Did it do this before you changed plugs, dist cap/rotor, carb???????


If it has only done it since you changed plugs, cap/rotor--------check out the order of the plug wires--not only at the cap, but in the proper order and location-It is not unusual for a novice to mix up some or all of the wires-can be just a couple switched, all put on cap starting wrong place, wrong rotation. Chev common to have # 1 & 2 mixed-also at plugs easy to cross wires as each pair on a small block are equal