Model 12H802 Type 2611-B1 Code 01032659. This mower starts and runs great and works well in light to moderate thickness grass. But if you get into heavy grass the engine boggs down. It won't die but seems to work extra hard. I have installed a new air filter, spark plug, fuel filter, sharpened the blade. I removed the carb and completely cleaned and rebuilt. Installed carb with new mounting gaskets. The engine has 60lbs compression which I know is on the low side, but I have had many engines running strong with 60lbs before.

Any thoughts or Ideas about the mower? Thanks for all the help.

Sounds like the engine may just be a little tired...

Check and make sure all the wheels on mower deck are set to the same cutting height. If the rear wheels are set lower then the front, this effectively doubles the load on the engine.

I would check out your valves, take your head off and clean up a good amount of the carbon buildup. Then remove your valves and clean them on a fine wire wheel, lap your valves using a valve lapping and fine compound. Reinstall valves and check clearances. you clearances at 1/4 past TDC are .005"-.007 Intake and .007"-.009 exhaust.

Following on to 30year's advice, ensure underside of deck is completely free of caked-on grass crud, this makes a big difference. Sharpen blades, don't mow when it's wet, raise deck a notch, go slower, etc.
Note: clearance checked with piston 1/4 inch past TDC, on compression stroke.

Thanks for the replies. Everything checks ok on the mower. (except the compression). Customer has a new mower so will use this one for the weeds until it dies.

Thanks for the help.