Small Engines (Lawn Mowers, etc.): Briggs and Stratton 18.5 twin Loud knocking, briggs and stratton, spark plug wire

Question
I have a riding lawn mower, with a Briggs and Stratton 18.5 twin cylinder. Model 42A707 Type 1238 01 Code 9602165A. It has a loud knocking sound that does not go away when running. I'm thinking it might be a stuck valve. What are your thoughts and if I took it to a repair shop what would your guess be on the price to have it fixed? Just asking cause I don't want to get raked there. If it's an easy fix, could you tell me how to fix it? Thanks

Answer
Kevin:

A loud knocking sound on this engine usually indicated a broken connecting rod. the engine is running on one cylinder and the other "broken" rod is just flapping around in there.

You can troubleshoot this yourself by doing the following:

With the tractor turned off,  Take off one spark wire and start your engine.
Does the engine start?  

Turn off the engine and reattach the plug wire you just removed.

Remove the other spark plug wire and start the tractor.  Will it still start?

If it only runs on one of these you have a bad cylinder.

the other thing that causes an engine to knock sometimes is an excessive carbon buildup, but we dont see much of that anymore since unleaded fuel came into play many years ago.  

Taking this to a shop will be expensive, but worse yet, Briggs and stratton quit making this engine many years ago and parts are hard to find.  I would be looking for a replacement shortblock or tractor if it is in fact a serious engine problem.

Sometimes it can be repaired with a new connecting rod and gasket kit, but its very labor intensive.  I bet you could easilyu be looking at $500.00

Unlikely its a valve problem.

Hope this helps, let me know what u find.

Dave