Cam chain TDC position

TFR Aussie

XS650 Enthusiast
Messages
39
Reaction score
31
Points
18
Location
Adelaide Australia
The problem I have is I can't center the cam at TDC, which has been checked at the crank with 2 different rotors and TDC tool so TDC is correct.
The first pic is where I've been running, see the pin hole is out to the left, or rear.
Engine is a bit of a mix being a 1972 256 but fitted with a 1975B head but with the 256 cam & chain.
Which I've been told here by the OG's is ok.
Any thoughts?
1725084246020.jpeg

I move it 1 tooth, and I am out the other way. (Putting tension on chain with screw driver, not in pic)
1725084501308.jpeg
 
I would set and run it like in your "moved 1 tooth" (2nd) pic. As the chain wears and stretches, the cam will roll backwards and come into perfect time eventually. If the chain is new (and tight), it will loosen up quickly and this will happen pretty fast.
The cam chain has about 4000 miles on it. But I shall set it the second way. Thanks.
 
I would set and run it like in your "moved 1 tooth" (2nd) pic. As the chain wears and stretches, the cam will roll backwards and come into perfect time eventually. If the chain is new (and tight), it will loosen up quickly and this will happen pretty fast.
So today I removed the pistons, pulled the rods vertical which I imagine would have to be TDC and this is what my timing mark on the rotor looks like. I am using the rotor from a 1975B on a 1972 XS2 engine (eng # s650 129 120) but according to both part manuals it is the same for both. Now I'm getting confused...
1725175164311.jpeg
 
Something seems amiss. However the way you got the crank to TDC is not that accurate.

I would go back to basics and recheck things. First check your rotor has a key installed , check the location dowel in the stator housing is installed and correctly located. Double check you have matching cam and crank sprocket tooth numbers.
Then after confirming all that I would refit the pistons and barrel then using a positive stop and degree wheel find TDC and check this against the factory marks.

If you take off the right engine (clutch) cover you can turn the crank with a spanner on the nut that holds the crank pinion gear on. To help your set up when you have found the correct TDC use a marker pen to mark a tooth that aligns at TDC . Makes finding a close to TDC a lot faster later on.

From your photo moved one tooth, I think it would have probably come right when the head gasket was torqued down and compressed. Although you didn't say if you had torqued the head down or not. For checking you can use old used head gasket.
Also install the cam chain tensioner when checking.
In the past I had a terrible time finding a cams lobe centres, the problem was I forgetting to tension the chain after adjusting the sprocket position.
 
The cam chain has about 4000 miles on it. But I shall set it the second way. Thanks.
It might be worth doing one more easy check before you put the engine back in the frame. Once you have reset everything, set the valve clearances and torqued it all down, take a dial gauge and check the point at which the inlet valve opens (either of them).

On my 77 650 it's starting to open 36 degrees BTDC and it would be obvious on the stator timing marks if it was out by one tooth. There's a write up in the shop manual how to do it but it is really simple and quick to check.

I also believe you need to accurately recheck the TDC mark. At or very close to TDC, the piston is approaching stationary and accelerating the other way after TDC. Which means for a very small movement of the piston, you see a disproportionate amount of rotation at the crank. So it's really essential you verify TDC accurately and remove any doubt about it. Hope that helps.
 
It might be worth doing one more easy check before you put the engine back in the frame. Once you have reset everything, set the valve clearances and torqued it all down, take a dial gauge and check the point at which the inlet valve opens (either of them).

On my 77 650 it's starting to open 36 degrees BTDC and it would be obvious on the stator timing marks if it was out by one tooth. There's a write up in the shop manual how to do it but it is really simple and quick to check.

I also believe you need to accurately recheck the TDC mark. At or very close to TDC, the piston is approaching stationary and accelerating the other way after TDC. Which means for a very small movement of the piston, you see a disproportionate amount of rotation at the crank. So it's really essential you verify TDC accurately and remove any doubt about it. Hope that helps.
That's my plan
 
Back
Top