More than one DL50 was having sputtering problems at our field and after finding the timing advanced 11 degs too far on one we set it back to 28, retuned, and it's ran great since. We later found a couple of others off by roughly the same amount and they were running quite bad as well. Setting the ignition timing and then retuning them fixed the problem with them as well.

In each case with the DL's they sounded horrible in the low and mid range and ran pretty good at WOT. T.O.M., with you having so much dyno time running these engines maybe you can help explain why the 11 degs of extra advance would cause them to run sooooo rough in the low and mid range and then they clear right up after setting the timing and retuning the mixture.