If it's Synchro Balanced I bet the bearings on the balancer and dog bone are what's knocking.They usually get worn out long before the rod starts to knock.I don't know if it's possible but you might just be able to Remove the balancer.Usually the balancer makes a mess of the crank where the bearings are.They beat the crap out of the crank till the dog bone breaks and jams up the crank and rod causing it to snap the con rod.
The 11hp in the old Bolens G11 didn't have a balancer in it.I had to replace the governor gear and sump gasket in it.
G11 was made in 78 and had Points ignition and was not synchro balanced.Yours looks very similar so I'm gona say it was made mid to late 80s.
Model numbers should be stamped on the blower housing near the spark plug.
It will look something like this
Model 252707 Type 0621-05 Code 91081611
Your Briggs is a Model 25 but I can't tell ya exactly what year it is without the numbers.
Edited by EricFromPa, June 29, 2015 - 04:13 PM.