Possibly the greatest cause of carbon build-up is restriction. Try a diffrent pipe for a while.
With regards to the fuel I've been running CP 30% nitro on my YS's for years. Having tried lower contents with varing results I've come to the conclusion that nothing makes 'em happier than the 30%. Secondly, I land with a FAR greater fuel reserve (an understament really, more like 2 times as much) with the 30% compared to the lower levels.
Yes it's more expensive but in my case, it's been by far, cheaper to run a YS 1.40 on 30% than a OS 1.40 on 15%. Considerably.
What was the question again.