Last year I had this problem. This year I'm having a problem with the mulching mower not mulching. The grass is clumping up and clogging whatever you call the 'chamber' where the blade spins and is supposed to chop up the grass.

It sounds like it's running high or faster, based on what I did last year with the throttle/governor spring. Could this have anything to do with it?

I've had the blade sharpened and balanced recently by a friend and it is plenty sharp. But I think he only sharpened the front main edge. Are there edges on the back edge where it's turned up that also need to be sharpened? Does this actually cut, or just force the cuttings up so they can be chopped up more?

I generally mow when it's dry.

I usually cut it pretty long, on the 2nd or 3rd highest setting, going by those notches and ridges where you manually change the height of the mower near the wheels. I've read you're only supposed to cut 1/3 of the height. I'm not out there measuring it, but sometimes it's been longer with all the rain we've had.

  • and, just in case, don't cut grass when it's wet as it will clump no matter what size it is chopped to
    – kevinskio
    Jun 3, 2016 at 16:32
  • What brand/model is the mower? Can you take a picture under the deck and of the clumping? The fins on the back of the blade do not need to be sharp. They just help the blades stay upunder the deck to get chopped finer. Jun 3, 2016 at 16:33

5 Answers 5


I think the 1/3 cutting rule is mainly for the health of the grass, not the proper function of the mower. If you let the grass grow long it may choke up the mower even if you are only cutting 1/3. Try setting the deck on a higher setting to see if it goes better.

Also are you moving the mower too quickly? Not sure if yours is a push mower or has an adjustable speed. If the grass is long you may need to take time to work through it.

  • it's a push mower; I didn't think I was going too fast; I slow down over the thicker/longer areas too; sometimes I've resorted to only mowing about 1/2 or 2/3s of the width of the mower when it starts clumping and slowing down
    – user26270
    Jun 3, 2016 at 19:53
  • @user26270: So it sounds like the mower is basically working fine. You are just biting off more grass than the mower can chew. Slow down, mow in narrower strips, mow more frequently, or get a more powerful mower.
    – Hank
    Jun 4, 2016 at 19:48
  • Yep. I did that (slowed down and mowed in narrower strips) and it worked fine.
    – user26270
    Jun 6, 2016 at 0:51

The biggest reason for clumping grass is mowing when wet and or not mowing frequently enough. If you're not mowing when grass is wet or trying to mow too much grass at once and your blade is sharp and motor is running fine then that really only leaves one thing.

Make sure the underside of the mower deck is clean. The design of the deck is important for mulching ability and a build up of debris and grass clippings under the deck can reduce how well the mower mulches. Remove spark plug wire, tilt mower so you can access the deck and scrape the deck to remove debris. It's good to do this at least once a year. I use a plastic putty knife and a brush. I prefer to do it after I have removed the blade and quadruple checked the spark plug wire is disconnected. Use a dust mask.

If you have a mower that has a port to attach a hose for cleaning, use it to help keep the mower deck clean in between thorough cleanings.

  • oh the underside definitely has a build up of grass; I've cleaned it out 3 times in the last month; just scraping off clumps of grass with a screwdriver; and last time I did this just before I mowed, and then it was clumping up again shortly after I started mowing
    – user26270
    Jun 3, 2016 at 19:50
  • @user26270 in that case pictures and more information on the make/model of the mower would be helpful. Jun 3, 2016 at 19:58
  • 1
    If the surface has gotten rusty and crusty it may need to be cleaned and repainted. Wax or silicone spray may help once it's clean. My mulcher also has vents under the motor - if yours does too, check that they are not clogged. I found that one to be more prone to clogging if using the discharge chute, so it's been in mulch mode for a long time...
    – Ecnerwal
    Jun 4, 2016 at 2:46

I've found that you I alleviate, not completely solve the problem, by raising the front wheels of my 20'' LawnBoy a notch or so higher than the back wheels. I guess this effectively double cuts the taller blades, and the shorter remnants seem to mulch and fall away better. The overall height of the cut remains about the same and looks pretty good.


I am often forced to cut the grass when it's long and it always jams the blade ( this is a horizontal spinning blade ) if I mow too fast. I don't use the collection bag at the back of the mower because I want the cut grass to be dumped back on to the grass. When the motor dies, I have to unplug it (electric), turn it on its side, remove all the wet packed grass from the inside the stomach and pylorus, spin the blade manually, and then reconnect it up again.

But I've found that if the grass is too wet, and even if I'm on the highest setting off the ground, it's simpler if I mow with the front wheels off the grass so I just cut the tops of the grass letting much less grass into the stomach of the mower, and this stops the jamming.


You probably have a slicing mower blade (standard blade sold with lawn mowers).

What you need is a mulching mower blade.

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