Quote:
Originally Posted by Viperbmw69
Can you explain that a bit more? How do I know if it's at the top? What's the pencil for?
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I hope i can help with the explanations to what everyone is saying. First off the pencil or screwdriver or chopstick or straight piece of wire is put down the spark plug hole so that you can feel the top of the piston, so when you rotate the engine you should feel the piston move up and down. This is a very crude way of confirming top dead center or TDC.
Next : with these being 4 stroke engines the crankshaft and piston assembly makes 2 full revolutions and the camshaft ( controlling the valves ) makes one revolution, so the camshaft is geared to turn at half the revs per minute (rpm ). It therefore takes 2 revolutions to make the complete the 4 strokes needed for a complete cycle.
With this being said the piston gets to TDC twice to complete this cycle At the TDC of one of the cycles both valves are closed briefly so that the piston does not hit them , the exhast valve is closing and the intake valve is about to open. If you set the valves at this TDC then the valves will usually be very loose. The next TDC is when both valves have been closed for a longer period of time so that the air in the cylinder can be compressed and ignited by the spark plug . It is at this TDC that you should set the valves.
If you watch the valve action while turning the engine you will see a time when the exhast valve is closing and almost immediately the intake valve starts to open ( don't set the valves here ) When the intake valve finishes closing then the piston is about to head back up to TDC, When it reaches TDC this is the time to set the valves. As a confirmation you can put your thumb or finger over the spark plug hole and actually feel the air in the cylinder getting compressed.
I hope this helps.