The "jerk back" indicates a sheared flywheel key. That allows the flywheel to slip on the shaft and throw off the ignition timing.

From the service manual-
"In order for an engine to run effectively and efficiently,
the spark must ignite the compressed air-fuel mixture
when the piston is in a specific position to deliver
maximum power. This position is known as Before
Top Dead Center (BTDC). If the mixture is ignited too
soon, kickback can be experienced due to preignition.
If the mixture is ignited too late, loss of power can be
experienced due to retarded spark. A partially sheared
or the wrong flywheel key can cause kickback, loss
of power, or an inability to start the engine with a good
spark. Check the flywheel key if one of these
symptoms exist. See Chapter 9 under "Service" for
the disassembly procedure."

Here's a link to the service manual

Here is a link that might be useful: Link

"A partially sheared
or the wrong flywheel key can cause kickback.. See Chapter 9 under "Service" for the disassembly procedure"
I don't get it
The manual say only that and nothing else. What is "flywheel key"? I'll disassemble the engine again and look to see if there has some misalign.
Thank you

Well, I checked for movement between the flywheel and the shaft and there is no movement.

The flywheel (Woodruff)key keeps the flywheel (on top of the engine) from rotating on the crankshaft. It is designed to shear if the crank stops abruptly, since the heavy flywheel wants to keep spinning. The magnets in it help generate the spark to the plug

The flywheel can be too snug to move by hand and still have sheared the key-
Thing is- you pretty much have to hit something with the blade to shear the key. IF it was running fine when you stored it, the key should be good.
Did anybody else "mess" with it?

To check it, you have to remove the flywheel.
Page 65

From the comments thus far, it sounds like a sheared key,
Tec part# 611004. Also, these engines utilize a compression
release mechanism which is PART OF the camshaft. Remove the
spark plug wire, remove the valve cover, and rotate the
flywheel in a clockwise rotation by turning the blade. There are 2 rotations of the engine when piston reaches the
top of its travel. You can "feel" this by putting a small
screwdriver or other probe in the plug hole. With each TDC
position, the magnet on the flywheel will have just passed
the 2-legs of the ignition coil and will be to your left at
the 7 o'clock position of the flywheel rotation. If the flywheel magnet is located in the center of the coil legs
when you find TDC of the piston travel....flywheel key IS
SHEARED. In in the "PROPER" 7 o'clock position....then the
kickback is cause by an improper valve adjustment OR the
mechanical compression release is NOT WORKING. Valves at TDC compression stroke should be @ .004 each. MCR problems
requires replacing the camshaft.

T-Man

Well, T-Man,
I did what you have told me to do. At TDS is the other side of the magnet (180 degree), actually the weight balance. IÂll take apart the engine to see why.

I didn't go too fare to figure it out. the key is completely gone.

Is the engine getting any fuel?

Where'd the key....GO ?? If it shears, there's usually
part of it still in the keyway of the crank or flywheel.
Nonetheless...CAN'T RUN WITHOUT ONE...replace it with a
Tec# 611004 key.

T-Man

popping & back fires when trying to start. pulled valve cover off &adjusted valves. they were so loose they fell off. after adjustment it started and ran great.