[jmsnyder]

Hello everyone.

I'm working on a friend's car. It's a 75 911 with the 2.7. The fuel injection has been removed and replaced with Weber carbs by the previous owner.

When my friend bought the car and brought it to me, the set up was a bit of a mess. The car didn't run very well and still had all the wiring for the fuel injection laying around in the engine bay.

I eliminated the unnecessary wiring and did a typical tune up. I adjusted the valves, changed the oil, new plugs, new distributor cap and rotor, and put the carbs back to the default setting, tuning them from there.

After all of that, the car ran better but still not right. Sometimes it would run damn near perfect and other times it would clearly be running on 5 cylinders. What has been confusing me is that the misfire seems to be in either cylinder 3 OR cylinder 4. When one was running, the other would not. I could tell by adjusting the mixture screw on each individual cylinder until RPM was affected. If the screw didn't do anything, then it lead me to believe that cylinder is not firing. At any more than 1/4 throttle, the engine ran smooth, and pulled strong.

Next I removed the points to inspect them (Forgot to check the dwell angle first...) The points looked just fine and I put them back in gaping them to .012 and confirmed with the dwell meter which showed 39 degrees that the dwell was in spec.

After the dwell adjustment, all 6 cylinders are firing! But there's still occasional popping out of the exhaust at idle. Sometimes not at all then sometimes constantly for 20 seconds or so. This is where I need some insight. The car runs great otherwise so this is the last thing I need to work out.

For added insight, I did a leak-down test showed 98-99% in all the cylinders. Also, the car is running the original distributor with no vacuum line attached. I have the initial timing at about 5 degrees BTDC. I know for peak performance the distributor should be re-curved but I don't see why it wouldn't idle perfectly with the current distributor. Also, I did clean the idle jets in the carbs which didn't seem to make a difference.

Thanks for the help!