# Does Pop-off pressure change per the altitude.?

Been struggling a bit to wrap my head around this, I know you would generally adjust H/L needles and such if you have a major change in altitude… but…

I had a carb services by EC and they set the pop-off pressure there for my location. When I received it back I tested the pop-off and it was different then what the had suggested it was set to. I did test against 3 different pop-off guages. (I also did a DIY-test validation of the gauges to a prisma tyre gauge just to see the reeding difference which there was slight delta but would not account for the gap).

So I guess my question is when you set a pop-off in a low altitude situation to a specific number, if you then test with the same pop-off gauge in a location that is 2000ft+ higher, would it still read the same or different?

secondary, if they recommend a specific setting for you altitude and set it at their location. should I be readjusting to those numbers at mine?

example.
EC says 12pop/8rest for my location
I test in my location (2000ft+) and it shows as 14pop/9.5rest

Should I be adjusting to to those 12/8 numbers to just sum it up to differences in gauges

At 2000ft I would expect the pop off pressure to measure about 1 psi higher than at sea level. Assuming your pop off pressure gauge is measuring gauge pressure, which most do. Gauge pressure is a measure of the pressure relative the atmospheric pressure. At 2000ft atmospheric pressure is about 1 psi lower than at sea level, so this will this will show as a 1 psi increase on the gauge. Put another way there is no change to the absolute pop off pressure, just the reference to which you are measuring.

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Sounds like you are going the way my mind was thinking as well, maybe it’s more relative setting. So in the theory of it, the pop-off is set correctly for all elevation, but just registers differently if I understand you correctly.

Ie. you wouldn’t adjust the current pop off to what they set It to even though it registers differently

bump, can anyone confirm this?

This is what ChatGPT said.

Does the pop off pressure change in a diaphram carburator with altitude?

Yes, the pop off pressure in a diaphragm carburetor can change with altitude due to the change in atmospheric pressure. The pop off pressure is the pressure at which the carburetor’s needle valve opens to allow fuel to flow into the carburetor.

At higher altitudes, the atmospheric pressure is lower, which means there is less force pushing down on the diaphragm in the carburetor. As a result, the pop off pressure will also decrease. This can result in a leaner fuel mixture, which may cause engine performance issues such as rough idling, poor acceleration, and reduced power output.

To compensate for this, some carburetors have an altitude compensation system that adjusts the pop off pressure as the altitude changes. Alternatively, pilots may manually adjust the carburetor’s mixture setting to ensure the correct fuel-to-air ratio at different altitudes.

Let’s be wary of posting ChatGPT answers without a disclaimer.

Pop-off would change with elevation I would guess as noted by Alan above and we do sometimes adjust pop-off for big differences in elevation, as well as make carb adjustments to compensate for the air density.

Hey @tjkoyen
Agree on that a change would generally be made, just trying to wrap my mind around the relative science/math.
my example being, if my carbs optimum setting is 12pop/8rest and that was set at their elevation. When I receive it at my elevation its tests different(multiple gauges and shop tested) which I would expect based on my reading and validates with yours and Alan’s posts. Should I be aiming for those target numbers they mentioned which would mean I would swap out the spring to get ther. or is it set correctly and thats how it is…
assuming im not the first one to come across this scenario… having an engine builder in a different elevation…

based on carb tuning app’s that have location awareness… the numbers they are suggestion align. but it would be hard for them to compensate for the elevation im assuming on their end.