[[]font="Times New Roman"][/font]I have a YS F120 that runs great when it runs. I'm using 30% helicopter fuel. The problem is that some times the engine seems to be lean and does not respond to the needle valve or throttle effectively. The first time this happened I sent the engine to YS for evaluation. They replaced some bearings and replaced some of the fuel pressure components and said the engine tested out satisfactorily. The engine ran great for several months. Now I have the same symptoms again. Any suggestions as to how to correct the problem