Re: Evinrude 1976 135 683D back fired and I just put in new plugs
The timing certainly can get off if the flywheel key shears. Repeated shearing of the key (with the proper 100-105 ft lbs of torque) would be a good reason to lap the flywheel/crank tapered surfaces and reinstall the flywheel. Also, if the power pack was ever changed out, it's possible to get the leads crossed going to the 4 coils. (assuming that the coil wires go to the correct plugs) So you've got two things to check. The later crossflows used color coded wired to differentiate the upper coil leads from the lower coil leads on each head. On your model, you don't have that identification option. You just have to make sure the spark plug coil lead goes to the correct scew terminal on the power pack itself. If you get two coil/power pack leads switched, the engine may run, but with lots of violent backfiring. For further clarification, after just starting a cold engine, the fuel mixture is lean. You will get some minor "sneezing" of the engine (a mild form of normal non-violent backfiring) till the engine warms up-usually 2-3 minutes. If this is your issue, periodically activate the choke. Usually for 3-4 seconds ever 10-15 seconds for the first minute of cold operation. Also, the two lower plug wires run close to the two lower cowling latches at the back of the engine. Make sure the plug wire is not jumping spark to ground when running.
