Pop off question

Pete Muller · Posted: Tue Aug 11, 2009 12:05 am Post subject:

It's pretty simple -- Steve explained it on the first page, but I'll give it a shot in different terms.

How much fuel is coming into the fuel chamber (the area under the diaphragm) is determined by how far the inlet needle is lifted off its seat. How much the inlet needle is lifted off its seat is determined by the position of the diaphragm.

The position of the diaphragm is determined by 3 forces --
1) The spring pressure on the fulcrum arm which pushes up on the diaphragm (yes: it could be argued that fuel pressure counteracts part of this force, but in this explanation we can just assume that's a small constant).
2) The pressure drop under the diaphragm -- which is a function of the pressure drop in the venturi acting through orifices of an adjustable cross-sectional area (the jets).
3) Atmospheric pressure, which pushes down on top of the diaphragm.

So... if you assume that atmospheric pressure is constant (on a given day during a given test), and the pressure drop through the venturi is constant, the only remaining variables are the spring pressure (popoff pressure if you like), and the drop in pressure under the diaphragm (by changing jet opening). (it's probably more accurate to say a drop in force on the diaphragm since there is only fluid under it?)

In a nutshell, if the spring pressure is increased (upward force on the diaphragm), that needs to be "balanced" by slightly increasing the pressure drop under the diaphragm. The only method to accomplish that is to open the jets slightly.

Raise spring pressure, open jets a bit. Lower spring pressure, close the jets a bit. (this assumes that the goal is the same amount of fuel passing into the bore of the carb).

PM

al nunley · Joined: 13 Nov 2006 Posts: 3038

Steve O'Hara · Posted: Tue Aug 11, 2009 10:51 am Post subject:

Taking Pete's very well stated explanation into consideration one might ask... "if a change in spring tension (popoff pressure) can simply be offset by an adjustment in the size of the orifice (open or close jet) why does is matter? The answer is that it doesn't make any difference if the engine speed is constant. If all you ever did was run static speed tests on a dyno you would not find any benefit to changing the settings but dyno tests and racing are dramatically different environments. The dynamic conditions of on track racing are the challenge for any carburetor and those changes in speed and load influence two of the variables in Pete's explanation. One of the variables is the fuel pump which tests show produces different pressure at different speeds. The other is the strength of the signal at the orifices in the venturi. As the engine speed increases the strength of the signal at the orifices gets stronger due to greater velocity through the venturi and the output pressure of the pump first rises and then falls at the very high end of the rpm scale. The two variable elements are what make the popoff pressure setting a factor in the way the engine performs through the dynamic conditions experienced on the track. Good tuners have learned to manipulate the popoff pressure, jet settings and to a lesser degree the maximum lift of the inlet needle (arm height) to generate different fuel delivery curves to suit the racing conditons they are dealing with. If an engine tuner never gets in the drivers seat and only tests using a primative dyno he will never see or understand the effects of adjusting popoff pressure.
Simple.
Steve O'Hara

Pete Muller · Posted: Tue Aug 11, 2009 11:15 am Post subject:

Yep... I agree completely.

As a matter of fact, on a dyno at a steady speed and load, you wouldn't even need a carburetor. You could probably have a gravity feed fuel source, and a petcock to adjust the fuel flow dripping into the airstream.

One issue with the Walbro that makes it more difficult is that the "signal" through the high speed circuit comes from a hole at the wall of the venturi... the worst possible location to sense a pressure drop due to airflow.

JIM SILVERHEELS · Posted: Tue Aug 11, 2009 12:32 pm Post subject: The Irish Gang

Ya know Al, every once and awhile I'd like to git a hired gun to beat O'hara and Muller just once on these carbs..... but who could we possibly get? they're too darn smart... tee hee
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al nunley · Joined: 13 Nov 2006 Posts: 3038

Pete Muller · Posted: Tue Aug 11, 2009 3:26 pm Post subject:

Jim,

I actually learned a significant portion of the carb theory from Steve. He was doing things for fine-tuning McCulloch carbs in the mid-70's that haven't even been done to this day.

JIM SILVERHEELS · Posted: Tue Aug 11, 2009 4:03 pm Post subject: PETE

Pete, you and Steverino obviously got yer poop together and I thank ya guys so very much for helping me and all of us. In all honestly, without being an apple polisher my hat is off to you guys and my buddy Al.
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Don't get stuck in someone else's discarded chewing gum with your thought process.