nipplebandit45

Ok, I keep hearing abou people putting on Holly fuel pumps, Carter fuel pumps and the like. Most peopl also put on a fuel regulator so they can set the fuel to a certain pressure. What will happen if you are running to high a fuel pressure? When my sock pump went south about 9 months ago I swapped in a cheap aftermarket pump that runs at about 6 or psi. I think. could th3e extra pressure cause me to run rich? or will the extra pressure bleed off through the return line?

Mike

Felix Wankel

It can damage your carb too.

hanman

It will cause your carb to flood. The pressure will keep the floats from sealing. Regulators are cheap, buy one and mount it on the firewall near the hoodlatch.
just my opinion,
hanman

riffraff

i think that you can safely run 4-5psi through the stock carb before you have to worry about it damaging anything.

Sterling

Your float bowls will seal just fine unless you've got a 40 psi wonder-pump, or your needles are rotten!
What will happen is that your emulsion tubes will become useless, and all of your mixing will occur in the venturis, and not beforehand.
As you enter into the higher RPMs, the fuel level should be dropping in your float bowls so that is uncovers some of the holes in the emulsion tubes, allowing air to be drawn in thru the air bleeds, and mixed with the incoming fuel. Too much pressure, and this will not happen. The float won't drop to where it should, and you'll run very rich; and very "not-so-powerful".

If your needles are truely crappy and worn, then perhaps I can see fuel oozing out the carb. But it only makes sense to me that with more fuel pressure pushing the float upwards even harder than before, it also would push that needle valve into the seat harder...thus sealing off the flow easier. Of course, that's all if your completly maxing out the ability of your return flow orifice.

In the fuel return set-up is an orifice for fuel back-pressure to relieve itself into the return line. In effect, it's a simple pressure regulator. Fuel is constantly being circulated thru the top portion of the carb at whatever rate the pump or the regulator allows. But because your pressure gauge tells you you're getting 4.5 psi into the carb does'nt mean that the float bowl valves are seeing 4.5 psi.
There's that little hole allowing the pressure to release.
Dig?

When you adjust the pressure on your regulator, it's really to set your float drop at maximum RPM. Not only that, but because you have a return line releasing pressure, a 1 psi increase in pressure from the in-line is'nt really a 1 psi increase in pressure at the needle valves.

Some people "dead-head" their return lines, or put the pressure gauge there. What this does is give an exact reading of the pressure. They are relying on the regulator to stop the pump pressure. Though I have'nt tried it, I'm inclined to think that it's a better system because there would be no inconsistancies resulting from the "double regulator" set up at any given RPM. The pressure would be "BAM"...it's there; no softness about it. The only thing regulating the pressure after the regulator, would be the float drop (engine needs).

So you should get a regulator. A pressure gauge can be the most crappiest piece of crap out there on the market for $5...It only serves as a reference for you to increment your changes untill you find the best float drop at high RPM.
This is not to say that there's nothing to be gained by having a truely accurate gauge. An accurate gauge will be expensive, but you'll have no doubt that the 3.5 psi it read before will still be 3.5 psi when you switch it back. (According to Paul Yaw, many "top - of - line" gauges are pretty inconsistant, and not calibrated very well.)
Drop the gauge, and you've just recalibrated it, BTW.

I have a Carter 7 psi pump and a Holley 4 psi regulator feeding my Yaw modded Nikki. I run 2.75 psi with very big jets.
The gauge is a moderatley expensive no-name that Yaw tests and sells. They come straight from a measuring tool supply company.

brownmound

So how does all this discussion apply to EFI? I am considering replacing the fuel pump as a matter of course when i get the Racing Beat exhaust system installed. Do I need a regulator also?

Sterling

No clue! You have two pumps, no?

V8kilr

how do you run at 2.75 psi?i though the stock nikki requires 4.5 psi.and yours is modded,so you would think regulating to 4.75 or 5 psi.lmk

Sterling

The stock carb for an 85 is supposed to run on 3.5 psi (I think). 4.5 for an 83; same carb. Go figure. I think Mazda wised up by 84 when they changed the pressure. What they "wised-up" to was that people were starting to realize the potential of their little rotary-rice-rockets, and running much higher rpms than the previous years' more cautious purchasers.
The jets and emulsion tubes in both model carbs is the same. Though I've never explored the possibility that the return line orifice (about 0.1 inch diameter) might be different sizes...but I doubt it.

