Confusion with camshaft timing mark- 2 identical engines.
 

I have 2 Mercedes with the M130.923 engine (carburetor versions).

I rececently installed a new Timing chain on the 250 model with the camshaft aligned to 11 degrees BTDC (as per shop manual)

The 250 doesn't run very well, even with the rebuilt carburetors.

So, I check the camshaft mark on my 280 model, and it is 9 degrees ATDC (the timing chain is almost as tight as the new installation on the 250).
This 280 runs real sweet set up this way, and the carbs on it are way overdue for rebuilding.

So why do I have a great running 280 with the camshaft timing set incorrectly at 9 degrees ATDC?

The 250 is set to Shop manual specs and it runs poorly at 11 degrees BTDC.

And yes, all tune up parts and valve clearances are new and adjusted.

I think you are talking ignition specs ..
Cam timing is set at TDC. If it is not at TDC when the cam index is aligned with the tower indicator, the chain is streached.

Checked the tensioner, problem found
 

It separated into 2 peices. The worn tension ring allowed the inner to seperate from the outer. It is keeping the chain too tight, by not allowing the tensioner to vary with load. In other words, it is a fixed length.