Looking for some help diagnosing a problem. I have an engine miss at idle and through the throttle range, with low power. Engine is a 1991 Mercruiser 5.7 with Rochester carb and Thunderbolt IV ignition.
Pulled all my plugs and noticed number 5 is black and visibly different than all the others. Did compression check all down the port side with engine semi-warm but not fully up to operating temperature. All 4 port side cylinders were between 165-170 including #5. Plugs, wires, cap and rotor are all close to new. Is there anything that can cause this other than a bad plug or wire?
pic below is the number 5 plug between two normal ones.
Pulled all my plugs and noticed number 5 is black and visibly different than all the others. Did compression check all down the port side with engine semi-warm but not fully up to operating temperature. All 4 port side cylinders were between 165-170 including #5. Plugs, wires, cap and rotor are all close to new. Is there anything that can cause this other than a bad plug or wire?
pic below is the number 5 plug between two normal ones.
