77 2.7 with sc cams need help

[sgalloway]

I have purchased a 77 911s that has undergone a partial engine rebuild, but has not run right since. I have recieved may suggestions on what to do, but need to decide. Here is what I know of the engine mods:

-All recommended 2.7 upgrades were done(stud inserts, fan, ect.)
-has stock (rebuilt) CIS fuel injection for 77.
-Cams have been ground to SC specs.
-Has orignal pistons -compression 125/130/145/125/145/150.

The car runs rough (some backfire, stuttering) at lower rpm's Above 3000 it is smooth. There is a consistant exhast popping sound during acceleration at higher rpms. Some guy called it 'Flapping' and said it was normal with cammed engines?
-I had a porsche guy here go through the system to solve. After chasing electrial and fuel, his final take on it was that the cams were not compatible with the injection system. He had trouble with fuel pressure fluctuations. His suggestions was to change to carbs.
-Another guy suggested changing back to stock cams.
I have read that this injection system cannot take any cam mods and doing so will cause sensor plate fluctuations, therefore the rough running.
-Another guy suggested that the 2.7 with CIS and sc grind will work, but only with high compression pistons.
-Another guy suggested that the compression readings are all over the place because of the cams and overlap.

Is somone out there familiar with this setup and symptoms, and able to point me in the right direction to finally sort it all out?

Thank you,
Steve.

[kodioneill]

Those compression numbers are all over the place. But I would start with checking for intake induction leaks and check fuel pressure. Make sure the pop off valve in the air filter housing is sealed. ( I have seen many of these leaking) .You may want to check the valve adjustment. Also a cracked air box is possible. You have to trace down the problem and it could be difficult. A good mechanic can almost tell by the way the car sounds what's wrong. You may want to enlist the help of a good tech.

[4sd911]

1) "SC" cams will not cause any problems.
2) "Compression readings all over the place", need to expand on this. Does the compression change in the same cylinder reading after reading? Or are the readings different in each cylinder?
3) "Fuel pressure fluctuations", that's not good, could be fuel pump, fuel distributor, regulator.

[ossiblue]

Originally Posted by 4sd911

1) "SC" cams will not cause any problems.
2) "Compression readings all over the place", need to expand on this. Does the compression change in the same cylinder reading after reading? Or are the readings different in each cylinder?
3) "Fuel pressure fluctuations", that's not good, could be fuel pump, fuel distributor, regulator.

Agree with this. I had SC cams in my 76S CIS 2.7 engine and it ran perfectly and smoothly.

Fuel pressure fluctuations are a major red flag for your symptoms, IMO, and are the easiest to address.

Tackle one issue at a time, resolve it, then move to the next. If it were me, I'd begin with the fuel pressure issue and if that doesn't fix the rough running problem, move to vacuum/air leaks, then on to compression.