I must be adjusting my engines consistantly wrong. I have two engines, a Zenoah G-26, and Chinese SV-26, and both are behaving in exactly in the same way. They start easly, and run up good in the pit area. Idle to full throttle without hesitation. However, fly for 5 minutes, and then try some touch and go's, and they both spit,sputter, and die. They refuse to go from idle to full throttle.

Which needle valve do I need to turn richer, the low or the high ? Or what else should I be concerned with ?

Thanks for your help. > Ed

My guess...its a low voltage thing...no .....hold the phone. If the engines run up good when coldd...you have them to rich. Rich is good on warm-up....but then when fully warmed up,then the too rich is not good.Tune them when fully warm. Capt,n</p>

typically with the low speed too lean or too far "IN", the engine will not accelerate when you give it the throttle

If the low speed needle is too rich or out too far, the engine when the throttle is advanced will sputter and four cycle for a instant before picking up speed.

Your comments re easy start indicate to me that the low end needle is pretty much OK
Since both engines exhibit the same symptoms, I'm tempted to look at the installation or other common causes.
Is it possible the airflow and subsequent cooling is lacking?
You would get these symptoms on an engine that is overheating

That's my vote also.

A lean top end would lead to overheating after the motor got warm.

+1 for overheating.
Easy starting indicates a rich idle setting, and good power indicates decent overall tuning.
Overheating will show after a few minutes running.
An overheating engine makes proper tuning impossible.

+2 for overheating....

Would just leaving the settings alone, where they are set now, and flying it without the cowl tell me anything ?

Ed

A photo of your aiplane from the front &amp; bottom of cowl will help us figure out what you have going on. Does the engine smell hot????? How high is the air temps there where you fly? Thanks Capt,n

Yes it would. Checking the condition of your plugs should also give you a fair indication of what's happening inside your engine.

Karol

Karol...what you just said...very good point too! Take Care Capt,n

Edited,

Ok, do what Tired Old Man says.

There's enough in the previous post that I disagree with to just leave it alone.

this gets confusing!

Hey ED....you want simple....wait for a cold/much cooler day and fly it....since you are not telling us much. Just park it for now. Capt,n