I got a chance to work on 440 running on natural gas that is on an emergency backup generator. This machine hasn't been able to pass a load test for a while. It had a full tune up, plus points changed out for mopar electronic kit, but was still failing. This is essentially dyno testing the motor at a fixed speed (3600 RPM).
Ran the machine as is, runs perfectly fine at idle, but slowly bogs down and powers out as load as added, producing around 27 kw.
I inspected the first set of spark plugs(brand A), showed same color as new spark plugs(Brand B), and motor compressioned out at 110 psi hot, engine is stamped as 1977.
After a few checks and fiddling doing nothing big, getting to 30 ish KW , I convinced the owner to let me change the plugs to a different Brand, brand C. Machine now produces and holds 47 kw. We make a change to
the fuel pressure, we get 54 kw, we got our hands on a new mixer and the machine makes about 60 kw watts, which is the derated for altitude spec for this machine which is now about 42 years old.
Brand C is NGK.
I learned quite a bit about vapour fuels from that, even when we found it was overly lean, it never popped back through the carb like a gasoline motor would, and it never stumbled either, it just reduced the overall power output. Interesting that they only carb'ed a 440 with a 388 CFM carb as well. The engine got hotter making low power with bad mixture and plugs than it did once it was running at full power, which was interesting, I am assuming a lot of unburnt fuel combusting in the water cooled exhaust manifolds.