Lawn Boy runs very rough after the blade hit a roc

[wally6934]

The mower was running very well, i hit a rock and it died. I tried to start it and it would start but run very rough. I decided to replace the blade and set the blade timing per the Lawn Boy manual and see if that helped, but it ran the same.

At this point i thought the keyway must have been sheared a little. I pulled the flywheel and checked it out only to find it was in perfect shape.

At this point i have no idea what could be wrong, and when i say it runs very rough i mean it will barely stay running, cough, cough, spit.

C-78 engine. mid 1960‘s

 

[LargeCarManX2]

It might have stopped/hit the rock right when the spark was burning the fuel. Could the piston rod have been bent? I cracked a engine case/block on a lawn mower hitting a rock many years ago.

 

[spasm3]

Bent crankshaft? That might put it out of time.

 

[wally6934]

Originally Posted By: LargeCarManX2
It might have stopped/hit the rock right when the spark was burning the fuel. Could the piston rod have been bent? I cracked a engine case/block on a lawn mower hitting a rock many years ago.


That sounds very bad, the only way i can think of checking for a bent rod is to disassemble the engine.

 

[wally6934]

Originally Posted By: spasm3
Bent crankshaft? That might put it out of time.


It runs rough but i can not say it feels out of balance like a bent crank would feel.

 

[fdcg27]

Unfortunately, I think you're right about the crank.
If it ain't the blade, it probably is the crank.
A shame.
Maybe you can find a cheap parts donor?

 

[GreeCguy]

You can check for a bent crankshaft by first REMOVING THE SPARK PLUG, (I type this in caps as you don't want it starting on you and chopping your hand or arm off).

After you have removed the sparkplug, tip the mower on end and rotate the blade, if it wobbles, your crankshaft is bent. Even if it just wobbles a little, your crankshaft is bent.

Usually however, if your crankshaft is bent, the mower will vibrate as it will be out of balance.

Are you sure the key way isn't damaged in anyway?

 

[LeakySeals]

Wish it could be something simple like flipping it over with the oil in it to check the blade. I did that once, caused me pain!

 

[Donald]

On the positive side. Now you can ask for something for Fathers Day.

 

[wally6934]

Originally Posted By: GreeCguy
Are you sure the key way isn't damaged in anyway?


I looked the key way over twice because i could not believe it was OK.

 

[Panzerman]

I am still betting its the keyway. Unless you actually remove the flywheel. It can look deceiving to look down on it. They will shear without being noticeable and just slide over.

 

[expat]

Originally Posted By: Panzerman
I am still betting its the keyway. Unless you actually remove the flywheel. It can look deceiving to look down on it. They will shear without being noticeable and just slide over.

 

[GreeCguy]

Originally Posted By: Panzerman
I am still betting its the keyway. Unless you actually remove the flywheel. It can look deceiving to look down on it. They will shear without being noticeable and just slide over.


Ditto.

 

[147_Grain]

Blade is out of balance at a minimum. Resharpen and balance the blade or replace it. IF engine doesn't smooth out, you might have a bent crankshaft or damaged lower end.

 

[Warstud]

Since you changed the blade you would have had to tilt the mower. Maybe oil leaked all over the air filter and clogged it...so check that first (clean or replace). If that didn't help.. check the muffler and spark plug for oil leakage to - (clean). If that didn't help... it's probably your flywheel key.

 

[GreeCguy]

Originally Posted By: Warstud
Since you changed the blade you would have had to tilt the mower. Maybe oil leaked all over the air filter and clogged it...so check that first (clean or replace). If that didn't help.. check the muffler and spark plug for oil leakage to - (clean). If that didn't help... it's probably your flywheel key.


If it was flooded with oil, it would smoke like Bogie on Key Largo.

 

[wally6934]

Originally Posted By: Panzerman
I am still betting its the keyway. Unless you actually remove the flywheel. It can look deceiving to look down on it. They will shear without being noticeable and just slide over.


I did remove the flywheel.

 

[wally6934]

Originally Posted By: 147_Grain
Blade is out of balance at a minimum. Resharpen and balance the blade or replace it. IF engine doesn't smooth out, you might have a bent crankshaft or damaged lower end.


I did put a new blade on it and i balanced it. I am thinking lower end damage.

 

[wally6934]

Originally Posted By: Warstud
Since you changed the blade you would have had to tilt the mower. Maybe oil leaked all over the air filter and clogged it...so check that first (clean or replace). If that didn't help.. check the muffler and spark plug for oil leakage to - (clean). If that didn't help... it's probably your flywheel key.


It is a 2 cycle, no oil to leak out.

 

[GreeCguy]

Is it the deck or the motor that's made from magnesium on the old Lawn boys?