How to find the compression stroke TDC to adjust valves on 155cc Yamaha single cylinder engine

[XB_Fan]

Good evening!

I have never adjusted valves before so here goes. This is on a 2020 Yamaha SMAX 155cc 4 cycle scooter.

I have identified the exhaust and intake valves and can easily turn the engine with a socket.

My big issue is being 100% I am adjusting these on the top of the compression stroke at TDC.

I inserted a clean wooden dowel and cranked until the piston came all the way to top. Am I right in assuming that I have found the compression stroke if the rocker arms have a little play in them? And if I am on the top stroke of exhaust, they are tight and don't wiggle much at all?

ETA: Yamaha has no timing marks visible unless you remove the radiator and the water pump. That's crazy, so I am finding another way to determine.

 

[FowVay]

The compression stroke will be when all valves are closed. You can use something as simple as a plastic straw inserted in the spark plug hole to determine when the piston has come to the top of its travel by slowly turning the crankshaft with the appropriate tool.

Turn the engine over slowly as you watch the straw come up and out of the plug hole. Once it stops traveling upward you're at TDC. If all valves are closed at this time then it's TDC on the compression stroke.

 

[dnewton3]

OP - Yes; you're on the right track.

To assure yourself 100% accurate, turn the engine over a few times. Observe the valves relative to the dowel action. The exhaust valves will always open prior to the intake valves. So rotate the engine a few times and watch for the exhaust cycle to end as the dowel comes to its highest point. That will be TDC at the beginning of the intake.

 

[NibbanaBanana]

You're gold. It's that simple.

 

[Cujet]

It is good to know that at TDC, compression stroke, both cam lobes are evenly split, pointing away from the follower's roller.

The picture below shows the orientation of the cam lobes at TDC, compression stroke.

 

[XB_Fan]

OP - Yes; you're on the right track.

To assure yourself 100% accurate, turn the engine over a few times. Observe the valves relative to the dowel action. The exhaust valves will always open prior to the intake valves. So rotate the engine a few times and watch for the exhaust cycle to end as the dowel comes to its highest point. That will be TDC at the beginning of the intake.

Thanks for the tips! This doesn't seem to terribly bad. On the plus side, the access to the valve cover on this scooter is literally two screws on the floor board and there it is.

 

[XB_Fan]

It is good to know that at TDC, compression stroke, both cam lobes are evenly split, pointing away from the follower's roller.

The picture below shows the orientation of the cam lobes at TDC, compression stroke.

Thanks for that pic. I can visualize that. I'll just take my time.