Just to clarify....i'm sure the manual states a max of a 40:1 fuel mix yes?..( I could be wrong!!).....anyways, replace the dci & plug.
On both my crrc pro 26's i started as manual states.....25:1 for a nice rich mix for run-in...one gallon...on cheap synthetic comma oil,...steady air flying..& occasional high speed etc..
Then second gallon i ran fully synthetic putoline oil & a mix of 40:1..as manual....using a 4.8v 2700mah sanyo batt pack...& jumped to a 17"6 prop as opposed to a 16"8 for run in.
Dci's & plugs are a consumable item...they wear out/degrade & break down over prolonged usage.