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1978 70 hp Runs on 2

bill_w

bill_w

Advanced Contributor
An older friend of mine has a low hour 70 hp 1978 that will start well, idle well, and run great cold, but after 15 minutes or so starts running on 2 cylinders. I work on OMC's and don't have a lot of experience on Mercs. To me this sounds like either a coil or a switch box breaking down after getting up to temperature. Does anyone else agree, or should I be looking at something else?

Thanks in advance for any help.
 
Check spark to see if its actually dropping a cylinder, if so check trigger wires for insulation breakage and if ok 90% of the time its the switchbox..
 
Thanks. It's always dropping the top cylinder, when he comes in it's soaked in fuel. I'll check the wires, and I did find a good switch box from another buddy.
 
Bad ignition coil! Got the same problem with mine. You can verify this by swapping wires to see if the problem jumps to another cylinder. Swap the green low voltage and high voltage wires between # 1 and # 2 as a test.

Jeff
 
Thanks Jeff, I'll try that first. Are all Merc coils pretty much the same? If it fits, and the wires are the same will they work regardless of part number?
 
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