piston slap...I think....B&S 6 HP

[Schmoe]

With the wife getting laid off here pretty soon (don't use any Hertz rental cars.....) and going back to school to become a nurse, I got to thinking about some of the higher ticket items I may need to purchase within the next couple of years. The mower came to mind. Mulching, 6 HP Briggs, MTD Yard Machines. Been a really good one, bought in '98 and still cranks on one pull. However, when cutting the thicker grass, I do notice a piston slap. Well, that's what I call it. Sounds exactly like the newer Chevy's we have at work that are doing it. Kind of like a loud tick sound. Only does it under load. Using 5w30 Pennz and a little bit of 15W-50 M1 to thicken it up some. Wonder if I need to go to a thicker oil or start looking for a new mower? We all got story's on these Briggs engines lasting for 20 plus years, but they weren't built this same way in the 70's.

 

[Bill Kapaun]

I'd just stick with 30 weight myself.
Are you sure it's not an exhaust noise?
Some engines WILL make a rod knock sound if you bog them too much. Like to the verge of stalling.
Keep the blades sharp, the underside of the deck clean and don't mow too fast. You can always mow a narrower swath if the ground speed is too high (self propelled).

 

[Turk]

Here's what you can (highly recommend!) do:

1) With it warmed up fully, pour some Seafoam down the carb slowly, then let it stall. Wait 5-10 minutes, start it up and run it for a while until it's done smoking.

2) Get that oil outta there. I run Delo 400 15w40 HDEO with great success, but if you want to stay on the lower side, go with 10w30 Rotella.

3) While you're at it, pull the Plug, replace if necesary, clean or replace the air filter and of course like prevoiusly recommended, replace or sharpen the blade.

"Please" post when you're done doing these things, especially the Seafoam & the oil...

Good luck,

 

[Rickey]

My 6 HP Briggs has been making what I think is a similar sound while heavily loaded since new. Thats over 10 years making this noise with no problems.
Maybe its built just a little loose.
My 20 HP Kohler also slaps a little under load when cold.

I believe that some air cooled engines are purposefully built a little loose in the piston to bore spec to compensate for the extra expansion caused by the higher operating temperatures that they must endure when loaded to the max for extended periods.

Rickey.

 

[tmorris1]

I think all engines make that same noise when they are under heavy load. Don't worry about it, its a lawn mower, they make a lot of noise.

 

[Schmoe]

Thanks all. Rickeys suggestion seems to make a lot of sense. Being air cooled, it could be subject to a wide variety of temperature swings which means expansion. Have to admit, don't really notice it as much when it's hotter outside, say in the 100's, as opposed to being a lot cooler. May try the Seafoam, worked well in my truck. But something tells me, if it ain't broke, don't fix it.

 

[Master ACiD]

alot of these little mowers with overhead valves need valve adjustments just like cars do. i have a 14hp briggs engine on a craftsman mower and every 5 years or so it will start making a ticking or knocking sound. this signals when its time to adjust the valves. its pretty easy to do. 4 3/8 bolts on the valve cover. to adjust the valves its an allen head wrench and a half inch box end wrench. when im done its alot quieter and SEEMS to use less fuel. but i dont have a fuel economy gauge or anything so this is just a mental observation.
if youre mower has overhead valves, give a valve adjustment a shot.

 

[Turk]

Schmoe,

PCMo isn't really the best oil running in mowers. I'd get the PCMO out in and put in HDEO.