I have a 20hp B&S (461707 type 0143) that I just replaced a connecting rod in. When I reassembled the tractor and tried to start it the enging turns over freely but will not get combustion. I'm troubleshooting that this week, but my real question is what could be causing the knocking? I lined up the timing dots on the crankshaft and camshaft when I put it back together and the rod bolts are tight. I read in another post that someone had air in the lifters that caused knocking and they just needed to work themselves out. I'm hoping that is my problem also, but if it is not won't running the engine (if I get it running) for 15 - 20 minutes damage something? My oil level is also just over the full mark and I'll drain it a little tonight, but could that cause knocking?