Volume versus Pressure...
If you have a fuel pump that's maxing out at 5 psi, and you poke a hole in your carb (IE open the throttle) then that pressure will suddely decrease. The pump can't put out more volume to ensure that there's a steady 5 psi going to the carb.

If your carb won't let more than 5 psi build up in it (due to a regulator or a fuel return line) but you have a 7 psi pump feeding it, that pump is trying to stuff 7 psi into the carb no matter what. Poke a hole in the carb ( open the throttle) and less fuel pressure will be registered at the return line...You'll have a steady pressure of 5 psi at the carb though, until your needs overcome the volume the pump can push.

Just because a stock carb is set up for 3.5 psi does'nt mean it's really utilizing 3.5 psi at the float needles. The return line is sieving off pressure always. But the float drop will occur much later than if you only put in 2.5 psi. And that stock float drop rate may not be the optimum for performance for the carb.

A stock pump that is crapping out, however, will not only be lowering the float drop level to a better rate. It'll also be unable to push the volume needed to continue the rate. The engine becomes fuel starved.

Put it this way...
If you went and did the math, and calculated pump psi along with hose diameter average, and took that as the true rate of which your fuel was leaving your tank in volume of fuel consumed, you'd be getting about ten gallons to the mile!

Float drop is a tricky thing in that the difference between trying to shove 7 psi and 8 psi into your carb is nil. But get down to the "theraputic level", and it's a bit like fine tuning your idle...The difference between 3.25 and 3.5 psi becomes very important.
You must also understand that the good folks at Mazda needed this car to get more than 17 MPG for it to be marketable. That fact combined with two others, is why the carb was'nt set to it's maximum potential. They are; the fact that noone that bought a new rotary was revving the thing to 8000 RPM, at least in this country. Only rotary enthusiasts of the day knew it was OK to do so. Anyone elses experience, including the majority of previous owners of rotary sedans knew only that the best engines of their day would'nt last for long beyond 6.5 K RPM before throwing a rod thru the pan!
The other is the emissions control system. It works on a knife edge! EVERYTHING has to be just right for it to work correctly. Screw around with jetting and pressure, and it's quite possible you would have to recalibrate some stuff. (That's just a guess.)

Float drop is very important for performance in the higher rpm range because at say 3500 rpm, your stock engine might be taking in a little over 100 CFM (air). You need the fuel to go with that. The floats are still high.
However, at 7000 rpm, you're using much more than just twice the CFM. More like two and a half times the amount at 3500 rpms. That's fine because your carb is designed to deliver it. You now have four barrels open, but you can't expect that kind of draft to suck out only the right amount of fuel. Just because there's twice as much air flowing thru the carb, does'nt mean there's twice as much fuel. In fact, the more air flow (rate), the higher the fuel flow (rate) will climb...until you've maxed out your jetting.
It'll suck way too much out; so you need emulsion tubes and air bleeds and such. They mix in some air with the flow of fuel from the jets by allowing a "bleed-off" in the fuel stream. The bleed off does'nt need to happen at low rpms, but at those rpms, the flow is'nt as furious, and the fuel level is actually higher around the emulsion tubes.
At the bottom of the tubes are holes, and at low rpms, those holes are submerged in fuel, and no mixing of air is happening. Everything in the carb is calibrated for driving at low RPMs.
If it were'nt for the air bleeds, you would get way too much fuel from the four fuel jets at high rpms. If the emulsion tubes are'nt working correctly, the same thing will happen. The way they "work"?...
Float drop level.

The drop level governs the amount of fuel that needs to be sucked out thru the jets before level of fuel in the carbs circuits uncovers some of the holes at the bottoms of the emulsion tubes.

The fuel pressure governs the float drop.
Too much pressure, and the level does'nt drop to where it should to expose the emulsion tubes. The engine runs too rich to make power at high RPMs.
Too little pressure and the emulsion tubes are'nt even a concern. There's just simply not enough fuel.

