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With advice from Scrubcadet10 I got a 16 year old Briggs 3.5 running. It clattered very steadily and very loudly. I replaced some oil with motor honey, no change. I assumed the rod was knocking and figured I'd beat my money's worth out of it and started mowing. About an hour in, the clatter stopped abruptly and completely in light grass.

I'm not complaining, but I don't think a rod would do that. What else could possibly make that much noise?

Make sure that the engine mounting bolts are all tight.

pull the spark plug wire off of the spark plug.

then lift the mower with the front towards the sky, and put something heavy on the handle, take a look under the deck... see if you see any impact marks from the blade hitting it.

Mounting bolts are rock solid and not a trace of blade/deck contact.

The engine had sat for a couple of years, is it possible that a bearing could knock and then stop? Perhaps a spun bearing insert?

Time to have a think.
If the clatter was at engine speed then it ha its origin in the movement of the piston
If the clatter is 1/2 engine speed then the problem was with the movement of the valves .
Things like rust on a cam follower or valve stem will cause a lot of racket till it gets knocked off.
Right now, change the oil , dilute it with petrol and have a good look at what is in it that s not oil .

bertsmobile1 said:

Time to have a think.
If the clatter was at engine speed then it ha its origin in the movement of the piston
If the clatter is 1/2 engine speed then the problem was with the movement of the valves .
Things like rust on a cam follower or valve stem will cause a lot of racket till it gets knocked off.
Right now, change the oil , dilute it with petrol and have a good look at what is in it that s not oil .

Click to expand...

Bingo. The knock was at about 300/minute, so probably half engine speed. There was a little crumb of rust in the oil.