Cam sprocket align By pretendentas22 (Read 6,295 times)

[pretendentas22]

Hello All,
I have small problem I had to replace crankshaft on my GY6 125cc and now can't set up timing. When piston is at TDC the chain doesn't allow to align cam sprocket, I mean cam sprocket marks are or half tooth too early or half tooth behind. Any Ideas how to match TDC with camshaft?

[Random Hero]

the way i do it is to get the piston TDC then put the chain on. Mine is not dead even but it runs(when not leaking oil ) just fine.

Look at this thread and start with post #39 it might explain it a little more.
scooterdoc.proboards.com/index.cgi?board=150cc&action=display&thread=6379&page=3

[Alleyoop]

Yes, the Case pointer will usually be a little to the LEFT(advanced) of the "T" when you bring it up to TDC and align the cam sprocket.
Alleyoop

[pretendentas22]

I found TDC by physically checking piston’s move and problem is if my cam sprocket is aligned correctly and piston at TDC I can’t put on the chain without turning cam sprocket to the left or right. My question would be is it better to have cam sprocket aligned before piston reach TDC or after?

[Bashan]

Here's the deal, when your crank is on the upstroke there's a lot of piston movement for each degree of rotation. When the piston is at top dead center there's a period of rotation where the piston is almost stopped while the crank rotates significantly:

Ideally your piston would be at TDC with the T mark on the timing tab but that won't happen, that's theoretical, not practical. Get your T mark on the timing tab with your piston at the top. Get your cam sprocket under the chain in this position paying close attention the the timing lines (red arrows):

The flywheem will drift some as Alley said, it will probably end up something like this:

A: Get the piston at TDC
B: Get the T mark on the tab
C: Get the cam sprocket oriented paying close attention to the timing lines
D: Don't worry about a little flywheel drift, it doesn't matter

Here's another thread to read over. Rich

[pretendentas22]

This is the picture where is my TDC.


At TDC this how I am able to set cam


if I align the cam - the mark is moved to this position


It is first time I am involved into timing and don't want to smash the valves.

Is it safe enough to leave it as it is?

[jct842]

Have you removed the cam chain tightener before trying to get it in time?

[tvnacman]

make sure the chain is seayed in the gear on the crank . Look inside there and make sure .

John

[pretendentas22]

Yes, the tensioner is off and chain on the gear.

[Alleyoop]

Is the head tighten down?
If the head is tighten down then that is the best it is going to get. If the head is NOT tighten down then you are off 1 tooth.
Alleyoop

[pretendentas22]

Yes the head is tighten down. Is it save to run like this or it will cause me problems?

[Bashan]

When the cam sprocket is in the position with the timing lines parallell to the head and the large hole exposed the cam lobes aren't even close to touching the rockers. If you look closely you can see the lobes lurking about near the head:

In the worst case scenario a valve is held open when the piston comes up, you're not even close to that so don't worry. Also, think about this, your options when setting the cam are to move an ENTIRE LINK one way or the other which would result in a boatload of flywheel rotation. The flywheel rotates TWICE as much as the cam sprocket. There are no small increments in this process so even though your flywheel has hinky marks I'm sure you're OK.

However, ALWAYS triple check things before you button it up, you don't want to have to tear that puppy down again. So leave the plug out and rotate the engine several times and make sure the valves are clearing. Something to be aware of is the cam lobes will actually result in some crank resistance as you're turning the engine, especially if you put in a hotter cam. So don't freak out and think it's a valve hitting. Take it slow and watch the valve mechanism while you're rotating the engine, you can see if things are oriented like they should be. I keep stressing if members understand exactly how these mechanisms work it greatly simplifies things. Rich

[pretendentas22]

Thanks for the advice I will definitely check it.
Other thing which concerns me is non-adjustable ignition time. The current gap between ‘F mark’ and actual TDC is longer than factory’s is set and I think it will cause an advanced fire. Is there any way around it?

[Alleyoop]

Two ways you can change the timeing on it:
1. Take the Trigger Pickup coil off and Extend the screw holes, make them as SLOTS so you can move it Left or Right.
2. You can buy Woodruff Keys that are Off set so many degress, and you can either Retard or Advance the timeing by just flipping the Key with the Offeset to the right or left.
But I would not worry about it. The way it is right now actually looks like mine.
It is advanced and will retard a little more anyway. You will notice a little more power.
Alleyoop

[pretendentas22]

I like the idea with screws very good.
Thank you all for the help all I need now is to get last part from China and see how it starts.