Small Engines (Lawn Mowers, etc.): 16hp Onan engine, twin cylinder engine, thick grass

Question
I have a Case mower (1980) with a Onan 16hp twin cylinder engine. Over the past year, after starting the engine, I have had to use more and more choke to keep it running. It started with 10%, then 25% now its up to close to 100% just to keep it running. When I hit thick grass (more of a load) the engine boggs down. If I reduce the choke the engine dies. What is my next step? I've replaced air filter's and spark plugs but to no avail. Thanks for the help.
Bob H

Answer
 Hello Bob:

 What is the Compression of the Cylinders? From your Description it Sounds like the Carburator needs to be Cleaned and Rebuilt. The Varnish has Built Up Inside the Carburator and is Slowly Restricting the Gas Flow to the Carburator Jet. It Could also be that the Valve Lash needs to be Adjusted. I would Do the Carburator First if the Cylinder Compression is Good. Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. Thanks.

 Good Luck

 Respectfully

 John