KT100 Fulcrum Arm spring length

[Pete Muller]

Al,

It's not difficult to visualize why different pop-off pressure settings "change" the way the carb acts.

I'll give 2 scenarios that show what changes.

The first one is simple: a steady-state RPM and load on the engine (like a simple water-brake dyno that "stalls" the engine at one rpm). Let's imagine the carb being set up at 7psi popoff pressure, and at 20psi popoff pressure. Not much will change in this example *except* for the jet setting(s). The reason is because there is much more "tension" on the diaphragm (caused by the spring pressure under the fulcrum arm) when the popoff is at 20psi vs. 7psi. A greater pressure drop is necessary under the diaphragm in order to lift the inlet needle to the same location when running higher popoff pressure (simple physics: 2 opposing forces need to balance at a precise location of the inlet needle). In this steady-state example, that's *all* that changes -- the engine is at one rpm at a constant load, so a given amount of fuel is required. 2 different jet settings for the 2 different pop-off pressure settings, and the engine will probably run exactly the same.

Now to the interesting example: Transition. Varying load, rpm, and throttle position. In the configuration of high popoff pressure and jets "more open", the carb is very quick to shut off the inlet needle when one lifts off the throttle. The difference between the pressure on the diaphragm (from the inlet spring) and the lack of demand (when the throttle is closed) is significant. Also, when one first hits the throttle, the diaphragm has to deflect (I'd almost call it "deform") enough to once again left the inlet needle off the seat to allow fuel into the space below the diaphragm (where it can be drawn into the venturi). If the popoff is low (back to our 7 psi example), the inlet needle is not quite so quick to close (less differential between spring pressure and pressure under the diaphragm), but is also a bit quicker to respond to demand from the venturi since much less pressure drop is required to get the inlet needle off the seat.

A perfect example of this is Steve O'Hara's explanation of how the Walbro functions in the pits when set up with high popoff pressure and jets set correctly for the track -- the engine will actually idle cleanly. Why? -- because at an idle, there is not nearly as much pressure drop under the diaphragm, so the diaphragm is "fighting back" a bit (for lack of a better term). The inlet needle is not lifted off its seat hardly at all, and there is more "tension" on the diaphragm… this causes the engine to run much leaner at an idle (so it actually *will* idle!).

The differences in high and low popoff pressure appear at an idle, and during transitions (because the response of the diaphragm is "different" -- not better or worse, just different). If someone takes the time to run dramatically different popoff pressure and pay attention to the "feel", it becomes apparent that there are differences.

If anyone remembers the McCulloch carburetors, they had a duckbill circuit (actually a compensation for the very thing outlined above). The compensation allowed "tuning" of how responsive the diaphragm is to the transitions, and also to the overall fuel delivery (roughly equating to more compensation at higher crankcase pressures). Steve was the master of those carbs, and he can explain in more detail if anyone is interested.

Bottom line: popoff pressure makes a difference. No one setting is better or worse, but make a change from 7psi to 20psi, and it becomes very clear that it changes how the carb acts.

PM

[Walt Gifford]

I can see where the inlet needle could meter the fuel at idle if set just right but I can't believe it would meter any fuel at higher rpm unless the pop off was extremely high. Sure in theory there would be a larger or smaller plenum of fuel for different pop off settings but the flow would be more than adequate to feed the metering needles.

Gif
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FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience

[al nunley]

I’m no carb expert, I’ll say that up front, but I have seen a few different carb setup’s, and I have a few ideas on that subject.
I was checking the popoff one day and I got this idea of trying to touch the fulcrum arm, where the diaphragm hooks up to it, without losing pressure. No matter how hard I tried, I couldn’t touch it light enough and still hold pressure. Now that was with 10psi popoff only. I wish I had tried 20psi. I have a feeling I wouldn’t have seen anything different.
I have been told, and have seen, popoff’s from 3psi to over 20psi from people that win races all the time. Seems to me the popoff wasn’t the reason they ran so well.
I know that I’m a minority voice on this subject, but I had some measure of success and didn’t worry about popoff. I set it, but I didn’t stress over it. I have this idea that, no matter how much time you spend on finding the very best popoff, it’s not going to do much, if anything, in your quest to go faster.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

[Pete Muller]

Al,

It's simply another tuning item -- absolutely insignificant compared to the condition of the tires (or the driver, for that matter). Is pop-off pressure (or anything else for that matter), the key to faster laptimes? No way. But stating that: "A setting of 10 psi is enough and more does nothing" is incorrect.

As far as not "feeling" the difference between 10psi pop-off and say... 20psi, that's understandable. The inlet hole under the needle on a Walbro has just under .003 square inches of area. That means that at 10psi, the physical force required to lift the needle off the seat is 3/100ths of a pound. The actual force at the point where the diaphragm attaches is probably half that due to the ratio of the fulcrum arm. In real terms (pushing with a finger), the difference would be in a few hundredths of a pound.

The carb, however, knows the difference.

PM