True Tdc

[79XS11F]

I have the head off my bike now in this on going effort to remove all question marks about why it dose not run as I believe it should. I want to know what is TRUE TOP DEAD CENTER. When I spin the engine in the correct direction I can see and feel the #1 piston stop moving upward and then it does nothing before starting back downward. There is about 2 or 3 degrees on the timing plate where this is the case. I can see that in the past the pointer has been removed because the paint at the screw is gone.
SO is TDC the middle of this range of no movement or at the beginning or at the end? I will guess that it is likely the middle but want to be sure before I reset the pointer.
Rob

[mro]

it is likely the middle

Good question.
I've never checked anywhere but I've always used "middle" for "TDC". Seems to have worked just fine on the few XS's I have had apart and on a bunch of engines I have rebuilt over the years.


mro

[prometheus578]

"Starting the day agreeing with MRO... surely a bad omen."

I'd go fer the middle, too.
TDC is just... well, not totally but almost, a meaningless reference. It only comes into play when setting up cam timing, and a few degrees play will not be enough to move the cam chain a link or two either way.
The "fire" mark, as far as timing goes, is more critical. Rotating in the proper direction, one sets timing when the mark lines up with the pointer.

[79XS11F]

Thanks for your input guys. I have adjusted the pointer to the middle of that dead spot at TDC, I had to move the pointer about 1/8 of an inch to correct it's position. I double checked it at #4 as well and the pointer is now what I will call correct.
Being that when I check my timing to that pointer could that pointer being out by 1/8 of an inch be partially responsible for the way the bike has been running? I had to move the pointer 1/8 of an inch clockwise to correct it's location.
Rob

[skids]

Re: True Tdc

Right in the middle.

Originally posted by 79XS11F

SO is TDC the middle of this range of no movement or at the beginning or at the end? I will guess that it is likely the middle but want to be sure before I reset the pointer.
Rob

[mro]

could that pointer being out by 1/8 of an inch be partially responsible for the way the bike has been running?

Have not been following or missed your recent posts about any problems. The pointer being off 1/8" should not have a noticeable effect.


mro

[mro]

"Starting the day agreeing with MRO... surely a bad omen."

Bitc...Bitc...Bitc...
What's up thar Prom,
Vespa not running and have to ride your moped???


mro

[skids]

The sprocket on the crank has, I believe, 16 teeth. The sprockets on the cams each have 32 teeth. The difference means that the rotational speed of the cams are half as fast as the crank. One tooth off on the cam is about 1/32 of a cam's revolution or 11.25 degrees. This would be 22.5 degrees on the crank.

Originally posted by mro

Have not been following or missed your recent posts about any problems. The pointer being off 1/8" should not have a noticeable effect.


mro

[79XS11F]

I know that the 1/8 inch difference on the pointer would make no difference in the cam timing but being in the wrong position would have an effect on the ignition timing wold it not. If the pointer is in the wrong position the timing would be ether retarded or advanced wouldn't it?
Anyway i am content that the pointer is where it should be and I have just finished cleaning about 1/8 of an inch of carbon off the combustion chamber surface and the tops of the pistons. I will soon be cleaning the valves which are also pretty much caked with carbon on their faces and first inch of the shafts.
I will install new valve seals and have the head back on the engine pretty soon.
Rob

[motoman]

Rotational speed of cams is TWICE that of the crank!

[skids]

Nawwww...Think about it. If you are adjusting the valves and the lobe tip is at the shim, it takes two full revolutions of the crank to get the lobe back to the shim again.

Originally posted by motoman
Rotational speed of cams is TWICE that of the crank!

[prometheus578]

the rotational speed of the cams are half as fast as the crank.

Skids is correct.
The crank has half as many teeth as the cam.
One rotation of the crank equals 1/2 a rotation of the cam.

79XS11F: Yes, only the ignition timing would be affected

[79XS11F]

So I've had my fill of wrenching for today. I will torque the valve cover tomorrow morning and install the exhaust. I'll post some compression numbers and see if this was worth the effort.
I'm going to use the recently cleaned carbs from the 79 midnight special with the jets, floats and bowls from my current setup. The fuel tank didn't lose a drop of gas all day so I can confirm that the petcocks are closed with no vacuum. I will test for proper flow when a vacuum is applied tomorrow.
If I'm luck this engine will fore up and run as it should.

Rob

[randy]

The ignition timing is only dependent on where the pickups are in relation to the reulctor (crank). Regardless of where the pointer is.

You have the sequence right, set the true TDC, then tighten down the pointer. You can then set the dynamic timing using the timing light on the pointer and rotate the pickup base plate to adjust the timing.

[79XS11F]

Yes ... I will adjust the timing with the base plate as soon as the engine is running. I figure the was it was set up my timing was incorrectly advanced across the board .
I just hope all this work will make the bike run properly. Anyone with time and some cash can put the bike together the way it was made .... but making it run the way it should is proving to be a little more interesting.
Rob