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. Available sizes are: 38/120, 42/160, 45/120, 45/140, 48/160, 50/120, 55/160. 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. These are available in sizes 42 and 50.
Jetting is something that takes trial and error. See our wiki page of for more info.
- Applies to the following models:
- Genuine
- Stella 2T 150
- Vespa - Vintage
- VBA 150
- VBB 150
- VBC 150 Super
- VLA GL 150
- VLB 150 Sprint
- VLX P/PX X150
- VNB 125
- VNC 125 Super
- VNL 125 GT/GTR/TS
- VNX P/PX 125 / T5
- VSB GS160
- VSC SS180
- VSD Rally 180
- VSE Rally 200
- VSX P/PX 200