kwbrown

I have an OS Surpass .91 four stroke in a Great Planes Big Stik .60 with a 12oz Sullivan tank. This engine runs terrible with the pressure line attached to the muffler. With the line attached it will idle beautifully but at top end it surges and runs very erratically. Once the line is disconnected, it runs a lot better. Does this indicate a problem with my engine? Will it harm my engine to run it with the line disconnected? Thanks for any help.

kobago

Hi. I think as follows.
It runs well because a main needle is loose after a pressure line is removed. A pressure line is connected under this condition, and it runs by the low speed rotation well because a low needle is thin. Once, loosen a low needle more, and adjust a high rotation stage with the main needle. After that, you will have only to adjust a low needle again.
By the way, a pump isn't attached to this engine.
kobayan.

Bax

My Feedback: (11)

Basically, richen the idle mixture, and adjust the engine at full throttle, then reset the idle mixture.

Surging at full throttle usually indicates a too-rich high-speed needle setting.