Dellorto carburettor M6 main jet Size 90 for all Dellorto carburettors from 17.5 to 24 mm. When tuning the engine, always install an oversized main jet. At full throttle the engine should start to stutter (4-strokes). Now you should jet down in increments of 5 until the engine runs smoothly. IMPORTANT: If the main jet is too small, the engine will run too lean, which can cause the engine to seize. The main jets have an M6 thread Please select the relevant sizes from the list below