I have a G- 38 that I am running on 100 av gas at 40:1 and 18x6-10. and getting 7200 to 8000 rpm depending on the weather. The last few times that I have flown, at about 4 minutes into the flight the engine starts to sag, get it on the ground and check rpm it has lost between 1500 and 2000 rpm . (been flying thes bird for 17 years sacound 38)

First thought was lean so gave the hi needle a full 1/4 turn rich, seemed to help just takes longer. with another 1/8 turn it goes rich and come up to speed, tryed leaning the low speed needle 1/8 no help there.

Have now checked carb and cleaned (did find some gravel in the screen about 10 granes), back in the air and the same thing, at about 5 minutes it still sages off back to the ground it has lost about 1500 rpm at top end.

So has any one got a clue of what might be the problem, outher than the nut trying to run it.