1991 Mercury 75hp 2-stroke power head "knocking" or "banging" noise
Engine starts and runs at all speeds OK (not really smooth acceleration) - would describe noise as tappet noise if the engine had valves. Engine is oil injected - may have a weak fuel pump. The noise seems to occur once per revolution assuming the idle RPM is around 900 from looking at the audio waveform because the noise happens every 65ms or so. I guess this also means that there is a problem with only one cylinder or the crankshaft? Earlier in the season, it did run on no oil for a few minutes - clear oil tubing cracked and drained the tank. Also, (not connected directly with the oil problem) the engine would stall at high RPM unless someone pumped the primer bulb. I bought a fuel pump rebuild kit and some new filters. Replacing the filter seemed to solve the problem so I haven't rebuilt the pump yet. When I took the video just before leaving the lake the primer bulb was not firming up even after a lot of pumps.
I live about 6 hours from the boat, had to leave before I could look at the plugs or do a compression check. I'm hoping that from the description above and the youtube video someone with some experience might be able to give me an idea what is likely the problem so I might be able to order the parts that are likely the issue and have them when I get back up there.
Thanks in advance,
Greg
1991 Mercury 75hp 2-stroke "knocking" or "banging" noise at idle - YouTube
Engine starts and runs at all speeds OK (not really smooth acceleration) - would describe noise as tappet noise if the engine had valves. Engine is oil injected - may have a weak fuel pump. The noise seems to occur once per revolution assuming the idle RPM is around 900 from looking at the audio waveform because the noise happens every 65ms or so. I guess this also means that there is a problem with only one cylinder or the crankshaft? Earlier in the season, it did run on no oil for a few minutes - clear oil tubing cracked and drained the tank. Also, (not connected directly with the oil problem) the engine would stall at high RPM unless someone pumped the primer bulb. I bought a fuel pump rebuild kit and some new filters. Replacing the filter seemed to solve the problem so I haven't rebuilt the pump yet. When I took the video just before leaving the lake the primer bulb was not firming up even after a lot of pumps.
I live about 6 hours from the boat, had to leave before I could look at the plugs or do a compression check. I'm hoping that from the description above and the youtube video someone with some experience might be able to give me an idea what is likely the problem so I might be able to order the parts that are likely the issue and have them when I get back up there.
Thanks in advance,
Greg
1991 Mercury 75hp 2-stroke "knocking" or "banging" noise at idle - YouTube