Lawn Mower Losing Power, Cutting Poorly

Question

I have an old John Deere 111, circa 1980, that seems to lose power when the blades are under an insignificant load. I'm not cutting very tall or abnormally heavy grass -- just your run of the mill yard here. The blades have been sharpened on a bench grinder and tested for balance. I've test the deck pulleys and they swing freely. Oil changed, cleaned the carb, and replaced the fuel filter.

When I engage the blades and without lowering the deck, they run at full power -- though it does leave an uneven, choppy cut on the grass. Now, when I lower the blades less than an inch, I can sense the blades not rotating at full power. The quality of the cut looks about like that of a mower being overloaded and choked out, but the engine runs fine the entire time. The quality improves when I reduce the speed to a snails crawl (2 of 5), but it is by no means perfect.

Does anyone have some advice on to why my mower is cutting poorly and also losing power when I lower the deck? I know this is a smaller JD, but this is ridiculous. Thanks for your help.

Answer 1

The motor runs at full power is a good clue here. While I, too, would reach for the low compression as a first guess, that seems less likely if the motor is running fine (and also if it starts fine - my mower that needs a rebuild is an absolute bear to get started when cold.) If you happened to have, or to know someone who has a compression tester that will fit, that would not be a bad thing to check, but it seems less likely.

So, the rough cut and acting up when lowering blades points to a problem in power transmission - bad bearings, mis-adjusted belts, old/stiff belts, something like that. I don't know your particular model that well, but If the power put in by the engine is not reaching the blades reliably, that's the part of the system where the problem is - dive in and look/feel for bearings that are either not rotating well, or that have excessive play in directions other than the rotating one. Evidently whatever is wrong is exacerbated when the mechanism is lowered, so you may need to arrange to be able to investigate that.

Thinking further, the rough cut makes the bearings the blade pivots on seem most likely - and movement from those will degrade their belt contact, and thus power transmission. You say they pivot fine, but what I suspect is "slop" - movement in directions they should not be moving in. They are 36 years old unless you have already replaced them.

Remember to disconnect the spark plug wire (or take out the plug) when mucking about in the deck/transmission area. Finish with the same number of fingers you start with.

I don't know if mechanics would be particularly helpful - outside of the stack exchange structure there are several sites dedicated to old tractors, some including or even primarily lawn tractors, where you might find other people who have the same exact model and are familiar with common problems and symptoms for that exact model. I would suggest you look for those.