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Trying to understand and find TDC of the compression stroke

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I'm trying to find TDC of the compression stroke.

1. "If the exhaust valve opened as the piston rose then it is the exhaust stroke" - Is this before or after I see the T mark as I turn the engine counter clock wise?

2.  Immediately after the T mark is displayed, as I turn the engine counter clockwise, should the exhaust valve move up after TDC of the compression stroke?

3.  When running does the engine run counterclock wise or clockwise?

The manual reads, 

Quote

Using a 12 mm socket on an extension inserted into the crankshaft cap opening and large ratchet or breaker bar, slowly and carefully turn the crankshaft counterclockwise while watching for timing marks in the crankcase cover upper hole. Align the mark for the T (not the T itself as shown in the photo, but the small line to the left of the T) with the index mark (slot) in the top of the hole in the cover and make sure the piston is a TDC of the compression stroke (not the exhaust stroke) and both rocker arms have some play.

Note: Probably the easiest way to tell if the motor is actually at TDC of the compression stroke (and not the exhaust stroke) is to watch the valves as the crankshaft it turned. At TDC of the compression stroke both valves are closed (so that there is a sealed combustion chamber for the power stroke which comes next). If the exhaust valve opened as the piston rose then it is the exhaust stroke and not the compression stroke (and the crankshaft needs to be rotated another full turn).

 

Edited by wazzabie

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With the intake valve cover removed rotate the engine counter clockwise with a T handle wrench while watching the intake valve rocker arm.  Watch as the intake rocker adjuster screw goes down and opens the valve then comes back up to close the valve.  From the point where the intake rocker arm stops moving back up continue to slowly turn the engine in a counter clockwise direction until the t mark on the flywheel lines up with the mark on the case.  This will be TDC on the compression stroke.

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15 minutes ago, oldman said:

With the intake valve cover removed rotate the engine counter clockwise with a T handle wrench while watching the intake valve rocker arm.  Watch as the intake rocker adjuster screw goes down and opens the valve then comes back up to close the valve.  From the point where the intake rocker arm stops moving back up continue to slowly turn the engine in a counter clockwise direction until the t mark on the flywheel lines up with the mark on the case.  This will be TDC on the compression stroke.

Thank you for the help.  I followed what you wrote.  If I continue to turn past the TDC on the compression stroke in a counter clockwise direction should the intake value go down immediately?

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Turning CCW past TDC on the compression stroke is called the combustion stroke. Both valves stay closed until near BDC, then exhaust valve opens and exhaust stroke begins. Just remember suck, press, bang, blow, repeat.

Edited by FightingRed

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I followed the instructions 

1 hour ago, oldman said:

With the intake valve cover removed rotate the engine counter clockwise with a T handle wrench while watching the intake valve rocker arm.  Watch as the intake rocker adjuster screw goes down and opens the valve then comes back up to close the valve.  From the point where the intake rocker arm stops moving back up continue to slowly turn the engine in a counter clockwise direction until the t mark on the flywheel lines up with the mark on the case.  This will be TDC on the compression stroke.

I followed these instructions but the intake valve went down after I went past TDC of the compression stroke.  The intake valve immediately started to go down.

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Then you were at TDC of the exhaust stroke and needed to go around another 360*.

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Right. Compression and exhaust are “up” strokes while intake and combustion are downward.

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All of the above work but here is a more visual method:

Remove the CDI cover, turn over the engine in a forward direction (CCW) until the rotating magnet (Advancer) aligns with the CDI sensor; that is 10 degrees before TDC compression stroke (and the "F" mark should be visible on the rotor).

Slowly turn the crank another 10 degrees until the T mark aligns with the case, engine is now TDC for the power stroke. And both valves should be closed and the rockers lose.

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Finding TDC on compression:   All you need to see is the intake rocker arm, and be able to rotate the crankshaft.  Rotate the engine in direction of rotation until you see the intake rocker jussssssssst start to move (start of the intake stroke).  Then turn the engine again in direction of rotation one complete revolution (360 degrees), and you will be at top dead center of the compression stroke (TDC on compression).  Fine tune lining up the T mark exactly and you are there.  This is the method that is usually taught in service schools and works on all types of 4- stroke engines large and small, gasoline or diesel.  :thumbsup:   

Old School Al

Edited by Old School Al

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8 hours ago, Chuck. said:

All of the above work but here is a more visual method:

Remove the CDI cover, turn over the engine in a forward direction (CCW) until the rotating magnet (Advancer) aligns with the CDI sensor; that is 10 degrees before TDC compression stroke (and the "F" mark should be visible on the rotor).

Slowly turn the crank another 10 degrees until the T mark aligns with the case, engine is now TDC for the power stroke. And both valves should be closed and the rockers lose.

Hi Chuck -  Thank you for the visual description.  I followed your directions and took a picture where I think the engine is at TDC of the compression stroke.  The T aligns with the the case and there is some play in the rockers.  Using my finger I can wiggle the rockers some. The exhaust has more play then the intake.   

I think the problem before is that I needed to rotate 180 degrees.  When I checked before I had no play in the rockers.

roto.png

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There is a hundred correct answers to this but I think the rightest of all is to get a full grip of what the functional process of a 4 cycle engine is. Its basic and you can apply it to any 4 cycle engine you need to work on.  I like to watch the valve train whenever possible. Another thing I like from many old manuals from back when Floyd Clymer and the like were publishing them, is to insert a long straight wire in the spark plug hole. On a vertical engine you can see precisely where the piston changes direction.

Also understanding that most, if not all valve lash adjustments are to be done when the lifter or rocker arm is on cam base circle. I.e not on lobe at all. All types of directions, which there are many, are just to assure that you have it in the right spot for adjustment. There are many degrees of a cam lobe that are the base circle, there is not one magic spot

These types instructions are most valuable on multi cylinder engines because you can save time and adjust more than one cylinder before having to rotate engine.

Just having a grip on where an engine is in process and what is supposed to happen there will help anybody with troubleshooting and tuning.

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