I have a 73.5 911t that has PMOs on it. The car was located and tuned toan altitude of about 4000'. When I drove the car home(about a 1,000 mile trip)andit ran finebut the carbs would pop, burp and hiss at startup. They did this when I picked up the car as well as when I got it home. I took it into a "Porsche Specialist" for a tune-up, valve adjustment, and carb re-jet. The car starts and idles much better than it did, but, now it starts to miss (very badly) at about 2800 rpm. I took it back to the shop that worked on it and he tried everything to get it to run right. He changed ignition modules(MSD) and tried to put a set of webers on the car. He tried 3 different distributors, check fuel pressure and flow, new gas, and it still runs horrible when trying to maintain a constant idle above about 2800 RPM.
The mechanic says my camshafts are not compatable for the carbs, but my car never missed like this prior to my expensive "tune-up". It's really not drivable.

Any Ideas?

 

Good chance the C/O (the fuel distributor adjustment) will need to be checked and reset, and that is likely the cause of the backfiring you’re getting. You'll need to do this with a C/O machine as it is very difficult to do this accurately enough by ear. You will likely also find it necessary to set the C/O higher than the original specs due to the different gasoline we have now, about 4 to 5%. Also, check your fuel filter at the bottom center of the tank and make sure it is clean.