Originally Posted by afm

First, you need to attach the degree wheel to the engine. I take a broken prop, and cut both blades off. This allows you to use it as a spacer, and mount the degree wheel on the crankshaft. Tighten your prop nut tight enough that the wheel won't slip unless you want it to. You also need a reference pointer. I used a large metal paperclip--the kind with handles, sort of like a clothespin--and soldered a stiff wire to it. You can clip the paperclip to a cooling fin, and bend the pointer so it's pointing towards the center of the degree wheel. The whole idea is to be able to accurately and repeatable measure the rotational position of the engine. The more accurate your clip is, the better your numbers will be.

It's time to measure now! Take the glow plug out, so the engine turns easier. A flashlight will make measuring slightly easier--if you shine it in the spark plug hole, you'll be able to see the ports better.

Exhaust port first. Turn the engine counterclockwise until the piston top is going down, and just opens up at the top of the exhaust port. Write down or memorize the number on the degree wheel, or turn the degree wheel (but not the engine) so the pointer is at zero. You want this point to be the point at which the gases will just be able to flow through, with the piston top at the top of the port. Now, turn the engine to move the piston down, and keep turning until the piston has gone back up, and stop just as it reaches the top of the exhaust port. If you set your pointer at zero, you now have the exhaust port timing. Typically, lower timing numbers are for lower rpm. The higher numbers, are typically for higher rpm.

Repeat this step for the intake port (crankshaft window), which is in effect a rotary valve. You will measure from the instant the port opens until it's fully closed. Rotary valve engines have longer timing because they don't open until the piston is moving up at BDC--creating a vacuum in the crankcase--and they close just after the piston reaches TDC.

Now, do it again for the transfer ports. Usually, there are two mirror image ports, one on each side of the exhaust port, then there is a third transfer port that's opposite of the exhaust port. This third transfer port is called the boost port, and it's common to have different timing than the other two transfer ports.

Now you've measured the port timing on your two stroke model engine.

AFM