My old Troy Bilt tiller has an original 8 HP Briggs flat head engine that has run flawlessly for years . (well cared for and minimal use).  I start it up and go towards the garden without even dropping the tines in dirt and it makes a horrible screeching sound followed by very loud backfire and then dies and would not restart.
 
I take it apart thinking there's catastrophic damage but the cylinder/crank/bearing looks like new and valves operate as they should. The only problem I found was a sheared flywheel key.  I decided to lap the vales, adjust the valve clearance, adjust coil clearance, replace flywheel key and clean it up.
 
I started it up removed from the tiller and the same screeching noise happened.  I don't think the noise is the starter recoil assembly.  What could it be and what should be the next steps? I would like to repair this engine if I can.  The flywheel key must have originally been sheered by the backfire or else it wouldn't have started being this far out of timing.  It did not sheer again this latest time. 
 
Does this old engine have a history of issues like this? I'm grasping for theories/next steps.
 
TIA

I suspect the engine is OK, aside from the flywheel key. Here's my guess at the sequence of events:
 
1. Tiller mechanism is stuck, not the engine.
2. Squeal is the drive belt against the pulley. Since the mechanism is stuck, the belt can't move.
3. Flywheel pin shears.
4. Because engine is now out of time, you get the backfire, and it stops.
 
See what's impeding the tiller mechanism, fix it, then replace the flywheel key.
 
Edited later on: Sorry. I only now notice that you wrote that you tried the engine off the tiller and it's still squealing. Just forget everything I said.

+ 1 ^^^^ on Conrads diag..
 
I would now remove the pull starter and inspect for anything dragging, touching that shouldn't be, etc
 
I believe your starter assembly, BOLTED TO THE RIGHT END OF THE CRANKSHAFT Is aluminum.
There's I believe 5 little balls inside (not your issue), BUT that squarish part should rotate nice and smooth. There's a tiny hole in the middle for OIL. See if it spins smoothly. Lubricate inside that hole with engine oil in any event.
 
The clearances in there are pretty tight but if rusted up, dry, could make noise.
 
You can also pull off the starter cover, while the engine IS RUNNING, and see if the noise goes away..
Obviously, very dangerous so bolt down the engine.
 
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But what would cause a big backfire event after the screeching sound?  I assume the backfire caused the sheer key to sheer because it wouldn't have started and run normal up to that point with the flywheel timing that far off. The backfire sounded like a gun being fired.
 
I did check the starter and it's clean inside and no sign of any damage. The ball bearings engage normally and free wheel in the reverse direction.

I agree with Conrad, a backfire will NOT shear the key.
 
Something else caused the key to shear (sudden stop, loose flywheel, something externally
acting against the engine rotation) and then the subsequent backfire.
 
And the type of squealing you note, again as Conrad noted, sounds like a belt or idler gone south.
 
I'd pull the pull starter off while running just to rule it out.  
 
Can you post any pic's or upload a YOU Tube video of the machine acting up?
 
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Is the squeal really the same with the engine connected to the tiller and removed from the tiller?

Almost the same squeal.
 
The keyway could have already been sheared or damaged (walking wounded) and maybe the flywheel rotated out of the correct timing position and that caused the backfire.
 
Maybe whatever caused the sudden squeal caused the flywheel to loose it's grip on the shaft???
 
Internally, all I can think as possible causes of the squeal would be bearings, connecting rod, or cam?? I really don't think it's the starter.  I'll have to dig into it some more but I need to make a better testing platform for the stand alone engine.
 
Thanks for the replies.
 

Can you leave the engine bolted to the tiller and just un-bolt and remove EVERYTHING 
from the end of the crankshaft?  
 
(The tiller will be the mount only).
 
As for rods, cam, etc  screeching,   very, very un-likely.
 
Try turning the engine over by hand (or pull starter) slowly listening for anything unusual..
 
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I finally figured out what was going on.  There were two issues that cropped up a couple months after I rebuilt the starter due to the pull rope not engaging reliably.
 
1. When I reinstalled the starter and tightened it up to the flywheel there's a washer that goes between the two.  It looks like an ordinary washer but it's not.  It's called a Bellville washer and is slightly concave.  It's meant to go on only one way.  Cup side against the flywheel so the starter compresses against the flywheel to lock it in position.  I didn't even realize it was a directional washer and had it on backwards.  That resulted in sheering the flywheel key and the subsequent big backfire event.
2. When I rebuilt the starter I cleaned all the ball bearings and their slots.  These parts are not meant to be oiled.  However the starter's bore and the male crankshaft end need to be very clean and lightly oiled. Lack of that is what caused the squeal.
 
Thanks for the replies.

davefr
 
2. When I rebuilt the starter I cleaned all the ball bearings and their slots.  These parts are not meant to be oiled.  However the starter's bore and the male crankshaft end need to be very clean and lightly oiled. Lack of that is what caused the squeal.
 

 
Wish you would have done ^^^,  from my first post, but glad it's fixed...
 
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