I’ve been there. Thinking to myself, “Why is my riding lawn mower smoking?” If you think about it, the answers make sense.
There’s really two possible answers here. One is easy to fix; the other is a little more laborious. Both can be overcome.
Tools Needed (Commissions Earned)
Riding Lawn Mower Smoking Diagnosis
First, where is it smoking from? Is it the crankcase, or is the smoke coming from the exhaust? Furthermore, does it only smoke during startup, or does it smoke during a heavy load?
Smoke From the Exhaust
It’s normal for an engine to smoke a bit during startup until it warms up. This is especially true after sitting over the winter.
What actually causes the smoke? Well, that’d be the burning of oil. Something is causing the oil to be getting past the piston rings and into the combustion chamber.
This could mean worn piston rings; that’s increasingly more likely if the machine is older. With more wear and use, the rings start to wear. This allows the oil to get past the ring gap and into the combustion chamber.
As the engine heats up, the piston rings will expand and that gap will start to close. As it does, the amount of oil seeping past will reduce and the smoke should diminish.
However, it could also mean overfilled oil. Overfilled oil can create “frothing” and extra oil pressure that has to go somewhere. This can provide enough pressure to get past the piston rings and into the combustion chamber as well.
Check the oil first to make sure it’s at the correct level.
If it is, remove the spark plug and inspect it. Is it unusually carbonized? This can indicate burning oil. If you have a borescope, go ahead and insert it into the spark plug hole for an inspection.
You might find carbon buildup on the cylinder head and/or the exhaust valve especially.
If you don’t have a borescope, just simply remove the cylinder head for a visual inspection.
Smoke From the Crankcase
This simply indicates a dirty crankcase.
An engine can get exceedingly hot during normal operation. Most riding lawnmowers are air cooled.
They rely on the flywheel to circulate air past the crankcase fins. This helps to dissipate the heat.
If the fins are plugged with debris from grass clippings, or the crankcase is covered in the oil you spilled the last time you filled it, then it’s going to smoke.
Either the clippings will overheat and burn, or the spilled oil will burn. Either way, it will produce smoke.
This is especially true when the engine is under load. When cutting deep or wet grass, there’s extra resistance, so the engine has to work harder. This produces more heat.
More heat equals more potential for burning debris and more smoke.
Recap
If you’re thinking, “Why is my riding lawn mower smoking,” think about the process. Where is it smoking from? When is it smoking? Then you can identify what is causing the issue so you can rectify it.
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