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We had several of the 9.9s of this vintage at our resort and we had plug fouling problems .We switched to if I remember correctly to Auto;ite AE901x plugs and it helped a lot but not 100 percent.In about 1976 my good friend Don who was the manager of the propeller and engineering division at OMC came up to Canada for his annual fishing trip he told me there was a problem with the head design and they would be coming out shortly with electronic ignition.The reason they went with the energy transfer ignition was to make more room under the flywheel for battery charging .We sold off the problem 9.9s and bought new ones when the change was made they were great engines till OMC cheapened them with the cable throttle control.in the tiller handle and in a few other areas the ones from the early eighties wetre the best.