1997 Mercury 200 EFI
Serial number OG570955
This motor has run nearly perfectly for many, many years. I recently came across a broken down boat and towed them 3 miles back to the dock. I could only tow at 7 mph at 80% throttle the entire way. After dropping them off was the first sign of trouble, the engine was difficult to start and smoked more than usual once we got going. The smoke cleared up quick but the motor missed a few times for just a second at a time running about 40 mph. After that trip I changed the spark plugs and fuel filter and the next two trips had no trouble at all. I figured the plugs just loaded up from the tow. On the third trip the engine began to miss again while on plane and this has been randomly happening ever since over the past couple trips. I have about 8 hours on the engine since the tow. It can miss at low rpm too and has felt like it was bogging down for just a second or two only at low rpm. 90% of the time it runs perfectly well but then it starts to miss again. The miss is only for a brief second and full power comes right back so it feels electrical but it may do it 4 or 5 times in a row then not do it for the rest of the day. In retrospect, I should have towed them in at lower speed or maybe it was just a coincidence that the problems started then. So where is a good place to start diagnosing this issue given how it only happens intermittently? I generally do all my own maintenance and have the shop manual. Thanks!