I'd assume mtr mts would be both on accel and lift. I'd also assume, no driveability complaints about engine stumble on lift gone lean, then again, it would be hard starting lean think, so engine bolts/running is out of the way. Only problem occurs upon lift.
I now point to one variable is a drive belt? On accel the wear is on the drive side. On lift, drive belt meets and greets the change in direction, loose are the rubber teeth on the drive side, the coast side is the vibrator flopping back and forth all elongated are some belt to sprocket wear, that or press up and down on the belt tension go check.