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Author Topic: Valve Information  (Read 1499 times)

Thumper

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Valve Information
« on: September 15, 2008, 02:55:57 PM »
I recently had to replace a threaded pushrod adjuster. A few days later a person on another RE forum had adjusted his valves - but not while they were at TDC compression. They got messed up pretty good and one was loose enough to fall out of the cupped holder. I wrote up a post to help him out. The following information based on those two experiences for future reference:

Two sections are contained:
I Background information
II Replacing a pushrod or threaded pushrod adjuster

I Background information:
-------------------------
You can observe the engine cycles by watching the valves at the rockers.
Notice that the pushrods push against the rockers while the other end of the rockers push down on the end of the valve stem against the spring opening the valve up.
Slowly turn the engine over with your hand on the kickstarter and watch the rockers.

When both rockers are low at the pushrod end and high at the valve stem end here is what is happening:
The pushrods are *not* pushing up against the rockers and the rockers are *not* pushing the valves open.
Both valves are closed.
This is compression (TDC).
Note that both of the pushrods spin freely when the engine is at TDC compression.

The next stroke is combustion and there is no change to the valves.
The mixture is ignited and the piston goes down to BDC.

When, on the exhaust valve, the rocker is high at the pushrod side and low at the valve stem side here is what is happening:
The pushrod is pushing up against the rocker and the rocker is pushing down on the exhaust valve to open it.
The piston has come up and forced spent gases out through the exhaust valve.
Note that:
a) The intake valve is closed, allowing only one exit for the spent gases
b) The intake valve pushrod spins freely
c) This is TDC exhaust

When, on the intake valve, the rocker is high at the pushrod side and low at the valve stem side here is what is happening:
The pushrod is pushing up against the rocker and the rocker is pushing down on the intake valve to open it.
The piston has gone down and sucked a new air/fuel mixture into the cylinder.
Note that:
a) The exhaust valve is closed, allowing only one entrance for the new air/fuel mixture
b) The exhaust valve pushrod spins freely
c) This is BDC intake

(See also: http://en.wikipedia.org/wiki/Image:4-Stroke-Engine.gif )

So, how do you know when your piston is at TDC compression?
First, the ammeter technique does *not* work with the AVL electonic ignition.

Here is the technique I use (after removing the sparkplug, tank and valve covers as referenced in Section II of http://members.verizon.net/allofusmorrows/RE_maintenance.htm):

1) Using a small wooden dowel carefully hold it about 1.5 to 2 inches into the cylinder through the spark plug hole.
2) Slowly turn the engine over with your hand on the kickstarter and the other holding the dowel.
3) Anticipate the piston making contact with the dowel so that you can begin to raise it in concert with the piston.
   This care is necessary so that the dowel won't bind.
   First time will be the learning experience, after that you will have a feel for the technique.
4) Once the dowel is raised as high as it will go check the pushrod adjusters through the cam access panel.
   If both spin freely you are at TDC compression.
   If neither spin freely you are at TDC exhaust.

A couple of things to note at this point:
- You will probably have to rotate the engine over, raising the dowel, several times in order to get a feel for when it stops rising - and just before it 
starts to descend.
- Once you are confident, use a black marker to draw a line around the dowel to use for future reference.
  The mark will really facilitate the process.

Now:
Why do the pushrods spin freely at TDC compression and why won't they spin at TDC exhaust?
Well, the pushrods push on the valves - indirectly via the rockers - in order to open them.
At TDC exhaust both valves are open.
That is, the pushrods are pushing up against the rockers, through the rockers and those strong valve springs, and are holding the valves open - so the force won't allow you to spin them.
When it is at TDC compression, both valves are closed so the pushrods are not pushing up against the rockers and are therefore, loose.


II Replacing a pushrod or threaded pushrod adjuster:
----------------------------------------------------
Remove the sparkplug, tank, cam cover and valve covers as referenced above.
Put the engine at TDC compression.
Loosen the locknut on the threaded adjuster and fully screw it in.
Pull the pushrod right on up and out through the top at the rocker.
(Now you can replace it or the threaded adjuster).

You now can replace the pushrod:
Just fish it down in reverse fashion.
Pull the rocker arm upwards as far as it will go to give room for the pushrod to fit up and underneath it.
Use needle nose pliers to hold the pushrod at the bottom end in order to first seat it at the top.
Now pull it into place at the bottom with the pliers.
Using your fingers or 13mm and 8mm, thread the adjuster until the pushrod is seated properly.
(e.g., spins freely, no up/down and minimal side-to-side).

Now rotate the engine several times and put it back to TDC compression.
You are all ready to set the valves.
(See Section II of http://members.verizon.net/allofusmorrows/RE_maintenance.htm for details on valve adjustment).


Matt
« Last Edit: September 15, 2008, 04:48:44 PM by Thumper »

REpozer

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Re: Valve Information
« Reply #1 on: September 15, 2008, 04:24:32 PM »
Thanks , Thumper

I intend on using your notes and my Electra Shop Manuel on my next tune up!
2008 AVL Classic Bullet in British Racing Green
REA # 84 ( the first time)

1Blackwolf1

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Re: Valve Information
« Reply #2 on: September 21, 2008, 07:31:14 PM »
  Thanks for the detailed explanation, should make it much easier to adjust valves in the future.  Will. 8)
Will Morrison
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