my riding lawn mower knocks 3 or 4 times at startup and when turning it off, it has been sputtering lately while idleing and cutting the grass too it has been doing this for a while so i kind of ignored it dont know what is up with that either i have did a full tune up about a month ago spark plug, oil, fuel filter, air filter and mower blades. It either dosent have a oil filter or i cant find it so if it does have one it has never been replaced. Any idea whats up with it, it is a Muarry 13.5hp riding mower

 

Mine don't like the new gas. I use premium, and add a bottle of octaine booster I get from the Dollar Store

 

Carb may need cleaning too...

 

I havent thought about fuel, it may be the problem i will try the premiunum with the octane boost. I thaought it may be i bent something because i am always riding ove over gron stumps and buried cinde blocks just yesture day i hit a stump by mistake and a piece of it about 4in in dia. got caught imbetween the blade and the blade and housing and killed the engine but i removed it and started it up and it still had the knocking i mean it souds like some one it beating some thing witha hammer 3 or 4 times under the hood at startup and when i turn it off ?
I am sure it wouldnt hurt to have the carb cleaned or rebuilt i know i haven't had it done in the 5-6 years that i have had it, i think it is something mechanical though.

 

belt driven mowers seldom hurt the engine, but that can create a balance problem with your blade. Try cutting through those areas with the deck all the way up first to see what's in there before you give a trim cut

 

You may be able to adjust the carb so it runs a bit richer also...

 

Make sure the mounting bolts are tight...

 

Sorry, but, I'm having a hard time understanding this, but, if you are having 'knocking' sounds coming from the engine, here are some possible causes.

1. Loose blade or blade adapter. Remove blade bolt(s) and clean the threads and re-install using BLUE Locktite.

2. Possible loose connecting rod(s). Take the mower to a reliable repair shop and have them take a look at it.

3. Possible wrist pin issue. Take the mower to a reliable repair shop.

4. As crazy as it may sound, this was the problem that my brother-in-law was experiencing w/ his mower -
Loose bolts securing the engine to the mower!!!
My b-i-l gave away his expensive Zero-Turn rider, because the engine was making one heck of a noise!!!
Three bolts were loose!!!!

ETA: Possibly the crankshaft COULD BE bent, especially after it sounds like you're trying to mow roots, cinderblocks, etc.