Rebuild engine, replace engine, sell as is?

[Rickster]

You could be...or your might need to spend engine again to confirm you are on the compression-firing stroke , and not just half way to it!
You should feel a little bit of up and down on the adjusters if you are at TDC. If one or both are tight, you might not be at TDC.
Some use a straw in sparkplug hole to double check piston is topped up.
In short, if you feel NO movement in the adjustment arms....be very careful not to change the adjusters yet! Bring that hole in the sprocket around again....then check for movement.
Stig

Great idea to check the piston position, Stig. Believe me, I've spun that engine over at least a dozen times, but I understand that I must get it correct before doing any adjustments.

[Rickster]

Thx, Stig. I rotated the flywheel as you suggested and no T mark. I've spun that bugger at least 20 times, so I'm confident that there is no T. I did see timing marks, but that does not help me. But, I really liked your idea of putting a straw in the sparkplug hole to make sure that the piston is at the top. But now I have new questions.  What is the shape of the top of the piston?  The reason that I ask is that when I first stick the straw in the hole, it hits something a few centimeters beyond the plug hole depth, but then the straw slides toward one side and goes down another 3 inches. I've repeated this "finding" several times and it seems very odd to me. Without pulling the engine and the head off, I cannot get a good look inside the plug hole to see anything informative. Attached is a photo of a GY6 piston from a YouTube video, and by the looks of it, the piston is essentially flat. So, now I'm concerned that I have a hole in my piston. Thoughts?
Thx,
Rick

[Stig / Major Tom]

Doubt you have a holed piston.
What did you find with the rocker arm movement when your big hole on sprocket is perp to cylinder head?
Movement,  yes/no?

Hole 90° and rocker movement = check your valve lash🙂
Stig

[Ruffus]

Iahawk, another newbie question if I may?  Because I have the valve cover off, I thought I should check and adjust the valve clearance, if needed. Per YouTube videos and per the service manual (see pic), I turn the flywheel until the T mark aligns with the index mark on the left crankcase cover. I swear that there is no T anywhere on that flywheel!!  So, instead, I turn the flywheel until the hole in the gear is TDC. I'm fairly confident that this is the correct position for adjusting the valves, but I'd like some assurance that my assumption is correct?
Thx!

Your mark on this flywheel should look like that.
Pls see pics (arrow) and piston.

I don't doubt your words, but it would be unusal not to have such a T- mark on it. To see this, the second plug has to be removed.
A straw is not really advisable to check TDC
(turns aside/ bends), a more rigid piece of something like a thin screwdriver shows better results (if neccessary).

Take your cellphone and shoot some pics through the sparkplughole in different piston positions.
This is how your piston looks like.

 For holed piston: I did not see any holed piston on a 4-stroke motor within last 25 years. Mostly 2-stroke engines are suffering from (lean gas mix/ bad 2-stroke oil/wrong sparkplug).
.

[Iahawk]

I've always thought that the reason you adjust valves at top dead center is because both valves are closed and you can do both at once. In theory (and Donyboy73, small engine Doc on youtube, has always done this) you can adjust either valve, intake or exhaust, as long as it's closed and there's no pressure from the cam on the rocker. I've seen him do this many times..if you can't find TDC then spin the motor so one valve is clearly open and there's no pressure on the other.  Check that adjustment and then spin the motor again to open that valve, and repeat.

and Rickster - your spark plug hole enters the motor at a 45 degree angle, so if you use something flexible, like a straw, it could very well hit the top of the piston and then slide across it until it hits the cylinder wall. A wood dowel would be ideal. It doesn't really matter where the dowel hits the piston, as long as it's hitting it. Turn the motor over a couple of times with the dowel in the hole and watch it go up and down. You'll see where the piston is at the top of its stroke.

TDC should be easy to spot. Cam sprocket in correct position (hole at top and horizontal lines parallel with head) and the cam lobes will be horizontal, not lifting up on either rocker arm.

[Iahawk]

and thank you, Ruffus, for always having relevant pictures!