Thanks for taking the time too look at this

whats happened is i have a 3 door cossie but got it all built back together had the valves and spring done i had it going and left it to tick over on the spot for about 30 mins was sounding great

so i gave it a wee rev to about 1500-2000 rpm after that the car cut out and it sound like its running on 3 cilenders

i took the rocker cover gasket of because there was like a rattling noise(pretty sure its a tappet) from there i am not really mechanicaly minded anyway there was loads off oil so it wasnt short on oil

and i checked the tappets all them turned no probs in a circle motion with out ease except one on the inlet side(4th one) i tried quickly turning the engine quickly with out the rocket cover and all valves look likes they are all moving but as i said the tappets were all turning except one(in circle motion)could the stuck tappet be doing anything?its going up and down but not turning circle like the other onesis there a way i could fix this and whats the best way to do so i am pretty sure this is whats wrong with it? i checked the spark plugs and the 3rd on was diffrent from the rest had black shit stuck to it i changed the plug but no diffrence i have also check the spark seems finecould it be i have blowed a hose off?i doubt it i check all but you's know better than me
thank you very much for taken the time to read the and would really appriate any advice or tips
cheeRS
Craig