This is the smaller jet that screws in next to the main jet stack. It regulates fuel/air mixture at the bottom 1/3 of the throttle. A combination of the idle jet and the main jet handle the middle 1/3. If your idle jet is too big or too small, it can create a "flat spot" in the acceleration. Idle jets have two numbers. The first number is the size of the bore that allows fuel in, and the second is the size that allows air in. So a 45/120 jet would run richer than a 45/140, because the 120 allows less air to pass. To make things more complicated, some vintage Vespas came with "plugged" idle jets, which have no air hole. Item D9892-07
Idle jet size: 52/140
Suitable for:
VESPA 125 VNA* 57 - 59
VESPA 125 VNB* 59 - 66
Vespa Super 125 VNC* 65 - 69
Vespa GT / GTR 125 VNL* 66 - 78
Vespa P125X VNX* 78 - 81
Vespa 150 VBA* 58 - 60
Vespa 150 VBB* 60 - 67
Vespa Super 150 VBC* 65 - 79
Vespa GL 150 VLA* 62 - 65
Vespa Sprint 150 VLB* 65 - 79
Vespa PX150E VLX* 81 - 06
LML Star / Stella LML*
Vespa GS 160 VSB* 62 - 64
Vespa SS 180 VSC* 64 - 68
Vespa Rally 180 VSD* 68 - 73
Vespa Rally 200 VSE* 72 - 79
Vespa P200E VSX* 77 - 82
* for SI carbs
Idle jet size: 52/140
Suitable for:
VESPA 125 VNA* 57 - 59
VESPA 125 VNB* 59 - 66
Vespa Super 125 VNC* 65 - 69
Vespa GT / GTR 125 VNL* 66 - 78
Vespa P125X VNX* 78 - 81
Vespa 150 VBA* 58 - 60
Vespa 150 VBB* 60 - 67
Vespa Super 150 VBC* 65 - 79
Vespa GL 150 VLA* 62 - 65
Vespa Sprint 150 VLB* 65 - 79
Vespa PX150E VLX* 81 - 06
LML Star / Stella LML*
Vespa GS 160 VSB* 62 - 64
Vespa SS 180 VSC* 64 - 68
Vespa Rally 180 VSD* 68 - 73
Vespa Rally 200 VSE* 72 - 79
Vespa P200E VSX* 77 - 82
* for SI carbs