1989 70 hp mercury definite miss at all speeds

Stetson

Cadet
Joined
May 24, 2021
Messages
17
1989 70 hp mercury definite miss at all speeds. Had all 3 carbs rebuilt in 2017, the mechanic was supposed to rebuild the fuel pump at that time but didn't. I had to later when it failed. Maybe it damaged the carbs? I replaced one failed coil and the engine ran ok but had a slight miss and not always firing on all 3. Replaced the other 2 coils and all 3 plugs and wires and got worst especially at idle. Seams to have good spark not sure how steady. Could it be a bad switch box? Compression is 110 psi on all 3. I disconnected the auto oiler when I bought it 20 years ago and adding my own at 50:1 or a little less. There is a long wait to get it in to be fixed so any help would be greatly appreciated. Thank you!
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
37,840
Hook up a timing light to #1.----Take boat out into the lake.-----Run to full throttle and observe the light.-----Repeat with #2 and #3.-----Should tell you which cylinder has an ignition issue.
 

Stetson

Cadet
Joined
May 24, 2021
Messages
17
Hook up a timing light to #1.----Take boat out into the lake.-----Run to full throttle and observe the light.-----Repeat with #2 and #3.-----Should tell you which cylinder has an ignition issue.
WILL DO. Thanks
 
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