Small Engines (Lawn Mowers, etc.): Briggs and Stratton shattered connecting rod, briggs and stratton, metallic sound

Question
I noticed a loss of power on my Briggs and Stratton 20Hp Intek V-Twin Model 407777 engine.  A compression test reveled piston #1 was not moving.  Upon disassembly I discovered the connecting rod was shattered.  I replaced the piston and connecting rod.  I reassembled the engine and it fired up right away with no hesitation.  I noticed a slight metallic sound at first and the throttle was higher than normal even at the lowest setting.  I began to cut the grass and the metallic sound became louder and louder until after 20 minutes it popped again.  After the break the throttle seemed to return to normal.  I have the engine apart again and sure enough the #1 connecting rod is shattered again.  I wonder if uneven carburetion would cause one piston to work harder than the other.  I am stumped.  Can you please provide me with your opinion on this matter?  Thanking you in advance.

Answer
Derek
Connecting rod failures will provide some of the greatest
challenges to your failure analysis expertise. Sometimes
the indicators will be pretty clear. However, in other
cases, they may be difficult to spot, or there may be two
or three indicators that seem to contradict each other.
The rod may be broken in such small pieces that it’s
difficult to find any failure indicators.
Your preliminary examination of the engine may provide
some valuable assistance where the rod failure
indicators are elusive or unclear.
To correctly analyze rod failures, you will need to identify
both. The similarities will usually help you determine a
general failure category (lack of oil, manufacturing
defect, etc.). The differences will help you distinguish
one from another, and often provide clues to the
circumstances or conditions that caused that particular
failure.
Where, then, should we look to determine the cause of
failure? Actually there are four areas that should be
scrutinized before a decision is made.

The first area to examine is the journal area
and the dipper (if it has one). Did the rod seize, causing
the aluminum on the bearing surface to smear and
transfer? Is the outside of the journal area discolored/
darkened? Are there burned oil deposits present? Do
the burned oil deposits extend down onto the dipper?
What is the condition of the dipper (intact, broken,
nicked or scraped, discolored)?
A fatigue break usually results from
damage done prior to, or during assembly. If the rod is
dropped on the dipper, or the dipper is bumped against
the workbench, a small stress crack can be created in
the aluminum. The forces of operation, along with
repeated heating and cooling, will cause increased
metal fatigue around the crack. The crack will spread
until the dipper finally separates and drops into the oil
pan.
Hopefully I haven confused you with this information I have done major extensive research in my business and I keep notes,mental and written.

Thanks