Im fitting some weber idf 40's but i keep seeing pictures of them with trumpets on, do I need these or will they run ok with just the filters on, if I do what size do I need and what size jets are best for 1641
You don't need trumpets (although they're cool) - you do need air filters, or you'll knack the engine. Can have trumpets inside the filters, of course...
ram pipes/velocity stacks smooth the air flow into the carb mouth the diameter should be the same as the carb mouth and height leaving around 25mm clearance to the top of the filter top