Thats sad. 30-40mph? my fathers jeep which is waay out of tune will hit 70 on the highway, and its a 2.5! Anyway, as for your problem, it seems that mis-typed. dieseling is caused by to far back, not advanced. my mistake Take your timing up to stock specs, and bring it up another 2-4 degrees. then have someone step on the gas when its running and run your hand under the exhaust. smell for glycol and unburned gas. As for your head question, it is possible that you mis-adjusted your valves. If memory serves me right, you are supposed to leave a gap of .002-.004 inches between the rocker arm and valve. But dont quote me on that. all my vehicles have hydraulic lifters. As for over-tightening, that would cause them to open too soon, not too late.