Just crossing off causes of my running issues and discovered that my breather pipe (with alloy cap) doesn't have any vent holes!! It appears it is a one way valve as you can suck but not blow. Shouldn't this breather pipe be completely open (like a standard piece of tubing that comes as standard) I reckon this is the cause of high speed engine spluttering and cutting out
You can either run the open tube breather - which can also let fuel out - or a valve on the end of the tube. This valve will let air in to replace the void left by fuel leaving the tank, but will not let fuel run out, should you do a stoppie or drop the bike etc. A short hose with a one way valve will do you just fine. If you have high speed spluttering I think you need to be looking elsewhere. A vent block will cause problems, regardless of speed.
