i have a new saito fa 125 four stroke engine(my first 4 stroke) which always stubbles a full throttle-is it too lean or rich? i've never had any problems with tuning my 2 stroke engines.

thanks

If it stumbles at full throttle it sounds like it's too rich. Are you getting a smoke trail at full throttle runs?

quite a bit. where should i tune it-low speed needle or the high speed needle?

If it's stumbling on the high end you'll want to turn it in a couple of clicks and then fly it somemore and see if it makes a difference. Sounds like it's off by more than a couple of clicks though. A couple of things for the low end. Try a pinch test on the line going to the carb. While it's at idle squeeze the line about an inch from the carb and see what it does. If it tries to stop almost immediately, it's too lean. If it takes a little bit to stop it's too rich. Adjust the low speed needle accordingly by turning the low speed screw in or out only about an 1/8th turn at a time. One thing you can do too is to do a somewhat slow fly by and and hit the throttle and see if you can hear where it stumbles or misses. See what range of the throttle it's in. Then after flying around for a bit bring the plane in and check the engine temp. Just make sure it's not too hot or it's too lean or it will deadstick.

Lean the high end till the motor reaches the highest RPM before slowing down, turn it rich to about a half turn. Then hold the nose up at full throttle, count to ten, if the motor revs up it's lean. Richen it up and keep doing this until the rpms don't change when the nose is up. Don't touch the low end until the motor has had a chance to break in.

Go to "welcome to club saito" in the glow engine forum. Lots of experience there.