[[]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

It may be full of rust again, and would have likely been the reason they replaced the bearings before. It could also be a bad check valve, bad fuel, bad plug, valves out of adjustment, lots of things to look at when they go bad. Did it happen all at once or gradually?

The engine ran great the weekend before. the problem symptoms then returned just like before that I described.

It has to be dirt in the regulator.

Thanks for your response. Can you advise what {location} I should clean?

Pull the reg off, noting that it should be re-installed with the little casting bump forward (it should be installed that way now). Disassemble it completely, including the regulator plunger. Wash it out in glow fuel and re-assemble. Reset the top of the regulator screw flush with the casting that it screws into.

Good Luck!

I'll try that over the next few days. Thanks.

Good Luck!