Gurtner main jet size 90. E2 Norm and other performance restricting measures have jets that are too small. Recommended when fitting a sports exhaust or 70cc cylinder. An increase of 10-15% is recommended for systems or 70cc cylinders. Check plug to see how the mixture is running.light brown - optimal, black - too rich (jet too large), grey to white-ish = too lean (jet too small) and danger of seizure.