If memory serves me properly , as a test I could get timing advanced up to 28.5 degrees BTDC .....This had the IP pushed all the way to the stop toward the engine...I just did this as a check to see what the entire adjustment range would be....I think the lower timing number was about 17-18 degrees BTDC....I then set it to the correct begining of delivery with drip tube which is 24 degrees BTDC...I was surprised how well the engines would crank and run across the entire 10-11 degree range (stop to stop)....It seemed to be about 25-26 degrees for best overall running.....I did this on my 1980 240D and my 1978 300D and the numbers tracked very closely on both vehicles....If 15 degrees BTDC is as close as you can get fully advanced...It does sound like you are off one tooth.....Like the earlier post says....Best to check crankshaft,valve train timing as a start before pump timing....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)