Do as the other fellow suggested & try resetting the lash on your adjustable rockers 1st, if your having a bit of job doing that & it hasn't been done: you may need your rocker screws that contact the valvestems defacing, I've learnt they take a bit of a durability beating, if not done, you'll find it hard to get the valve gaps to accept an accurate gap, although using a 'wire' type gauge meant for settingvplug gaps can be an extra help.