[Rich Biebel]

There is really no way to prevent this issue. Onan recommends decarbonizing the engine as a regular maintenace service. Myabe they thought using 4C cleaner would help and I am sure it would but running it is what cuases the carbon buildup. They run at one speed 1800 Plus or Minus about 50-100rpm no matter what the load is...or at least they should if the governor and engine are working well.

The only way to prevent the carbon buildup, is to not run it so that makes no sense....

Maybe some fuel additives might help.....but mostly they are snake oils to me for the most part.

I can tell you without a doubt that since these engines are air cooled. The hotter they get the more they coke up the oil and prodcue more residue buildup.
The higher the load you have on the unit.....the hotter they will get....So reduce your loads in hotter weather and make sure it gets a good supply of fresh air away from it's own heated exhaust air.

I just took a part some newer style over head valve single cylinder units. They are 4000watt units that run at 3600rpm. 4KY is the model and they are very popular .....One had a lot of isssues...burned exhaust valve.....lots of carbon....bad oil rings......2700 hours and was running poor for a long time and run hard....The oil ring was allowing a ton of oil in the chamber and basically caused all the problems......I think it was run so hot early in it's life it killed the oil ring expanders tension. So here was another unit killed by heat.....It was mounted under the back end of a van and I have worked on many of those. They are always in bad shape......Air flow!!!! Mobile Dog Groomer van.....They get used every day and those generators get a ton of hours.