Timing mark mystery, Please help me solve Options

[bbrock]

Cleaned up the impeller on the 2.0 liter today and need to repaint the timing mark. To my surprise, I found two marks to choose from (IMG:style_emoticons/default/new_shocked.gif) The mark on the right (nearest TDC) was cut with a file and is exactly 52.5mm from the center of the "0" for TDC. According to Haynes, that is the correct distance for 27 degrees BTDC. The farther mark looks to be original and is about 57.25mm from TDC. If Haynes and my math are correct, that would be around 29ish degrees BTDC.



Now I have a vague memory of cutting a mark on an impeller of one of the five Type 4 engines I've worked on, but too much alcohol has swashed through my brain in the 35+ years since that event could have happen to remember why. I can think of two reason: the original had a broken fin and I stole this one from a bus engine, or I just put a mark in where Haynes said it should be, assuming the other was wrong.

What really bothers me is this image from another thread has a pic with 27 BTDC marked exactly where my original mark is not at 52.5mm where Haynes says it should be, but TDC in that pic is not where mine is using the impeller blades as a guide. So can anyone help me figure out which frickin' mark is the right one? Is Haynes wrong? If it is the one I cut, any idea why the other one is there? Did buses have a different mark? So confused. (IMG:style_emoticons/default/confused24.gif)

(IMG:http://www.914world.com/bbs2/uploads/post-8384-1196294703.jpg)

[bbrock]

Thanks for that link! I think we may be getting somewhere. First, that thread confirms that my TDC "0" mark is right where it should be. I think the clue to the second mark is in the last paragraph of the second to last post where they say to just set total advance (as @MarkV suggested might be the case) at 32ish degrees.

Thirty-five years ago I rebuilt the bottom end of this engine with a reground carb cam to convert to dueal carbs with a 050 dizzy. I'm pretty sure I'm the one who filed the second mark in and it probably was to add a mark for setting timing with the full mechanical advance dizzy. I'm guessing I read some instructions to set a few degrees more advance than stock and thought I'd be clever and file a mark to make that easy. I'm not sure why I put it at 29 rather than 32 but at least it fits with the idea of compensating for lack of vacuum advance. The project stalled for several decades and now I'm finishing the job and scratching my head trying to figure out what the heck I did. (IMG:style_emoticons/default/huh.gif) To add to the confusion, I did such a nice job filing in that mark that couldn't tell which is factory. I'm also leaning toward not using the 050 because I'd rather have vacuum advance. So my cleverness may have caused confusion with no good reason. That sounds about normal for me. (IMG:style_emoticons/default/screwy.gif)