B&S 190cc hit a rebar, now knocks a couple times on start-up, shaft isn't bent, what's your guess?

[GAJoe]

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?

 

[fixit1ddh]

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.

 

[GAJoe]

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?

 

[ILENGINE]

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.

 

[GAJoe]

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.

 

[raider820]

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.

 

[yardiron]

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" .