exhaust elbow setup is prone to collecting carbon and is a very common cause of engine problems. It does not have to be totally clogged to affect starting/running. The problem does generally comes on very slowly and should not normally affect starting overnight (not saying it can't happen.) I know it is a chore to remove the elbow and clean ...or god forbid replace it, still about half the elbows out there could use a good cleaning regardless. Also, and I hate to even mention this, but you are making sure that these starting efforts are not filling your water-lift muffler and forcing a bit of water back up into the cylinders?