rough running at steady speed
 

77 911-s 2.7 CIS
vehicle does not have the power is should and stumbles at steady speed 2000-3500 rpm
valves =ok
timing adv 10-15 deg to make it run
ignition seems ok
new dis cap and rotor
do i need to adj the o2 at the injection head or what ??????

Lots of possibilities = lots of questions.

Did it always run this way?

Or did it start after you made a change? i.e.cap and rotor

What steps have you taken already?

You will need to start ruling things out. The engine needs the right combination of gas, spark and air. Chances are only one of those is messed up. Narrow it down to one of those three and you will be a lot closer to identifying the problem.