Surging

Hello! I'm typing this on my phone so please refer to my user account to get the serial number and model number of my Toro super recycler. This mower has a Briggs & Stratton what I assume to be a quantum. Has the air cleaner assembly that bolts to the side of the carb with the primer button and the solid aluminum OHV cover .. I bought this mower at a yard sale not running. I cleaned the carb and reinstalled without tearing any gaskets so I see no potential air leaks there. When I pulled the filter I found the top rubber piece that should be completely closed off had sheared off and laying down in choking off the air inlet. Most likely why it wouldn't run. This must not be the correct filter even though the filter base fits correctly in its seat. I doctored this up enough temp to go for a start. Started on the first pull and runs but surges and there is very little throttle control. All the way back to idle and the motor only slows down a little right at the off position surging throughout. The throttle itself is hooked up correctly with full range of motion at the carb . There must be something going on with the governor. The motor smooths out when the blade is engaged and mowing under a load but still surges a little even then. The front spring is there on the linkage hooked up correctly. At this point I had to stop working on it yesterday and will pick it up today after work. I'm guessing there is another spring under the top or somewhere I did not see yet before I stopped for the day yesterday. Where should I proceed from here?

Google: Toro lawnmower manuals, click on " Parts and Manuals-Toro", enter model number, scroll down page for your 1998 model & serial number manuals, service manual has information you need. I would also change oil, spark plug, air filter like you said & remove, clean & inspect blade for dressing up with file or replacement if cracked or distorted, use OEM parts.