Why keep getting wide different "duty cycle" readouts for fuel mixture???

[wawago]

Hi, I have some rough idling and found out fuel mixture needed to be readjusted. After it has been readjusted by my mechanic the rough idling is better but still remained. I try to adjust it myself so I bought a Sear Multimeter 82334 with duty cycle. This is the first time I have used a multimeter.

I set the function dial to frequency/% and select % in display.

Then I put the positive lead (red) into #3 in X11 and the negative lead (black) to the driver side's plate (ground) that hold the spring into place.


With the engine running, parked and temp at 82 I saw the # on the multimeter display fluctuated and then stopped (frozen) at a number and doesn't change anymore. I tried several times and keep getter a very wide range of frozen numbers 99, 100, 51, 27, 34 without any adjustment to the fuel mixture. So I don't know what is going on let alone trying to adjust it close to 50%.

Please tell me what have I done wrong??? Thanks very much.

Rick