I'm not 100% sure of the process of setting the timing on a Blazer but I believe it's necessary to put the computer in bypass mode to set the timing correctly. I replaced the distributor on my wife's old '92 Seville STS and it was necessary to put it in base timing mode before adjusting the distributor position. Otherwise the computer kept trying to compensate for where the timing was set.

 

There is a brown wire inside the passenger compartment where the firewall meets the transmission tunnel towards the passenger side. Pull back the carpet and you should find a connector on this wire. This wire must be disconnected to set base timing.

 

So I bought a timing light and the parts store magically produced the proper repair manual from the back. the timing was quite off, like an inch and a half on the clockwise side from the gauge. Here is the thing though.... I don't know where on the gauge that the timing mark is supposed to line up with. Looking at the gauge there are 3 "peaks" (pointing towards the front of the vehicle, like teeth) and 2 "valleys" and on the clockwise end of the gauge is a short little tube, I assume is for a magnetic timing light? The gauge has enough surface rust that I can't make out any lines or numbers that should be stamped on it. I've got it close, roughly on the center one of the 3 peaks and it runs good, but I think it could be better. Where on this gauge should the timing mark line up at? My Haynes manual (which sucks but that's what the parts store had), doesn't cover this style gauge.

 

You're correct about the tube on the timing mark plate. It's for a magnetic timing probe.

If you can find a factory manual or get access to the Mitchell on-line manual you should be able to get the correct timing data for those marks. I like to go to my local library and look up the Mitchell manual. That way I can get precise information.

 

I'd just line the mark up with each one, take for a drive, and note the performance. Then leave it on the one that performs to your liking, and doesn't ping real bad.

The '00 Vortec engine I put in my Blazer has one plastic notch on the top with no reference lines in it, as that engine wasn't designed for a traditional distributor setup. I just played around with it until it ran good. I do that with most vehicles anyway because usually I run a bit more advance than the book says for a little more grunt down low.