I checked my timing today and with the light it shows that I am at 10 degrees. I have it set at about 15 I thought.The adaptor will not show correct timing if I remember right. It even says in the instructions to check a engine that has the correct timing and see were it shows the advance is.It also seems like the injectors may be weak the light seems to jump around a few degrees.Sence it gets its signal from the fuel pulse when the injector opens.So if the pop pressure is week it may be firing early.The motor has 170k and it looks like the injectors have not been changed.

 

I use a pulse diesel timing light to check timing but never to adjust it.
They are highly accurate as long as you have TDC marked correctly and the correct timing tape on the right place on the dampener.
When I've run into a weak pulse removing the paint off the section of fuel line where the probe attaches usually improves it.

 

If you have yours set at 15 degrees by measuring lift of the pump will the light also show 15 degrees? We have a snap on light with the adjustable timing so you just bring it back to TDC and it tells you were the timing is.I marked the balencer at TDC by the timing pin.

 

If you are getting discrepancies between lift and light there lies the problem.
I quit using the timing pin after cross checking three trucks with a dial indicator on the #1 cylinder to find TDC.
All three were off from 2 - 5 degrees.
The pin is good enough to adjust valves but not timing.

 

Keep in mind, the light will vary depending on where you place the inductive pickup. If you place it right on top of the line at the pump, it will show more advance than if you place the pickup at the injector.

I have thought about doing this just to baseline the timing. Then, an annual check will tell me if the timing is slipping or not.