fuel pressure question

[chachi]

should your fuel pressure be the same with the engine running or with it not?

[Amskeptic]

should your fuel pressure be the same with the engine running or with it not?

36 +/- 2 with engine off

28+/- 2 with engine idling

Pulling off the vacuum hose to the fuel pressure regulator at idle will show you how the regulator is working.
Colin

[chachi]

...

[RSorak 71Westy]

No not as such, the pressure the fuel pump puts out is effected by the length of the pushrod. More gaskets under the pump or a shorter rod lowers the pressure.

[dtrumbo]

Fuel pressure for carbs is something between 3 and 5 psi. depending on what carb you're using. I'm pretty sure the Bentley manual spec's it out. The numbers Colin posted are for fuel injection engines.

[Gypsie]

OK call me confused....

Who is actually on first?

The original question suggests electric fuel pump, and a mechanical fuel pump would obviously not be creating pressure when the engine is off...So?....

Though I wonder....Could a carbed engine have an electric fuel pump with pressure regulated down to 3-5psi?
Or I'm just... confused.

[vdubyah73]

i use an airtex electric pump on both my vw's, with webers. spec'ed at 2.5-4.5 psi. just look for early 80's ford courier 2.0 liter fuel pump. around $50. they last for years the one in my buggy, with dual 44, idfs is 13 years old. it was my daily driver for 5 years before becoming a buggy. use the same model pump in my bus, w/ dual ict's for 6 years. have one of those facet cube pumps, that came with an engine from bernie bergmann. new in box, stashed in bus w/other road spares.

[SlowLane]

To the original question:
If you have a carburetor and mechanical fuel pump, then I suppose you could have fuel pressure for a short time after you shut down the engine. The one-way valve in the pump's inlet and the needle valve in your carb would create a seal at both ends of the system's pressurized section. But I expect that pressure would bleed off fairly quickly.

On the topic of adjusting pressure by shimming with gaskets or shortening the pushrod: it seems to be a common misconception that this works because it shortens the stroke. The stroke is independent of rod length. What does change is the operating region of the pump's diaphragm spring. By shortening the pushrod or shimming, the spring is allowed to lengthen to a more relaxed state, so it presses less strongly against the fuel in the pressurized section. It is this spring pressure which directly effects the fuel pressure.

Electric fuel pumps for carbs are generally not adjustable. If an electric pump delivers too much pressure, an external regulator between the pump and carb can lower the pressure delivered to the carb by "pushing back" against the pump. Note that this is a subtractive mechanism, so that you can't just "dial in" 2.5 psi on the regulator and expect to see that at the carb. Instead, you subtract the regulator setting from the pump pressure (or just tee a pressure gauge between the regulator outlet and carb and adjust the regulator until you get your desired pressure). Electric pumps should also be checked to ensure that they deliver adequate pressure and volume at the expected maximum load, since they don't vary their flow rate with engine speed as does a mechanical pump.

Chachi: If you're not getting the answer you're looking for, maybe you could give us a few more details about the particular system you're puzzling about?

[airkooledchris]

ive been following a FI bus owners attempts at lowering head temps and they eventually found that raising their fuel pressure dropped their head temps.
by swapping in a Bosch adjustable FPR they were instantly able to reduce head temps by 40*

"the fuel pressure regulator is off of a saab, i cant remember the specfic model but the bosch part number is:0 280 160 001. there is no vacuume port so your idle mixture will be a little on the rich side but it is a vary minor inconvenince compaired to the positive effects you will gain. i hope this helps. "


"Adding fuel pressure increases fuel atomization and is great as long as you stay 42 PSI or below." - plausible?

[Westy78]

I have an adjustable regulator from a 914. Looks the same as stock except for the adjusting screw. I'm running at 40psi right now. I need to check it again to see if it's stayed stable.

[airkooledchris]

I have an adjustable regulator from a 914. Looks the same as stock except for the adjusting screw. I'm running at 40psi right now. I need to check it again to see if it's stayed stable.

thanks for the reply. no vacuum port on it right? not a big deal if so?

that's 40 PSI without the engine running?

looks like this?

[Westy78]

Yep that's the one. 40psi with engine running tested through the port on the 3/4 rail.

[SlowLane]

I have an adjustable regulator from a 914.

ie. a D-Jet regulator.

Looks the same as stock except for the adjusting screw. I'm running at 40psi right now. I need to check it again to see if it's stayed stable

I can't imagine why it would make any difference if it stayed stable or not. At 40 psi, the fixed-pressure regulator will deliver too much pressure for L-Jet under all conditions. The vacuum adjustable regulator used with L-Jet provides a constant pressure across the inlet and outlet of the injector. This is necessary because L-Jet doesn't sense intake manifold pressure and therefore is unable to compensate for it.

D-Jet, on the other hand, relies on measuring manifold pressure, so is able to adjust for it. D-Jet needs to count on the fuel pressure not varying relative to atmospheric.

Put another way: since you're running at a constant +40 psi relative to atmosphere, your injectors are seeing roughly 48 psi across their inlets and outlets at idle.

How do your plugs look running with this regulator?

[chachi]

...

[Amskeptic]

sorry, mine was a vague question and it's easy to see why there could be confusion since i'm obviously a little confused. i'm trying to verify whether i'm getting fuel to my carb. this is a progressive with a mechanical fuel pump and the part where i'm confused about is...well...what does it mean to have pressure? if you saw my thred about the progressive you'll know the bus isn't starting and i'm skeptical that it's getting gas. i thought i could just move down the line and check everything in order. so to my mind, what i thought i'd do would be to install a cheapo pressure gauge and see what i was getting, but i guess what i'm hearing is that if the engine isn't running, you will not have fuel pressure.

caveat: i know you guys hate the progressive and i'm in the midst of rebuilding my solexs but it was running passably before so i'd like to at least figure out what's wrong and drive it while i get the solexs nailed down.

Just take the fuel hose off the carb, stick it in a jar, and ask someone to crank the engine for a few seconds. You should get pulses of fuel into the jar. Ergo! You have fuel to the carb! Pressure is just to make sure it arrives. Now check the carb itself by looking down the throat(s) with a flashlight and open the throttle rapidly. You should see/smell fuel being discharged. If yes, go check for a good spark. If no, your inlet needle valve may be sticky or the carb may be clogged all up.
Colin
(be *organic* and flexible with diagnosis, do not get too wrapped around numbers and specs)