If your engine is racing and not idling then u are too lean. But u said u checked the needles, so maybe there's an airleak, there's too much air in the chamber and that's making it too lean, the engine sounds like it's all or nothing. I had the same problem with an OS motor, and I gave up and just bought a new one. Listen if it's not a simple airleak, replacing engine parts can be half the price of a new engine, or the same price as buying a used one that is in good condition. So consider getting another motor if repairs and adjustments aren't working.
Cars: Tamiya TRF 417x, HPI Ken Block WR8, EREVO Brushless
Heli's: Blade 300x, Blade 130x, Blade mcpx BL