1990 2.0L 140HP OMC SeaDrive Erratic Timing Light

[richhills]

Hey Guys..when trying to set my timing I can't get a steady light to check spark advance, it fluctuates 2 degrees back and forth. What's causing this to happen? Checked the light, it's fine, checked connections, their fine. Idles fine, high speed fine but not right in between on acceleration. It has an erratic acceleration then clears up and runs fine. Plugs good too. As always, appreciate your advice you have given me over the years., Oh, by the way, I had a high speed overheat question awhile back which I forgot to let you know the outcome. Turned out to be my 2yr old water pump. Appears my son had been running the engine down over the summer when coming in and going out during camping trips to the lake. Mud/clay wear on the pump. Replaced pump, gave son instructions and been fine since.