valve check-cam and flywheel won't stay in place

[classacted]

I've worked on both 50cc 139qmb and 150cc 157qmj gy6 engines, and when I line the flywheel and holes in the cam gear to top dead center so I can check the valve clearance, for some reason every engine I've worked on the cam gear and flywheel always move one way or the other maybe about an eighth of an inch, and THERE it will rest. there seems to be something within the engine that won't let it stop and be still at the perfect spot. I tried removing the tensioner with no success.
is there any tool or trick that can help? it's funny how the online video's never seem to have this problem.
thanks.

[FrankenMech]

There is play between the rings, piston, wrist pin, rod, and crank, so it will never stay exactly where you put it but it is close enough.

[classacted]

thanks FrankenMech,
yeah, that's exactly what I ended up doing.

[GrumpyUnk]

I think it is permanent magnets in the flywheel. I KNOW a Honda CB350 will NOT stay in position to adjust valves or points. It drove me nuts trying to set clearances and have the thing just weasel its way out of position. I attributed it to the rotor magnets...
A solution for a geared machine is to put it in gear once you get to TDC. That won't work with a CVT...
I think a rubber-belt wrench could be wrapped around the flywheel or variator and jammed to the ground which seemingly should keep things in position once set. An alternate would be a box-end wrench on the fastener in the center of the flywheel propped on a block to keep things in position.
Given the only thing really adjuste(AFAIK) is the valve clearance, it is correct IMO, that you should be close enough to TDC that the valve clearance will likely not be affected at all. Both cam lobes should be on their heel or very close to it, which is where you want to adjust the clearance.
tom

[classacted]

thanks GrumpyUnk, the way this behaves, the flywheel magnets idea makes sense. you can't seem to wait until it goes away, and fwiw, the flywheel socket is a thin walled 13mm. I like the strap wrench idea, it's an underrated tool.

[FrankenMech]

Just don't buy strap wrenches at Harbor Freight, -they break even with my weak diabetes and age ravaged hands. Totally worthless. You shouldn't have to worry about it for a valve adjustment as Tom says.

[snaker]

Approximately half (180 degrees) of a cam is the heal and anywhere along the heal will give the same gap.
Some valve lash instructions call for turning the engine and watching your target valve close.
Turn maybe 30 degrees more just to be sure you're not on any part of the lobe or any possible decompression bumps and set the gap from there.

[GrumpyUnk]

What Snaker says is pretty much true for most engines. There is one exception that I have no explanation for, and that is a B&S vertical single that instructs to go some number of degrees past TDC on the compression stroke to adjust the valves. Makes no sense to me, but it's what they instruct.
Got any ideas?
tom

[FrankenMech]

I have no clue. I haven't worked on one of those B&S singles in 60 years. I do have two that need work but they are not on my bucket list.

To adjust most valves I just watch the motion of the rockers and adjust somewhere between when they close and open, somewhere on the heel of the cam and don't worry about a precise position.

[classacted]

FrankenMech,
I bought a strap wrench from Temu if I recall correctly the handle was red and the strap was yellow. it was quite heavy and well built. I was pleasantly surprised at how well it worked and it was super inexpensive.

[FrankenMech]

THANKS! They are a handy tool, I will give them a try. I had an old craftsman that worked but it eventually gave up, -just wore out. I even gave a thought to making one of my own at one time.