I haven't seen this mentioned on the board as a possible reason for poor running older motors. I recently redid a 1961 18HP that I got cheap. In the process I removed the exhaust cover and the whole exhaust was completely full of carbon. Not just coated, but actually packed full of black carbon that looked like the briquets you burn in a charcoal grill. I had to gouge it out of there with a stout pick. The points/plugs/condensers/coils all needed replacing, which I did, but no amount of new parts could have overcome the fact that the thing just couldn't breathe.
