Z1 timing vs compression question

1974 Z1A
In the process of removing cylinder head and jugs I set the crank timing at T for cylinder 1 (in case it made  a difference, which it doesn't on dis assembly) and noticed that the arrow on the exhaust cam sprocket was approximately 1 tooth ABOVE the gasket surface. (and I was so careful when I set the valve clearances! ).  So my timing was about 6 degrees behind where it should be. (30 tooth cam sprockets). Cams were 28 pins apart as they should be.  This got me wondering - would this +/- 6 degrees late be a factor in the compression being on the low side when I checked it beginning of the summer? (120 120 118 120)  Rings aren't stuck, cylinders have no scratches, ridges or damage, crosshatching still visible. No smoke when running.  Bore kit, pistons and rings have about 9000 miles since the work was done.  (A leak down test earlier in the year was inconclusive, we had trouble with the motor rotating some when air pressure was applied at TDC, maybe we weren't doing it correctly). I was planning on replacing the rings while the engine is open, but I may not be able to find rings for the Yoshimura 69mm pistons, in that case I'll have to do a slight ball hone for crosshatch and re use the rings.

Cam chain stretch will alter the cam timing marks. 

Ditto Warren's thoughts, aged peanut 219 cam chain. Another possibility, make sure that little chain guide that attaches to the foreword cylinder assy is still intact. I've seen plenty of those fracture and fall in the oil pan.

sf4t7 wrote:

1974 Z1A
In the process of removing cylinder head and jugs I set the crank timing at T for cylinder 1 (in case it made  a difference, which it doesn't on dis assembly) and noticed that the arrow on the exhaust cam sprocket was approximately 1 tooth ABOVE the gasket surface. (and I was so careful when I set the valve clearances! ).  So my timing was about 6 degrees behind where it should be. (30 tooth cam sprockets). Cams were 28 pins apart as they should be.  This got me wondering - would this +/- 6 degrees late be a factor in the compression being on the low side when I checked it beginning of the summer? (120 120 118 120)  Rings aren't stuck, cylinders have no scratches, ridges or damage, crosshatching still visible. No smoke when running.  Bore kit, pistons and rings have about 9000 miles since the work was done.  (A leak down test earlier in the year was inconclusive, we had trouble with the motor rotating some when air pressure was applied at TDC, maybe we weren't doing it correctly). I was planning on replacing the rings while the engine is open, but I may not be able to find rings for the Yoshimura 69mm pistons, in that case I'll have to do a slight ball hone for crosshatch and re use the rings.

With the "leak down" test once you have the motor in the position you want, select 1st gear and get an "assistant" to firmly apply the rear brake before you connect the air supply (in the car world we used the "E" brake) best if he rear wheel is clear of the ground just to be safe as it will still "kick" when you apply the air
Agree with everybody else ref cam chain "stretch" the only thing I would add is that if everything else (guides etc) are OK and it has stretched that much, even though you are counting the pins the distance will have changed which will effect the cams position relevant to each other

I had similar problem with timing on my bike .I replaced the camchain and sprockets. here is a pic from when i did the job the old camchain was about 5mm stretched
compared to the new.After replaced chain and sliders the timing was spot on.I did a quick writeup but have not posted it

Chains don't "stretch" as such, the extra length comes from wear in the pins and the holes in the links. If you lay the chain flat and bend it sideways this becomes clearly evident, the chain I removed from my diesel Jaguar had 45mm of side deflection at the centre of the "arc", the OE replacement had 15mm of deflection !!! worn pins is why they "break"

Yes that is true ,but if you look at my picks where i have compared old and new camchain they are longer to not just sloppy. Here is another camchain i replaced

Rolf I'm not disputing that they get longer when worn, just clarifying the reason for the "growth" is wear not elasticity (stretch)

Wookie58 wrote:

Rolf I'm not disputing that they get longer when worn, just clarifying the reason for the "growth" is wear not elasticity (stretch)

Correct.  Chains don't stretch, they elongate, due to wear on the pins and related parts.

Ok, i'll check the chain for extra length due to wear (probably a better term than stretch) and replace if needed.  Asphalt900, the front chain guide was broken at the screw holding the upper end but was still in position held by the bottom.  My main question was could the timing being off cause the lower compression numbers?  Yes Guy, we didn't hold the rear wheel with the brake but I did have it in gear. Guess i've been spoiled by cars being held while in gear by their weight. Unfortunately that ship has sailed, the engine is apart.
 

sf4t7 wrote:

Ok, i'll check the chain for extra length due to wear (probably a better term than stretch) and replace if needed.  Asphalt900, the front chain guide was broken at the screw holding the upper end but was still in position held by the bottom.  My main question was could the timing being off cause the lower compression numbers?  Yes Guy, we didn't hold the rear wheel with the brake but I did have it in gear. Guess i've been spoiled by cars being held while in gear by their weight. Unfortunately that ship has sailed, the engine is apart.

 

Yes, incorrect valve timing can effect compression

There is a method outlined in the manual to check for chain stretch.  You hang the chain vertically with a weight (I think it's 5 kg) and measure 20 links against the standard length in the book.  

In general, more than 1% stretch is excessive.  For bicycle chains the stores sell an easy measuring device.  Perhaps someone can come up with a similar device for timing chains?