Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

My h75 has been working exceptionally well and it's a joy . However few days ago I noticed a very rough andnintsable start، idling and lack of throttle response. As usual I checked the carb , and intakes , compression and spark plugs to make sure they are all fine and they seemed to.
What I noticed on the spark plugs were this black shiny deposit and I didn't think much of it as it wasn't a soot or wet, until I during my tests I replaced the plugs and engine worked perfectly again.

I'm surprised on how short lived the plugs were, specially as I replaced the PK ones with RCEXl which has been working amazingly and without the black deposit on my R90s .

Has anyone else with H75 noticed low life span of plugs ? I must add I'm using mixture of coleman fuel and Lucas oil treatment which is same as MMO. I believe the carb setting is correct . I do noticed that it's quite hard to get all 4 cylinders to run on low idle . Could this be caused by over heating of spark plugs ?