Dyno Dave only runs 1.75 psi ! (I think...I know for sure that it's really low)
You could run as low as that, or higher, and still change when the floats begine to drop simply by bending the tabs on the loats, and causing the fuel level in the bowls to be further down. This would start to mix air in with the fuel at an even lower RPM. But you would have to know exactly what you were doing.

I believe that you could make a few air bleed changes, float tab adjustments, ect, and run your engine on one psi! You would need a regulator and a fuel pump that could garantee the pressure at hi volume consumption.
The problem with this is that your fuel delivery would be at the mercy of gravity, sideway G-force, and bumps and dips in the road.

peejay

Keep in mind that when you change pressure, you need to re-adjust float height. You have to adjust the floats "up" to get the level in the fuel bowls right, in effect making the floats "weaker".

This has benefits! When you go around corners the fuel moves around in the bowls. When you go around righthanders, the fuel all moves under the floats' pivots... this reduces the fuel's leverage on the float. The bowls overfill with fuel and you get a big fat PUHHH when you try to accelerate out. With lower fuel pressure the bowls don't overfill as much. (This is ENTIRELY my take on the "righthand bog" issue with Nikkis and I explain it with the understanding that I may be flat-out wrong)

That said, I used to run "over 5psi" through my '80 Nikki. I needed to, for it to get enough fuel to the engine. The 7000+ overrichness issue with a stock carb was unwittingly "fixed" with my half-assed intake manifold... the primary runners never moved enough air to get the primary barrels into the "excessive fueling" part of the fuel curve!

I learned a lot of lessons with that setup. And I can't wait to go faster with less work! (As in, decent intake manifold, and fix the mixture with jets/bleeds/booster tuning instead of just playing with pressure!)

riffraff

stock nikis require 2-3 psi

FJ

The regulator is mounted on the fuel rail. There is no need to change it or the fuel pump when you change the exaust.

-John.

inittab

I just installed a Mallory Comp70 fuel pump which is good for 70gph at 6psi. Even though the pump is adjustable I'm still gonna install a Holley regulator as soon as it comes in.

I'm a bit confused about the return line and the double regulator effect that comes with a Holley installed on the inlet side. So should I dead-end the return line or what? TIA

RotorMotorDriver

iTrader: (1)

What about with a Weber that doesnt have a return line? Obviously I only have one place to put the regulator, before the carb. But, does anyone know a good fuel pressure or float level to try first of all? Or does anyone know a good site where I can read about float levels and fuel pressures for Webers?

~T.J.

REVHED

This doesn't make sense to me at all. Fuel pressure as such is not pushing up on the floats but simply the level of fuel which isn't going to exert nearly as much. In my experience the carb will flood with as little as 7 or 8psi even with a brand new needle and seat.

REVHED

The float level is best left to the recommended spec as is the fuel pressure which is 4.5psi.

Sterling

Originally posted by Sterling
If your needles are truely crappy and worn, then perhaps I can see fuel oozing out the carb. But it only makes sense to me that with more fuel pressure pushing the float upwards even harder than before, it also would push that needle valve into the seat harder...thus sealing off the flow easier. Of course, that's all if your completly maxing out the ability of your return flow orifice.

--------------------------------------------------------------------------------
Nope- Dunno what I was thinking there. You're right.

I also got this backwards:

"Just because a stock carb is set up for 3.5 psi does'nt mean it's really utilizing 3.5 psi at the float needles. The return line is sieving off pressure always. But the float drop will occur much later than if you only put in 2.5 psi. And that stock float drop rate may not be the optimum for performance for the carb."

Lower pressure will make the emulsion tubes breathe sooner, not later.

However, I do disagree with this statement by peejay:

"Keep in mind that when you change pressure, you need to re-adjust float height. You have to adjust the floats "up" to get the level in the fuel bowls right, in effect making the floats "weaker"."

You should adjust the floats so that they measure about 9/16ths inch from the top of the closed float to the gasket.

Changing the float drop will dramatically change how the carb behaves. Raising the level will cause you to run rich at high RPMs, and lowering can starvation.

It is'nt something that should be fooled around with *****-nilly.