Oh for a portable four-gas analyzer.
The best I could do, if I'd just get out the torch and do it, would be to weld a couple of 3/8-pipe bungs on the down pipes and wire up the Isspro pyrometer left over from my 855 Cummins days. Then I could keep track of the exhaust temperature and that would tell me both if I was working the engine too hard and if the Mixture was too lean.
