04-12-2020, 03:24 PM
(This post was last modified: 04-12-2020, 03:27 PM by Ian McGowan.)
I seems to me that the likeliest explanation is that No. 4 cylinder is not burning all the fuel - possibly due to it receiving an intermittent spark?? I would begin by checking appearance of plug (compared to the others) check distributor cap for tracking from no. 4 lead, check condition of lead 4, and check for spark constant presence at no. 4 plug.