View single post by Esprit2
 Posted: 04-20-2020 05:27 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 536
I've had two 907s develop a problem with "ignition scatter". Put a strobe timing light on it, and the ignition timing could be seen jumping all over the place. Careful examination showed that neither the distributor nor the electronic ignition pick-up were the cause... so what was?

I decided to dig in deeper, and remove the auxiliary housing for a more thorough 'bench' examination. But when I pulled the auxiliary pulley off, I noticed that the keyway in the bore had been 'hogged' out to about 3/4" (19mm) wide. The Woodruf key is shorter than the bore is long, so both ends looked normal. But look inside the bore, and the damage was obvious.

Even when an engine appears to be running smoothly, the power comes in pulses. In a 4-cylinder, two pulses per revolution. The pulley hadn't been mounted with Loctite. And even though the bolt had been torqued to spec, the pulley eventually over-rode the wimpy little wire circlip, releasing all the clamp load. It was loose on the shaft, and the pulsing power delivery had worked the Woodruf key back and forth in the bore enough to 'machine' a wide grove. The pulley & shaft wobbling back and forth relative to one another... and to the crank... is what was causing the ignition scatter.

I've killed two pulleys that way, mounting both without Loctite. Sometimes I can have a rather flat learning curve, but this lesson eventually sunk in. Don't forget the Loctite.

Tim Engel

Last edited on 04-20-2020 05:28 pm by Esprit2