Re: 1975 70hp Evinrude trouble... pls help
Actually, your video is helpful - you just have to listen closely. What I think that I hear is a cylinder that is not firing. Mixed in with normal exhaust sounds is a hollowing "popping" sound that is indicative of lack of ignition in one cylinder.
Put an inductive timing light on the motor, one spark plug wire at a time. If you have lack of ignition in one cylinder, that wil tell you which one it is.
When you say that you replaced the "switchbox," I am assuming that you mean the powerpack. Having done so, however, does not mean that you have eliminated all possible sources of an ignition failure. You could have a bad sensor in the timer base or a bad coil. You could also have a bad spark wire or the clip assembly inside of the boot could be loose.
Checking a coil can be as simple as swapping it with a coil that is known to be good. As for the timer base, I recommend buying a factory manual for your motor. In the chapter for the ignition system, there should be a procedure for doing an ohms test on the sensor coils.
On my 1972 65hp Johnson, the values are 8.5 ohms plus or minus 1 ohm, when checking from the individual sensor leads to the common lead. It should also read "infinity" when checking to ground. That said, your timer base part # is different than mine. That may or may not be significant, because some of the part numbers on shop.evinrude.com are superseded part numbers, not necessarily different parts. I note that the CDI (aftermarket manufacturer) replacement part for both of our motors is their part number 133-1251 and the shop.evinrude OMC/BRP part number for your timer base is 581251. As such, the above test values are probably right for your motor.
Check the security of the spark plug wire in the coil too. The wires are inserted into the coil by screwing them onto a threaded post in the recessed cup of the coil. I've had them come loose, but not completely out of the coil, with a misfire as a result. If this is the case, clip about a 1/4" off the end of the wire and re-install.
Let us know how you make out.
