os40la
 

engine goes dead after about 3or4 minutes it is smokeing good an then the smoke stops an starts to loose power bad i can idle it down for a minute or two an then it will back run good for a few more minutes i have changed glow plugs set main needle at 2 to 3turns out an the low speed allway in it seems to run best at this setting motor does not seem to be runing to hot a have changed all fuel line took muffler off an resealed it runing 10% fuel thanks for help

os40la
 

tld,

It sounds like the engine's going lean and overheating. You may need to check for fuel foaming. That's where air is agitated by vibration into the fuel in the tank and then it's drawn into the carb. The added air leans the mixture. When you throttle down, the vibration changes, the air runs out, and the lower RPM allows the engine to cool. It runs nicely until it overheats again.

When the engine overheats, it loses compression, which causes the power loss.

You may have to add foam padding around the tank (if you don't have much), or remove some (if it's packed too tightly). You should also make sure the prop and spinner are balanced. If you have a very light airframe, it could be transmitting vibration to the fuel tank.

Sometimes, just changing brands of propellers can change the vibration mode and the problem will go away.

Vibration-caused problems can be devilishly tricky to track down.