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I have a motor from my old Toro Recycler 22" (10 Yr old deck cracked.) 7.00 190CC mounted on a Murry Ground Force 3 Deck with spoked high back wheels (got it for $25). It makes a strong push mower. I hit a rebar and it died instantly. The blade doesn't contact the deck anywhere. I checked for a bent crank-shaft and that looks good; the center bolt head stays centered when spinning. But it makes a couple knocking sounds when it fires up but then runs OK. I've used it a couple hours without a problem but that knocking on start-up concerns me every time I start it. It's such a strong mower and starts with one or two pulls now that I have the auto choke linkage freed up. I don't want to do any damage to it.
Any guesses as to what I'm hearing?
Keep running it as is?

Sounds like engine could be loose on deck now. Check all mounting bolts. And check the block for cracks around the mounting ears on block.

I checked all that and found nothing loose or broken. I wonder if something on the connecting rod broke that is letting it knock but not come totally loose?

If you killed the engine with the rebar I would check for a partially sheared flywheel key. Just because the crankshaft isn't bent doesn't mean the flywheel isn't damaged. Just thinking it could be a spark timing knock.

Thanks! I'll check that tomorrow.
I did a more precise check confirming the shaft is straight. I pulled the spark plug, laid it up on it's side, and removed the blade and adapter from the shaft. I then fastened a wooden 2x4 across the bottom of the deck. I ran a long wood screw through the 2x4 so that the needle sharp point of the screw was just touching the beveled corner of the shaft. I then spun the shaft and the distance from the screw to the shaft didn't change.

When you removed the blade adapter, was it completely intact? Your problem sounds like a problem I had when I hit a metal bar and broke a chunk out the adapter.

I had a 3.5hp Snapper last week that would knock on start up and when the engine was slowing down, but not at full throttle or a steady RPM.
This is what I found inside.


It had a brand new blade when I found it and it started on the first pull and cut grass just fine. It was a mid 80's model Snapper HiVac. I changed out the rod, checked he crank, replaced the base gasket and mowed my lawn with it.

When I first started it, it sounded like a loose blade or engine.

I had a JD with a 6.75hp a few weeks ago that had a boom-a-rang shaped blade on it and a bent crank that not only was bent at the bottom, but it twisted below the rod journal putting the crank counterweights out of phase. I had straightened the lower crank, checked the key and put a new blade on it, it started hard but ran. It had a vibration but not that bad. I actually had cut my lawn a few times with it but it would stall easily and was hard to start when it got hot.
I tore it apart chasing the the vibration and found the bent crank. The lower half of the crank was twisted almost 1/4" putting the lower counterweight behind the upper weight about 1/4" .