how I know (IMG:style_emoticons/default/smile.gif)

[7TPorsh]

What's the setup under the tank up front? My issue was a kinking hose just of the tank out let.

[reese5997]

possible fuel flow restriction prior to the pump. ever cleaned the the strainer located on the tank outlet? imho all the fancy regulators and pumps are overkill, not to mention the high end braided lines and fittings. in my experience a relatively inexpensive internally self regulated rotary fp is all you need. maybe a quality inline gauge to confirm the self reg pump output.

reese, i'm trying to figure out where the pump is installed. is that the rear trunk or front?

k

Rhodyguy,

The pump is located in the front trunk. I figured all the fancy stuff was overkill, however I got it to try to fix the consistent fluctuation. I had simple lines, filters and pump originally and I had the problem then too, so I figured all the old stuff was the issue. I got rid of it all and went for new stuff that looked hoping that the issue would be resolved and it would look nice too.
I bought a new strainer but never installed it. I saw the old one and it looked like someone sprayed gray primer on it so i'll pull the tank and install that.
In the next day or so, I will hook up my faucet where I have the holly, and put a gauge right behind it and I will let you know my results. Thanks for your help!

[reese5997]

What's the setup under the tank up front? My issue was a kinking hose just of the tank out let.

I have a steel braided line with enough slack to lift the tank out so about 1 foot; the other nipple for the return line is sealed shut.

[reese5997]

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

[reese5997]

Current set up

[reese5997]

Fuel injection is pressure sensitive, carbs are volume sensitive. I haven’t encountered a return with carbs before, which doesn’t mean it’s wrong but I’d suspect that the fuel bowls may not be staying full fuel because too much gas is being diverted.

I understand that ultimately the fuel bowls not staying full will make the engine run rough, but my issue is happening before the carbs I think. The fuel pressure goes up some times and down others even though I set my regulator so that the gauge reads 2.7 and it never stays there. The pic of my current set up in the post labeled "current set up" will give you some eyes on my engine compartment to show what I might be doing wrong.

[Mike Bellis]

Could also be the pump speeding up/down either due to current flow or heat problems.

I had a fuel pump in the front that came with 2 hole straps. If the straps were in the right place and the pump warmed up, it expanded the pump just enough to bind on the strap. My pump would just stop working for 5 minutes, but you could still have drag.

If is is a current flow or wiring problem, most likely is a bad ground or grounding point. The 914 electrical system does seem to spike quite a bit. This is evident to the guys that have a volt gauge and turn on the blinkers. larger size wire may also help. If you have a high performance pump on the stock wiring, this may also be a bottle neck for current flow.

You could try to connect the vacuum line on the regulator. At idle (max vacuum) it will reduce pressure. At WOT (least vacuum) the pressure will return to static spring pressure.

So you could use the vacuum to set the minimum pressure and at higher rpm, the pressure would increase. This only works if the diaphragm is still good, it should be if there are no leaks.

You could also try to seal the vac port and see if temp inside the chamber is an issue.

[reese5997]

possible fuel flow restriction prior to the pump. ever cleaned the the strainer located on the tank outlet? imho all the fancy regulators and pumps are overkill, not to mention the high end braided lines and fittings. in my experience a relatively inexpensive internally self regulated rotary fp is all you need. maybe a quality inline gauge to confirm the self reg pump output.

reese, i'm trying to figure out where the pump is installed. is that the rear trunk or front?

k

Listen to Kevin on this one. Ask me how I know (IMG:style_emoticons/default/smile.gif)

What he says makes sense. I took the tank out to clean it before installing my Raby 2270cc daily driver and saw the strainer looked like someone sprayed gray primer on it. problem was, I took it out away from my home and had to put the tank back in to get back. I ordered the part, its here. So I will get on that asap. So I should install a internal pump too? I feel obligated to ask; so how do you know? lol

[reese5997]

Could also be the pump speeding up/down either due to current flow or heat problems.

I had a fuel pump in the front that came with 2 hole straps. If the straps were in the right place and the pump warmed up, it expanded the pump just enough to bind on the strap. My pump would just stop working for 5 minutes, but you could still have drag.

If is is a current flow or wiring problem, most likely is a bad ground or grounding point. The 914 electrical system does seem to spike quite a bit. This is evident to the guys that have a volt gauge and turn on the blinkers. larger size wire may also help. If you have a high performance pump on the stock wiring, this may also be a bottle neck for current flow.

You could try to connect the vacuum line on the regulator. At idle (max vacuum) it will reduce pressure. At WOT (least vacuum) the pressure will return to static spring pressure.

So you could use the vacuum to set the minimum pressure and at higher rpm, the pressure would increase. This only works if the diaphragm is still good, it should be if there are no leaks.

You could also try to seal the vac port and see if temp inside the chamber is an issue.

The positive wire I have is pretty thin, and it runs from the pump in the front of the car through the car to one of the fuses that gets power when the key is turned one click so I can prime the carbs by tapping the gas. I forger which one it is connected to, but i will use a thicker gauge.

I never thought of using the vacuum line. I called the shop where I bought the regulator from and they said the port that sticks out is for a turbo connection; the regulator is less that 9 months old. Is this port the same as the vac port? If so, how do I proceed? I'm not sure what to connect it to.

[larryM]

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

[Mike Bellis]

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.

[reese5997]

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!