lawn mower knocks

[Choreboy]

I have a Craftsman push rotary mower with a 6-hp Briggs engine. It was bought in 1997. I use it to keep 3/4 acre mowed. It has been smooth and quiet and gets a lot from a quart of gas.

A couple of weeks ago I noticed it knocked and shook when I started. That seemed better the next day. (Maybe the oil was cooler.)

Yesterday it was knocking under a load (heavy grass). It reminded me of the sound behind a machine gun. I checked the muffler.

I get a knock if I disconnect the plug and pull the starter. If I turn it on its side and tug the blade, there doesn't seem to be slop in the main bearings.

If I turn the blade to the top of the compression stroke and move it back and forth a little, there's a knock. I guess it's the connecting-rod bearing.

The owner's manual has a parts breakdown for the engine. There are no connecting-rod bearings. I guess one simply replaces the connecting rod.

What's cheap and expedient? Will I suddenly wreck the engine if I keep mowing? Will 30W oil help? Should I assume the wear is on the rod and simply install a new one? Is it better to buy a new mower?

[Duane Bozarth]

.....

A) Cheap and expedient is to run it until it dies and replace it or at least the motor. B) Yes C) For a short additional time, maybe D) No--the rod and crank will both be gone by now E) Probably at least the engine--repair parts are normally nearly the price of the whole engine--it is worth looking at if you're able to do a competent repair yourself, but it's likely the cylinder walls are worn by this time anyway, as well as who knows what else...

[The Kearsley Curse]

x-no-archive: yes

I believe you should be using 30 W oil to begin with, it will be in your service manual. Also, the clearance between the rod bearing and the crank shaft should be about .002, if I remember right. You cannot just replace the rod and the bearing. You may have to replace the shaft too. If it's been knocking like that, then there could be damage to both parts, most likely is. Those little parts for those engines are not that cheap. Then you'll need gaskets and sealant. And maybe a flywheel puller. And you will have to check the clearance with plastigauge. You know how to do that? You'll also need a micrometer to measure the crank. Fun huh? It is, if you've got the tools.

A lot of people think these little engines are simple to repair and they are, if you have the right tools and the know-how. But like any other gas engine, two or four stroke, there are torque specs, procedures and manufacture specifications to follow. If you don't do just what the manufacturer states to do, kiss your attempts at repairs good-bye.

My advice? Run it until the rod pops outside the crankcase. If you want to speed up the process of destruction, then play around with the governor for a bit. See if you can get the little sucker to hit 10000 grand :D Then look for a used engine that runs OK, or buy a new mower, or a good used one.

[The Kearsley Curse]

That will do. If you can make a B&S go 10 million RPM, then you might just get a job with NASA.

I meant "10 Grand". :-)

[gfretwell]

Sounds like a death rattle that yopu are not going to fix easily. If you really like ripping into small engines you can order the parts and go at it but I bet a new engine is cheaper by the time you finish. At that point you have a new engine on an old mower deck. What will break next? If you can nurse it through the rest of the season it might be an interesting winter project tho. I really can't be without a mower very long (or the grass will take over) so fixing one is not usually an option if it taks very long.

[Edwin Pawlowski]

10000 grand is 10 million rpm NASA can use it.

[Choreboy]

A neighbor said in his experience there may be no wear on a steel crankshaft when an aluminum connecting rod wears out. A cousin said he had filed a connecting-rod cap to take up connecting-rod slack, and the Briggs engine had run fine for years afterward.

So I took the pan off. I could find no slack in the connecting rod. Everything looked amazingly clean and unworn. The clicking with the ignition off had come from backlash in the camshaft gear. It didn't look worn, so the backlash may have been correct.

Still not knowing the cause of the knocking, I removed the head. The valves seemed okay. I noticed the cam opened the exhaust valve very slightly for a moment during the compression stroke. Could that be a sort of compression release for starting?

Deposits can cause knocking, so I scraped away the slight deposits. I replaced the air filter. Now it mulches fairly heavy growth at an idle with only faint knocking.

It makes me wonder. Are modern mower engines made to knock a little for reduced hydrocarbon emissions? Could a clogged air filter cause worse knocking?