Cam Timing on 1977 911S
 

I have just measured the valve overlap on both cams to see if this is the cause of my running problem. These are the results. Left cam, checked on cylinder #1 0.25 mm. Right cam checked on cylinder #4 0.42 mm. Specifications are 0.47mm. I believe I will have to retime the left cam but I wonder if the right cam is within acceptable limits considering this engine has 200,000 km on it. Please give me your thoughts.

As you may know, the factory specs for the 2.7 CIS cam is .40mm to .54mm. .47mm being optimum. That said, I doubt this would be noticeable to all but the most sensitive driver.
With the mileage on your engine I would be looking at new timing chains, as well as close inspection of the cam and intermediate shaft sprockets.
Could you further explain your running problems?

Thanks Doug. That's what I figure with the right cam. Do you agree that the left one being at .25 mm is too far out? My problem has been that the engine will start and run at 3200 rpm for 1.5 minutes then start to slow down, miss, backfire and then quit after another 15-20 seconds. I can keep it going a little longer with the accelerator but it is not running smoothly at all. I have replaced a whole host of parts including points, distributor cap, coil, ignition wires, fuel filter, tested and replaced one injector, tested WUR, and fuel pump pressure and volume. I am checking the valve timing before I buy an MSD box. I have had lots of help from our PCA members up here but so far noone has come up with a solution. Thanks again, any further suggestions are welcome.

From your description I would focus on fuel pressure. Can you inspect the fuel filter? If cheap enough, I would replace it even though it is new. Has the car sat for a significant period of time? Old fuel? Dirty or rusty fuel tank? A dirty or rusty tank will continue to give you grief until it is cleaned out. What was wrong with the one injector? I would also listen to the fuel pump. It could be all dirty or contaminated. Might run for a minute or two then stop. I'd also loosen the gas cap to test the tank vent system.
Good luck,