Quote:
Originally Posted by paulsstag
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.
|
Very good explanation, except for the sneer about finding TDC with a chopstick or a straw, the way it has been done for - oh - 110 years or so. Super sophisticated methods of finding top dead center are not neccessary for valve adjustment. All you really need is to be around TDC on the compression stroke/power stroke. Why? Think about it. You aren't going to get any compression worth talking about if one or more valves are open on the compression stroke are you? And what kind of power are you going to get if one or more valves are open after the spark plug fires on ye power stroke, right? So, the simple, time honored way to do this is to jiggle a chopstick, or whatever, in the spark plug hole to feel where the piston is. Pull it back up, or out and advance the engine a little with your hand on the kickstarter. Check it again, and repeat until most of the stick is out of the cylinder. Now put the stick back in the spark plug hole and very slowly, using the kickstarter, watch the stick rise until it stops rising. If you keep going, the stick will stay up at the same height and then just start down. Half way between from where it stops coming up and starts going down is utterly and absolutley TDC. This is called TDC "dwell". Anywhere in the TDC dwell area is where one sets the valves. On the compression stroke, remember. How do I know when I'm on the compression stroke? Well, you put your finger over the spark plug hole and.....Jeez, do I gotta' tell you everything??? ARH
