Ok here we go simple and concise as I can make it.

Let us talk about the valve adjustment. Usually when these gaps get too big it doesn't affect anything other than top end power. Idle, transition and so on are not affected. Just the Max power output. Yes if the gaps get extreme then it can affect lots of things but anything from the desired 0.002" to about 0.010" will not affect idle or transition. Only top end.

Turn the prop on the engine to TDC. If you bring it up against compression and slowly turn it about 90degs the prop shaft will sit in a notch like position. It won't be up against compression and it will be only about 1/2 way through the compression area. This is where the crankshaft must be to adjust the valve clearances. If in doubt about what is going on watch the tappets move as you turn the prop shaft. This will show the tappets moving up and down as they open and close the valves. The valve must be close meaning these is some gap between the tapped and the valve stem when you are adjusting these clearances.



Look at the drawing below. This drawing shows the tappet, the valve, and the pushrod. The adjustment screw, and the locking nut.

To adjust the gap or the tappet clearance you loosen the locking nut, and dial the adjusting screw.

Couple of tricks. First some of the YS engines have a slotted adjusting screw for a small jeweler type straight blade. And some of them have a small allen or socket head set screw style.

Use the allen wrench or the slot in the screw head as an indexing mark. The adjustments here are very small.

Place the little wrench on the locking nut. Then place the screw drive in the slot. Loosen the locking nut without turning the adjusting screw. This is tough...that is why I say look at the slot as the index. Inmost cases the locking nut needs only 1/2 turn or less to give the required adjustment.

Once the locking nut is loose. Twist the adjusting screw to get your desired gap. The gap can be set using the feeler gauge or some shim stock that is 2 thousandths of an inch. 0.002" This is a trial and error adjust the screw and check the gap with the guage material. The gap should be be " snug on the 0.002" strip, but it should not be tough to insert between the tappet and the valve.

Once the screw is adjusted properly carefully tighten the locking nut. The adjusting screw will want to turn tighter with the locking nut...but you need to hold it in place at the right spot while tightening the locking nut. This is why you use the slot in the screw or the "L" of the allen wrench as the indexing mark.

The locking nut doesn't get torqued down like a Nascar tire lug nut. Its just tight...Don't over tighten or you will break the adjusting screw. I have done it...It suck...Easy to repair and replace the screw but it sucks.

So tighten the locking nut. Then use your gauge again and check the gap. If the 0.002" gauge is a little tight in the gap that is good....then wiggle the tappet and look at the gap with your eye, or feel the little click click back and forth as you wiggle the tappet back and forth. If you have a little click click or can see a gap the Valve is closed. This is GOOD and what you want.

This is the setting for max performance. To be on the safe side the 0.002" gap is fine. You might be down 100rpm, but the valve will no doubt be closed...this is what you want.

Caution it is very easy for the adjusting screw to put pressure on the valve and cause it to open. So you have to have a gap...that gap should be small. The instructions say 0.002" I try to set mine a little tighter than this with the above instructions.

If you use the OS kit...there are two little metal strips as seen in the photo. The thicker one is 0.1mm and the thinner one is 0.04mm So your setting should fall between these two for thickness.

Basically you make it tight enough that the thicker one will not slide through. And the Thinner one does slide through it.

these are the basics. Its and easy 5 min task.


Clean the valve cover gasket and engine head with denatured alcohol and re-install. If you are not careful when taking it apart you can tear the gasket. If the gasket sticks to one side or the other this is fine just leave it there. Valve cover screws again are not torqued tight...they are just tight...A little beyond snug.