Yesterday I fitted a brand new k&n cone filter and done every think to the instructions given in box ,looks great sounds great but when driving normally no problems but as soon as I press the accelerator hard its starts to boost but cuts off and back on and off and on again quite harshly ,I know it will need tuning again but is this norm or is somthing wrong or gone ant ideas plz

 

Does it sound like its pinking.Its probably blowing the spark out (det).Stick some new plugs in and make sure as they are gapped for your spec.Its the biggest killer of a Cosworth imo.My black one only did it once.Never keep your foot in if it does it because it will be game over,lean and a holed piston.Just to add I have been in cosworths and this happened with people who should of known better and the results were a meltdown.Any hesitation is a bad sign.