Timing advanced 25 degrees??

[rrs]

Okay, I just checked the timing for the first time on my recently acquired 1979 spider (2.0 liter carb). The car ran fine, but I replaced the timing belt and rebuilt the distributor so I wanted to make sure everything was set right. According to the timing light I was set at about 25 degrees of advance, which is obviously way beyond the recommended 10 degrees advance. So I dialed it back to 10 degrees and figured I was good to go. However, at 10 degrees of advance the car lacked power and backfired under heavy acceleration. I rechecked everything else - the plugs are new, the carb is a new 32/36 DFEV, no vacuum leaks. So I put the distributor back to 25 degrees and it runs great again. Any ideas?? I say 25 degrees because when I put the timing light on the crank pulley, the mark is about an inch south of the third timing mark (this is a 79, so the first mark furthest toward the driver side is TDC, the middle mark is 5 degrees and the third mark is 10 degrees advance. The distance from the first mark to the third is about 3/4", so if 3/4" is 10 degrees I figure 1 3/4" is 25 degrees. The car runs great when set at the 25 degrees but my MPG is horrible, about 16.5 MPG. I checked the plugs prior to adjusting the timing and they where black. After driving about 50 miles at the 10 degree adjustment the plugs where light grey. Any ideas?? Thanks in advance.

[azruss]

the first thing that comes to mind is you are using the wrong mark on the crank pulley. lt is a simple notch in the edge and doesnt align with any casting marks.

[rrs]

I'm not at the car at the moment, but I think I'm using a line across the edge of the pulley, not a notch. I will check tonight for a notch.

[So Cal Mark]

does your advance work?