Last Friday while doing a run-up on the new engine prior to take-off, I advanced throttle from 1500 to 2100 RPMs for the mag check. When it hit 2000 RPMs, the RPMs did not increase with a corresponding forward movement of the throttle. . .in fact, the engine began to stumble and lost 100-200 RPMs and then jumped to 2300 RPMs all at once. It did this twice and then ran fine.
Since then my mechanics have been able to consistently duplicate the behavior on a cold engine, but as soon as it warms up it runs normally. The mechanics have checked the fuel filters (two), removed and cleaned the injectors, checked the airbox, prop governor, mags, alternate air control, spark plugs. . .and called Lycoming, who indicated it may be a stuck valve and recommended an oil additive to lube the upper cylinders (similar to Marvel Mystery Oil).
Any A&Ps out there got any ideas? We'll be test-running it again tomorrow morning to see if the additive did anything. . .the engine has over 100 hrs. on it since installation and has run fine up to this point.
Since then my mechanics have been able to consistently duplicate the behavior on a cold engine, but as soon as it warms up it runs normally. The mechanics have checked the fuel filters (two), removed and cleaned the injectors, checked the airbox, prop governor, mags, alternate air control, spark plugs. . .and called Lycoming, who indicated it may be a stuck valve and recommended an oil additive to lube the upper cylinders (similar to Marvel Mystery Oil).
Any A&Ps out there got any ideas? We'll be test-running it again tomorrow morning to see if the additive did anything. . .the engine has over 100 hrs. on it since installation and has run fine up to this point.