I've read several threads on carburator fuelpressure especcially when Dell Ortho's are fitted. Recommended pressure is between 2 and 2,5 PSI.
My question is this; Is this pressure adjusted with the engine running at idle speed or is this recommended pressure the pressure the engine needs when pulling top speed? My quess is that the engine needs sufficient fuel for top speed running. So in that situation the pressure should be 2 to 2,5 PSI. Am I correct?
pressure regulator
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Huib
Re: pressure regulator
The purpose of a pressure regulator is two fold. One: to get the exactly right pressure. Two: to keep the pressure constant at all revs and all loads.
A fuel pump which is in order and is allowed to function properly (enough stroke, no restrictions in the fuel lines / filters) also delivers fuel at a nearly constant pressure but not necessarily at the correct pressure.
The fuel is sucked into the pump by the stroke and pushed out of the pump by the spring under de membrane. The pressure of the spring therefore determines the pressure at which the fuel comes out of the pump.
The stroke of the pump and the surface area of the membrane determine the maximum amount of fuel that the pump can deliver in one revolution of the engine.
One could also adjust the pressure by changing the spring inside the pump. Note that if the pump has to deliver more fuel, the spring expands further. Since the force of the spring is inversely proportional to its expansion, there is a second order effect that reduces the pressure somewhat at high delivery. In practice it can be neglected as at high demand the engine also makes more revolutions. The effect can only be observed when operating the acceleration pumps repeatedly and rapidly by hand at idle.
On some fuel pumps I observed that the pressure collapses at high demand but this is always caused by malfunctioning in or output valves in the pump or clogged filters or insufficient stroke.
It is much easier to adjust the pressure by turning a screw on top of the pressure regulator than changing the spring inside the pump by trial and error.
The way I do it is to set the pump to deliver fuel at a rather high pressure (not ridiculously high of course). Say between 0,4 and 0,5 bar and at maximum stroke.
Then adjust the pressure to get the right float level in the Dellorto's. At sea level in our climate in Holland this is usually 2,5 psi. At high temperatures and high altitude this is less.
A fuel pump which is in order and is allowed to function properly (enough stroke, no restrictions in the fuel lines / filters) also delivers fuel at a nearly constant pressure but not necessarily at the correct pressure.
The fuel is sucked into the pump by the stroke and pushed out of the pump by the spring under de membrane. The pressure of the spring therefore determines the pressure at which the fuel comes out of the pump.
The stroke of the pump and the surface area of the membrane determine the maximum amount of fuel that the pump can deliver in one revolution of the engine.
One could also adjust the pressure by changing the spring inside the pump. Note that if the pump has to deliver more fuel, the spring expands further. Since the force of the spring is inversely proportional to its expansion, there is a second order effect that reduces the pressure somewhat at high delivery. In practice it can be neglected as at high demand the engine also makes more revolutions. The effect can only be observed when operating the acceleration pumps repeatedly and rapidly by hand at idle.
On some fuel pumps I observed that the pressure collapses at high demand but this is always caused by malfunctioning in or output valves in the pump or clogged filters or insufficient stroke.
It is much easier to adjust the pressure by turning a screw on top of the pressure regulator than changing the spring inside the pump by trial and error.
The way I do it is to set the pump to deliver fuel at a rather high pressure (not ridiculously high of course). Say between 0,4 and 0,5 bar and at maximum stroke.
Then adjust the pressure to get the right float level in the Dellorto's. At sea level in our climate in Holland this is usually 2,5 psi. At high temperatures and high altitude this is less.
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Geoff Goldberg
Re: pressure regulator
I second Huib's recommendation of a fuel pressureregulator. I ahve been somewhat unwilling to put one on the older Lancias (Aurelias) for reasons of looks - its hard to find a small one that looks neat in the engine bay (anyone got any suggestions?).
At one time, a friend did some tests on an Aurelia engine supplied with only a mechanical fuel pump. By changing the gasket thickness from paper thin to thick wafer, he was able to adjust the fuel pressure from 0 psi to 7 psi!
That started us wondering if this might be a bigger issue. perhaps some of the older cars were washing out their rings due to over-pressurized fuel feeds. The Webers for Aurelias seem to want between 2.5 and 4 psi, so some care here is needed. Of course, the same would apply (within reason) to other Lancias.
At one time, a friend did some tests on an Aurelia engine supplied with only a mechanical fuel pump. By changing the gasket thickness from paper thin to thick wafer, he was able to adjust the fuel pressure from 0 psi to 7 psi!
That started us wondering if this might be a bigger issue. perhaps some of the older cars were washing out their rings due to over-pressurized fuel feeds. The Webers for Aurelias seem to want between 2.5 and 4 psi, so some care here is needed. Of course, the same would apply (within reason) to other Lancias.
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Peter de Wit
Re: pressure regulator
William,
I am sure you should measure fuel pressure while driving. I have a big high precission fuel pressure guage precisely for this. Complete with T-piece to put in the fuel line. Use it in the cockpit and observe. Bought from a competition team. I will lend it to you if you want.
I am sure you should measure fuel pressure while driving. I have a big high precission fuel pressure guage precisely for this. Complete with T-piece to put in the fuel line. Use it in the cockpit and observe. Bought from a competition team. I will lend it to you if you want.
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william
Re: pressure regulator
having used the gauge so kindly supplied by Peter I can report that the difference between top speed and idle pressure is negligible. Starting without regulator I had 6PSI. With the regulator screw max out I was not able to adjust below 3PSI. Assuming the gauge is not 100% accurate the pressure is anyway half of what it was. Running seems better as it starts and idles without using choke or the accelerator. Which it didn't do before. There seems less strain on the banjo gaskets with the lower pressure so my new Dell Ortho's will stay fuelstain free!