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.

1 Like