Metallic clunk noise - need help troubleshooting

[smitty078]

I need help troubleshooting the issue with this engine. It's a Briggs and Strat 128602-0304-E1 on a Craftsman mower. Here's what I've got so far:

• The engine does start and run
• There is a terrible terrible metallic clunk sound while starting and running

First thing I did was pull the spark plug cord off and pull the starting cord to see if it still made the sound: it does.

I have also done the following:
• Removed starter coil cover and verified that noise is not coming from the starter rope assembly (it isn't).
• Removed the flywheel cover and verify the flywheel key is not sheered and there is not debris anywhere.
• Removed the valve cover and verified valve operation.
• Verified the blade is not loose/bent/etc

All of the above tests checked out.

I finally discovered that if I remove the spark plug and crank the engine by pulling the starting cord, there is no noise at all. With the spark plug in, the noise is present. I tried several different spark plugs, same result.

My initial thought is that it is a broken rod, but I would think that with a broken rod, the engine wouldn't start.

What else can I do for troubleshooting beyond disassembling the engine complete? Any thoughts as to what this could be? The engine ran perfectly up until this noise started mid-mow out of the blue.

 

[bertsmobile1]

If you have removed the blades & drive from the engine thus isolating the engine from the rest of the mower and it still klunks then it is time to split the cases.
You can bugger around for days trying to decide it the rod cap is loose or the counter weight con rod is broken or the compression release is broken or the govenor is broken but in the end you are still going to need to split the cases.
If you decide not to go ahead with the repair you will get a much better price if you can show photos of the problem.
A used motor that goes "klunk" is worth scrap metal value
A used motor with a known & photographed broken decompressor is worth more to someone who needs parts or likes to resurect dead engines.