question about finding top dead center

[Dents]

Hello everyone,

I'm about to attempt to change my belts and do my own valve adjustment for the first time.  I've checked out Chris's video's and they are very helpful.  I'm not exactly sure how to find top dead center and I imagine for the valve adjustment I need to find it for both cylinders.  Also what do you use to turn the motor over when you do it.  Is there a way to do it without draining my oil? 

[Mr Earl]

1.  No worries on the oil - it stays right where it is.

2.  Either: put the bike up on a rear stand, upshift it to top gear and use the rear tire to turn the motor, or: buy an engine-turning tool to turn the crank directly.  The latter option is preferable but not mandatory.

3.  To find TDC-compression (not just any ole TDC): pull both plugs, put your finger over the plug hole of interest, rotate the motor until the compression pushes your finger off the hole.  Then insert a Phillips-head screwdriver (or equivalent) into that plug hole, contacting the piston, and SLOWLY bump the motor, noting that the screwdriver moves up as the piston continues to rise.  When it seems like most movement of the screwdriver has ceased, get a flashlight and look in the window on the left-side cover.  Bump the motor slowly until the mark lines up with the pointer.  That will be TDC-compression.

4.  To replace belts, you need only line up the crank and cam pulley witness marks.  That position will be TDC-compression for the horizontal cylinder.

[MongoReturns]

I really don't recommend sticking a screwdriver in there & turning the motor - Last time I did that to change the belts, the screwdriver got jammed in the hole.  Needless to say I freaked the heck out, backed the wheel off a bit to un-jam the screwdriver, and fortunately all was well.


As stated above, there are marks on the belt rollers to show when you're at TDC;  when I change my belts (new ones currently next to computer) I'll use those, and stick a chopstick in the hole to make sure.

[redial]

i would use the finger on the hole test, then line the mark up,
then with the motor still, use the screwdriver....



The first time i did mine, i only used the screwdriver method, and got the incorrect TDC for
the horizontal cylinder. (ie not the compression stroke)
The result? I sanded down the shims until i accidentally doubled my intended gap,
and had to go buy new shims and do that cylinder all over again. realllllly sucked.

but hey, learning from your mistakes is good. Learning from others mistakes is better
good luck!

[nomadwarmachine]

+1 on the finger trick.  Use your finger to plug up the spark plug hole and rotate the motor until you feel the air escaping from the cylinder.  When the air stops whoosing by, you are at TDC and you should find a marker on the flywheel sightglass window that matches the index needle.  That is TDC-compression.  

If you are unsure, keep rotating the motor and you should see the EXHAUST rocker depress as the fourth stroke (exhaust) in the cycle begins.  If the INTAKE rocker depresses, you were not at TDC-compression, and you need to keep turning.

Also note that, at TDC-compression, the opener shims on both rocker arms should spin freely.  (If they are very tight, they may not, but ideally they should both be unloaded).

Last word of advice.  If you feel confident in your abilities, and take your time, you will meet with better results if you do the adjustment with the belts off.  No danger of dropping the valves into the cylinder, and you can get the closers exactly right.

[Mr Earl]

It's easy to blow right by TDC - that's why I use the screwdriver.  If it makes you nervous or seems hazardous, maybe use a large zip tie or something similar.  I prefer to not rotate the engine backward if I go past TDC, so the screwdriver saves lots of rotating to reposition.

I agree with the belts-off valve adjust technique, after having tried it.  For convenience, I marked the vertical cylinder TDC position on the vertical cam pulley and the crank pulley with some green paint, corresponding to the witness marks on the case.  Makes removing the vertical belt at TDC foolproof.

[scott_araujo]

I agree with the belts-off valve adjust technique, after having tried it.

+1.  So much easier.  Also, If you leave the piston for the cylinder you're working on at TDC you can't drop the valve into the engine if you lose your grip on it.  You can always move the piston down a little if you need to spin the camshaft.

Scott

[Speeddog]

You don't have to have the pistons *exactly* at TDC to do the valves.

For the horizontal, line up the dot on the cam drive pulley with the hashmark on the case.
For the vertical, get the dot to about the 9 O'clock position.
You can fine-tune it with the marks on the flywheel if you're absolutely ADD.

Don't rotate the crank unless you're sure that all the valves are closed.

Remove and install belts with the dot on the cam drive pulley lined up with the hashmark on the case.

[Silver King]

For the vertical, get the dot to about the 9 O'clock position.

I have a question about this.  After I have removed the belts, the green dot on the vertical cam pulley wants to flop down to the 6 o'clock position.  The pulley won't stay at 9 o'clock because of the spring, right?  So......what am I doing or need to do?  Right now, I rock the cam pulley back and forth slightly (between 5 and 7 o'clock)to measure each of the openers and closers on the vertical cylinder.  FWIW, I measured clearances originally with the belts on and now I am fine tuning shims.  I just don't trust the method in which I am going about it. 

Thank you!   

[scott_araujo]

If you pull the belts the position it tends to flop to has both valves closed.  There is a tiny 'helper spring' on each valve that pushes them to closed.  Though the piston isn't at TDC the valves are out of synch since the belt is off.  Just rotate the cam around a few times, when you get to the longest part of rotation with no valves opening go to the middle of that span and measure.

If you need to adjust the valves, rotate the cam to get the rocker off the closer.  Then I recommend putting the piston at TDC before you take the closer shim off so you can't drop the valve into the engine.

Scott

[Silver King]

Thanks, Scott.  That's kinda what I did.  When the pulley is at the 6 (valves closed), there's enough play to get each of the openers to start closing.  So, I just set the pulley over to one side, measure intake, rotate it slightly the other way, measure exhaust.  I wanted to make sure because I over sanded one opener. 

Horizontal was easy.  I just couldn't get the vertical to line up at the 9.

[scott_araujo]

Yeah, getting the vertical belt back on is a bit of a trick.  I usually just leave the drive gear aligned, hold the cam in place with one hand, and wrestle the belt on with the other.  It usually takes a try or two to get it on aligned but nothing too terrible.

Scott

[Speeddog]

You've confused Top Dead Center positions with the position for installing the belts. 

I've clarified my previous comments, as shown below.

For Top Dead Center:

The cam drive pulley is the one on the crankcases, just above the oil filler.

TDC for the horizontal piston, line up the dot on the cam drive pulley with the hashmark on the case.

TDC for the vertical piston, get the dot on the cam drive pulley to about the 9 O'clock position.

You can fine-tune it with the marks on the flywheel if you're absolutely ADD.

Don't rotate the crank unless you're sure that all the valves are closed.


Belt Removal and Installation

Remove and install belts with the dot on the cam drive pulley lined up with the hashmark on the case.

The dots on the cam pullies (on the heads) need to be lined up with the dots on the belt covers, positions of these are different among the models.