question about pistons at TDC with regard to valve adj.

[emzdogz]

Last night I checked my valve adjustments (again) and I didn't need to change them.

I have just been using the mark to the right of "T" to find TDC. I do the valves for one side, and then wind it carefully around once to that mark again to do the other side. If I accidentally pass the mark, I go around again.

I read in the post about setting up a bike new to you (things to check) about feeling the top of the piston to determine which side is on TDC, or that I am generally at TDC. But when I stick a thing in (I think in the post I mentioned it referred to a knitting needle, but I used a thin flat screwdriver) there to touch the tops of pistons, really, they feel to be in about the same position on both sides. At the same height. Not like one is way down in the cyl and one is way up.
Is that the way it should be? That at TDC, both pistons are equally far "up"?
or if not, how much difference should there be?

Is it OK to just go by the mark by the T? or, why would it be important to check the top of the pistons by touching them to make sure they are "up" before doing valves on that side?

also, when you're winding the nut to the left with a wrench to turn the engine through, what is that pronounced "click" I hear just before the point where it wants to swing on through on it's own? It's like at the point just before I need to be careful to hold the wrench so it doesn't swing through and pass the mark by the "T".

thanks

 

[Naturil]

The most important thing is that you adjust whichever valves are closed at TDC [adjusters should be slightly slack] and then rotate around back to TDC again and adjust the other side (which should now be closed).

 

[emzdogz]

That's how I have done it. thx

 

[grepper]

>Is that the way it should be? That at TDC, both pistons are equally far "up"?
Yup, unless you have a rephased engine, they both go up and down at the time. It's 180 degree stroke engine, each side is doing something different. i.e One is on the compression stroke while the other is on the exhaust stroke, both traveling up.

See Bill's Valve adjustment video
It explains how you want to find TDC so there is not any pressure on the valve, plus it's normal for it to want to jump past TDC when your turning the motor.

 

[emzdogz]

Yup, unless you have a rephased engine, they both go up and down at the time.

thanks, that's exactly what I was wondering about.

Will re-watch Bills's video - have seen it already, and really appreciate his efforts.

edit: correction have not seen all the valve vids...so, will do

 

[grizld1]

No, the XS 650 is not a 180* engine. It's a 360. A 4-stroke cylinder fires every 720* (every other revolution). On a 360* twin, the pistons travel together and fire alternately (one every 360*). At TDC, one cylinder is primed to move down on the power stroke (valves closed, both rockers loose) and the other is set to move down on the exhaust stroke (valves open, both rockers tight). The valve action is controlled by the cam lobes. TDC is used as a reference point to be sure that the rockers are on the base circle of the cam on the cylinder that you're adjusting (the one entering the power stroke). You don't need a video to figure that out.

 

[cros36]

fwiw, i use a flash light through the spark plug hole to see TDC. then i use a small piece of 3/4 pvc to hold my wrench in place while i do my adjustments

look ma! no hands, or slipping past TDC!!

 

[grepper]

No, the XS 650 is not a 180* engine. It's a 360. A 4-stroke cylinder fires every 720* (every other revolution). ...... You don't need a video to figure that out.

Sorry, my mistake on the terminology...
Nope, don't NEED the video, but I think it always helps to see it.

 

[emzdogz]

What if:

so what would happen if you adjusted them at the same time? in other words if you didn't rotate the crank around before setting the other side. What if you set it to TDC and set all 4 valves at once, without rotating in between.

Would you then just have one dead cylinder?

(if this is the stupidest question asked on the forum today, I'm sorry)

edited to just say "all 4 valves", not "all 4 exhaust valves"...doh..

 

[Tim H]

You would probably set the cyl on the exhaust stroke to an unknown looser tolerance than the cylinder that is on the compression stroke.

As grizld1 stated, you are setting it to a point you know the rocker will be riding on the base circle of the cam. That is where you need to set your clearance.

If you set it in another place, you could be riding on the lobe and which you will be adjusting your rockers to varying states of too loose.

 

[5twins]

I don't think you'd have one dead cylinder but the valves would be very loose on that side and make a lot of racket. Posts pop up quite a bit around here from people who have done just that and are wondering why their motors make so much noise.

When I do a valve adjust, I don't look at the piston but rather watch the valves opening and closing. This tells me which cylinder is the one at TDC on the compression stroke and ready for adjusting. On the cylinder I want to set, I watch for the intake valve to open and close, then the exhaust, then the intake again. When the intake opens and closes the 2nd time, I know that cylinder is coming up on TDC of it's compression stroke. I then start watching the rotor mark and stop turning the motor when it reaches the "T".

 

[PHeller]

Is TDC based off the right or left cylinder?

 

[Tim H]

Is TDC based off the right or left cylinder?

Unless rephased they are the same.

 

[grizld1]

Em, what would happen is exactly this: you'd be unable to get any clearance at all on the exhaust valve of the cylinder entering the exhaust stroke, even with adjuster set flush with the rocker arm. Bottom line: confusion is impossible, unless you have a very serious talent for it.

 

[emzdogz]

"confusion is impossible, unless you have a very serious talent for it"

hahah! love it. OK, thanks.

 

[PHeller]

So both pistons are on the compression stroke at the same time? I'm thoroughly confused. Wouldn't that mean they act like a big single?

DOH: after looking around at the animated GIF of how the XS650 runs, I guess they do rise and fall at the same time.

Totally new to me.

Derp.

 

[Tim H]

The pistons are both traveling.side by side hand in hand. It's the cam that is off so when one piston is on compression stroke, one is on exhaust.

Both pistons will be physically at top dead center, but they will be at different stages of the cam.
This was explained a couple posts up better than I am.

 

[PHeller]

Hence why if you switch spark plug wires side-to-side, nothing should change.

Sad thing...it definitely change things when I switch plug wires.

 

[PHeller]

Why does setting the timing based off the #1 Piston Matter?

See this post:
http://www.xs650.com/forum/showpost.php?p=104102&postcount=2

 

[5twins]

What you linked to is unrelated to what's being discussed here. That's about making new timing marks when none exist due to the PMA swap. And I don't think it really matters if you use the #1 or the #2 piston, results would be the same except maybe if the motor was rephased. He probably chose the left side because you can more easily observe it while looking at the flywheel.