Small Engines (Lawn Mowers, etc.): Stihl FS85 trimmer will not rev up, carburetor body, stihl fs85

Question
I have a Stihl Model FS85 trimmer that will not rev up. It idles fine. If I try to rev it up it cuts off when the speed control is applied about 1/2 speed. This is after the engine is warmed up. The only way to get it to go any faster is to apply the choke about 3/4. Even then it does not get anywhere near top speed. It has 160 psi compression. I have changed the muffler screen, spark plug, fuel suction line, fuel filter, air filter, and rebuilt the carburetor. Adjusting the carburetor does not solve the problem. The muffler has been removed and there is no carbon blocking the exhaust. The combustion chamber has been cleaned with Stihl's carbon cleaner (it didn't look badly carboned up). It still will not rev up to speed. It has fresh gas/oil. What should I do next? Could it need a new magneto?

Answer
Hello Robert:

When the Muffler was Off, Did you Notice any Scratches or Gouges in the Jug or the Piston? What is the Compression? If the Rings are Stuck/Frozen the Engine will Operate Correctly Until the Engine Heats Up and Expands. The Rings Being Stuck/Frozen Can Not Expand with the Jug and The Compression Weakens and the Engine Will Not Rev Properly. Here is the Insructions for Setting the Air Mixture Screws. You May have them Adjusted Improperly. All the Carburetor Adjustments are the Same for Lawn Mower and Trimmers. There are Several Location for the Air Mixture Screws. The Idle Air Mixture Screws are Usually Located at the Top of the Carburetor Bowl and the Top of the Carburetor Body. If the Main Air and Idle Air Mixture Screws are Side by Side on the Side of the Carburetor, then the Idle Air is the Farthest from the Engine. The Main Jet Air Mixture Screw is Located in the Bottom of the Carburetor Bowl or Beside the Idle Air Screw on the Side of the Carburetor Body. Some of the Older Model Carburetors have the Main Jet Mixture Screw Straight in from the Top of the Carburetor Body (this is Rare anymore). If you have Cleaned the Carburetor (Disassembled and Soaked Overnight in Carburetor Cleaner (I Use Gunk). Then Blow Out the Passages with Compressed Air and Install a New Kit if Required. Now Setting the Air Mixture Screws:  ****Turn the Idle Air and Main Air Mixture Screws In Until SNUG **Don Not Jam** Then Reverse Both Screws 1 1/2 Turns. Holding the Throttle Full Open, Start the Engine. Turn the Main Air Mixture Screw Clockwise Until Proper Revs are Obtained. Occasionally you May have to Turn this Counter Clockwise to Achieve the Proper Revs. Now Allow the Engine to Idle. Set the Engine Idle Screw (Not Idle Air) so the Engine will Stay Running if Required. Now Set the Idle Air Mixture Screw so there is No Hesitation when Throttling from Idle to Full Throttle. Reset the Engine Idle if Required.**** Hope this Helps. I am Here if You Require More Assistance. Let me Know What Happens, Please. Thanks.

Good Luck

Respectfully

John