B&S 3.5 Vibration

[camrydriver111]

I have a B&S 3.5hp classic mower. It's probably 5 years old. It starts up on first pull, runs well, but it feels like the mower is transferring a fair bit of vibration to the handle. I feel like it should be running smoother than that. I did an oil change, air filter, new spark plug, checked the handle to make sure it's not loose, and it has fresh gas.

Want to get this mower running better. What could be causing the vibration in the handle?

It's hard to describe in words, it's not extreme vibration, but at the same time I don't feel it's normal, and I don't remember it like that last summer. It's not uncomfortable to use, but slightly annoying.

 

[c502cid]

Sounds like a bent crank. You hit anything real solid the last time you used it?

 

[expat]

Originally Posted By: c502cid
Sounds like a bent crank. You hit anything real solid the last time you used it?


Or the blade is out of balance.

 

[Merkava_4]

1. Blade unbalanced
2. Bent crankshaft
3. Missing fins off the flywheel

 

[Fordiesel69]

Originally Posted By: Merkava_4
1. Blade unbalanced
2. Bent crankshaft
3. Missing fins off the flywheel


Exactly........

Has nothing to do with how it is running.

 

[Kuato]

I go with the blade; had a couple mowers where a chunk was missing from the end of the blade, and spinning at 2000 rpms really makes it dance.

 

[Kamele0N]

check 3bolts with wich motor is bolted to chasis....if one or all get loose it starts to dance...had that issue two week ago

 

[TFB1]

Proper way to check crank for run out(bend) is with a dial indicator...

In lieu of that, remove spark plug and spin engine to see if it has visible crank wobble... Can also remove blade, reinsert bolt and fashion a pointer or straight edge near the bolt shank to see if there is wobble... Anything over .040-.050" is going to be noticeable...

 

[HangFire]

Run it with the blade removed. That will narrow it down to blade versus engine.

 

[Kamele0N]

Those OPE dont run without blade......because blade serves as flywheel!!!

 

[TFB1]

Originally Posted By: Kamele0N
Those OPE dont run without blade......because blade serves as flywheel!!!


True but they can be started... Bring it to compression stroke, pull as quickly as you can and be prepared for it to kickback... But even if crank isn't bent, they'll run fairly rough without the inertia of blade, may not tell you much...

 

[camrydriver111]

Thanks! The blade doesn't look damaged. I will look at the crank and the bolts next time before I use it.

 

[Warstud]

Originally Posted By: HangFire
Run it with the blade removed. That will narrow it down to blade versus engine.


I tried to pull start a mower once without a blade and it almost ripped my arm off

 

[HangFire]

OK bad advice...

 

[Al]

Check and make sure the blade is firmly seated on the adapter with the adapter pins in the blade holes.

I doubt its a bent crank..the shear key would have let lose...or at least deformed it so it would run rough if at all.

Overwhelming likelihood is the blade.

 

[TFB1]

Originally Posted By: Al
Check and make sure the blade is firmly seated on the adapter with the adapter pins in the blade holes.

I doubt its a bent crank..the shear key would have let lose...or at least deformed it so it would run rough if at all.

Overwhelming likelihood is the blade.


In a perfect world, maybe... But in the real world those 7/8" B&S cranks(every 3.5Hp B&S I've ever seen) bend probably 50% of the time and flywheel key is fine...

 

[camrydriver111]

I checked it out and there is no crank wobble. I didn't take the blade off and check for balance though.

Maybe I have too high expectations for smoothness?

I'm also running Shell VPower in it since it's ethanol free. Could that cause the problem? It does seem to run "stronger" with it.

 

[old1]

Take the blade off and hang it on a nail and see if it is balanced. If it is, and the crank is not bent even a little, It is probably normal. The cheap lightweight no name mowers are so light that they transfer more of the normal engine vibration to the user than heavy machines. These single cylinder Briggs engines aren't the most balanced engine.

 

[Merkava_4]

Originally Posted By: old1
Take the blade off and hang it on a nail and see if it is balanced.


A 1/2 inch drive extension works better.

 

[camrydriver111]

Originally Posted By: old1
Take the blade off and hang it on a nail and see if it is balanced. If it is, and the crank is not bent even a little, It is probably normal. The cheap lightweight no name mowers are so light that they transfer more of the normal engine vibration to the user than heavy machines. These single cylinder Briggs engines aren't the most balanced engine.


I think that may be the case.

What oil would you use? I'm running mower W30 and wondering if there is something better.