Tillotson carb question

Most all of the diaphragm type carbs recommend a “pop-off” pressure. In our case with the Tillotson HW48a for the mini ROK is 10 psi pop off with a maximum bleed of 16.
What is the effect if the pop off pressure is high like 12 or low of 9 ?
Does 12 psi, say, make the overall mixture leaner or richer? Or is the effect something different, like for higher altitudes.

I know there are different springs available to adjust the pressure just wanted to know what the changes will be and when to use them.

There’s an article linked in here that you can read. Basically popoff is “kinda” like float level. Higher popoff will make the carb run leaner, lower will richen…

I could be wrong on this, but pop-off is a subjective test number to test how much spring tension the fuel needle has against it. In other words, your fuel isn’t pressurized but this is used to check spring tension and also how well the needle is sealing. Considering how many cycles the diaphragm makes pushing against the fulcrum and spring, I have a hard time believing the difference between 9psi and 12 psi would make a difference.

I disagree. Like James said, the pop off acts like the float in a bowl style carb. If its too low you run rich and if its too high you run lean. More so on the lean side, because if you open your needles more than the pop off can supply, you are stil on the lean end of the spectrum. You can adjust out a soft pop off with the needles, but still have a drop in performance.

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anyone know why my tillotson carb is popping at 8 and holding at 6? seems a little low?

Did it change from a previous setting?

Pop off is adjusted with the spring tension and/or fulcrum arm height.

It does seem low but it depends on application and track to an extent. Lower pop off will generally run richer across the curve.

that was my first time ever testing it, it had ran pretty rich at the track on stock carb settings at 60minutes a piece, and also i recently had flooded the engine (i have no clue how it flooded i had just brought it back to the pits, it cranked up on the stand and when i went to go back on track it wouldn’t start) so from it running rich and the engine flooding i figured something is up, i just am not smart enough on these to know what it is.

Try to change the spring at the needle fulcrum arm .
I adjust or reset to normal the pop off pressure from there when i have low readings .

The springs also come at different grams so they give different pop off pressures when you apply them