1974 timing issues

[Rich Patonai]

Help needed.
I have a 1974 911 that I put zenith carbs on. I also installed an igniter electronic ignition. It runs fairly well but is not consistent throughout the power curve (great while accelerating but runs like something not quite right at the lower rpms) and I'm not sure of how to time it correctly.
The electronic ignition does away with the dwell requirement, correct? If so, do I need to hook up the vacuum advance that is on my Bosch distributor? Would a mechanical advance be better, if required? Also, the carbs were dialed in at sea level and I am at 4200 feet. Is that enough of a difference to warrant having the jets adjusted? I'd appreciate any comments or recommendations. Thanks, rich

[2.7RACER]

Rich,
I'm up here in Albuquerque at over 5000 feet. Yes your altitude is enough to warrant a retune.
Typically since the air is thinner by approximately 3% per thousand feet, you are getting 12% less oxygen to your motor.
This needs to be compensated for. Smaller main jets, larger air correction jets.
I'm not familiar with the Zenith carbs but that is where it is.
Your ignition timing can and should be advanced slightly about 3 or 4 degrees.
You can also run regular gas because your "effective" compression is down 12%. A 10:1 comp engine acts like a 8.8:1 engine at your altitude.
The issue here is whether you may be driving the car back to lower altitudes.
A carbureted car tuned and "optimized" for altitude will be lean and run hot at lower altitudes.
I know of a 356 from New Mexico that burned it's valves in California because the owner did not reset for lower altitudes.
Computer controlled fuel injected cars don't have this issue because altitude and air:fuel mix are monitored and adjusted continously.
So the answer is retune, how much depends on where you drive.