lawn mower engine knocks at startup

[The motor guy]

My exmark lawn mower seems to have started knocking slightly at startup and shutdown. The only thing I did was change the spark plug. It's pretty random. Sometimes it does it sometimes it doesn't. It lasts about 2 seconds and then goes away. The engine runs fine. Nice and smooth. It has an FJ180v kawasaki engine on it. 6hp.

 

[Eddie]

Check for a loose flywheel

 

[greystone]

Or even more likely a loose blade.

 

[The motor guy]

So you don't think it's a rod knock? If it was a rod knock would it get louder with more rpm?

 

[hemitom]

May be a loose blade , or slightly worn hub or hub key.

 

[CharBaby]

I have to agree with the "Loose Blade" This would be my first thought and then go from there!

 

[01rangerxl]

Originally Posted By: motor_oil_madman
So you don't think it's a rod knock? If it was a rod knock would it get louder with more rpm?


It is least likely to be rod knock, and that's the most difficult potential problem to diagnose and fix. Start with the things on the outside of the crankshaft.

As others said, the blade is the #1 suspect. I have seen tons of push mowers make a knocking noise at startup/shutdown because the blade has shifted slightly off the tabs on the blade adapter or they have been sheared completely. It's not balanced at startup, so it knocks, but once the engine builds speed it's balanced enough to run smoothly. You may need to take the blade all the way off to actually check this.

 

[The motor guy]

would just being off balance just to being dull cause this? It does need to be sharpened. Tomorrow i'll start with putting the old spark plug back in and see if it stops. I don't remember it doing this before the spark plug replacement.

 

[01rangerxl]

Dull blades don't usually cause imbalance (both ends wear about the same), but a bent blade definitely can.

If the knocking started immediately after the s/p change that's something to look into.

 

[yeehaw1960]

Did you see if the blade was loose? You can catch a little rod knocking noise on some mower engines if you run them, let go of the dummy bar and pull it back down after the engine is almost stopped. As the engine starts running again you can hear it if it, in fact, has some slop in the rod bearing.

 

[The motor guy]

well I took the blade off and and checked to see if it was balanced and it was. Then I noticed I could wiggle the part that holds the blade to the crankshaft. Ever so slightly though. There is probably 3 or 4 thousandths of play. I don't know if the key is worn slightly. I know there is a brass collar that supports the crankshaft I don't know if that could be worn. It knocks at startup and shutdown and seems to be getting worse. The knocking is definetely coming from the crankcase though. So i'm thinking it's a rod knock.

 

[The motor guy]

Oh I want to add that the blade mount wiggles and the crankshaft stays in place.

 

[The motor guy]

When does a rod knock occur? It basically only occurs at the around 200 or 300rpm. Wouldn't it be really noticeable at speed? It runs at 3300rpm

 

[The motor guy]

Originally Posted By: hemitom
May be a loose blade , or slightly worn hub or hub key.


Is that what that's called? the hub? That's the part that has the slightest bit of play in it and you can hear it knocking inside the crankcase when i turn it back and forth.

 

[clarklawnscape]

sounds like a partially sheared key on your blade adapter to me

 

[The motor guy]

Originally Posted By: clarklawnscape
sounds like a partially sheared key on your blade adapter to me


So that little bit of slack could be what's causing it to knock real bad at startup and shutdown? I suppose I could put a new key in their. Is seems like it's just the blade adaptor that has slack. The crankshaft is staying put when I move the adaptor.

 

[01rangerxl]

Originally Posted By: motor_oil_madman
Originally Posted By: clarklawnscape
sounds like a partially sheared key on your blade adapter to me


So that little bit of slack could be what's causing it to knock real bad at startup and shutdown? I suppose I could put a new key in their. Is seems like it's just the blade adaptor that has slack. The crankshaft is staying put when I move the adaptor.


If there is any slack in the blade adapter you will hear it at startup and shutdown. It will make a knocking noise that sounds very much like rod knock.

 

[The motor guy]

ok well atleast a key would be pretty cheap to buy. i'll try that. I guess the noise just transfers all the way up the shaft making it sound like a rod. This makes sense cause I forgot that on lawn mower engines the blade is part of the flywheel.

 

[hemitom]

If the key was shearing it would most likely be tight on the hub, The hub is the piece that slides on to the shaft and the blade bolts to it. Usually when a key starts to shear it binds on the hub making it tight and less likely to knock or be loose, until it complete shears off.

 

[The motor guy]

If the key was shearing it would most likely be tight on the hub, The hub is the piece that slides on to the shaft and the blade bolts to it. Usually when a key starts to shear it binds on the hub making it tight and less likely to knock or be loose, until it complete shears off. [/quo

I'm not sure if the hub is causing it. The key isn't sheared cause I haven't hit anything. The hub is tight on the shaft as far as side to side movement is concerned. But I can turn it back and forth ever so slightly. This was me wiggling the hub itself with the blade removed. I'm not sure if it's even the problem cause once you bolt the blade back on the bolt keeps the hub from moving spinning back and forth thus taking up any slack.