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.

People tell me they don’t like ngk and run the other xyz brands. Good for them. I’ll stick with the ngks. Interesting story.

My first motor home was a 25 Ft Ute Liner on a International Harvester 1 1/2 ton truck chassis with their medium duty 392 C.I. V8 that had been converted to Propane. It was horrible on power and fuel consumption, I had messed with the tune and got it to run a tiny bit better but not near anywhere acceptable When I research the different fuels available back then in the late 1980 and early 1990s Diesel had the most BTU at 230,000 per gallon, regular pump gas had right at 120,000 BTU, Methanol alcohol had 85,000 BTU and Ethanol had 84,000 BTU, Propane had around 55,000 BTU, Butane had 54,000 and natural gas had 53,000 BTU. This was before the emissions rules kick in here on the fuels in the U.S., the new eco friendly fuels, gas and diesel fuels both, with alcohol in them suck big time on power and economy on MPG
I switched back to gasoline and loved it after that.
I've been told that 95 % of the vehicles in Europe are diesel fuel powered and get way better mileage than the vehicles they export to us here in the U.S. Their big diesel powered V.W. SUV get 55+ MPG in Europe We need to be able to import them and make American vehicle makers catch up on making our vehicles clean and efficient as there's are

US is missing a major opportunity not switching fleets over to CNG instead of all the electric vehicle garbage.

I am working on a 440 for my RV that will run dual fuel- gas or CNG.

If you don't mind could you post the NGK part number you used?

XR5 3332

Or they should make propane and natural gas really cheap per volume and then we can turbocharge the snot out of things with it, they both have pretty healthy octane numbers.