ptallman

My Feedback: (28)

I have an older 120 twin with a two-needle carb. I'm having some trouble adjusting it. It seems the low speed needle has quite an effect on high speed, I get it adjusted and it seems to lean out after a flight or two. The high speed needle seems to be very sensitive as well. Any suggestions? This is in a Hanger 9 Cub, the tank seems to bein a good location and I am using pressure from one of the exhaust headers.
Thanks,

Phil

Bax

My Feedback: (11)

The main thing is fuel tank location. The centerline of your fuel tank should be level with the center of the carb, or a hair lover. Usually, the tank winds up too high with many twin-cylinder installations. This causes leaning in flight because you have to lean the high-speed needle to account for the higher fuel head pressure, and as the tank drains, the fuel head pressure lessens.

A very-sensitive high-speed needle usually means that the fuel pressure is too high. The FT-120 was almost always run without any kind of pressure feed to the fuel tank. Pressurizing the fuel tank would increase fuel pressure, and make the needle more sensitive.

The best way to get the carb adjusted is to start out with a very-rich idle mixture, get the top-end set, and then set the idle.

Also, try running without the muffler pressure. You'll likely find tha the high-speed needle will have a broader adjustment range.