911SC run intermitant rough run

[richardmoist]

My 1983 911 SC starts good, but sounds like it is missing but not quite....sounds like it might be running rich. Exhaust gas test on lean side(5%). Compression good. Have new o rings on injectors and injector holders. Timing good and advance works correctly. When running along it will clear and sound perfect, then if you lug it down it will sound rough again...then clear...???

[T77911S]

make sure plugs and ignition are in good working order. might not hurt to replace the plugs. take a look at the plugs and see if any stand out as far as looking rich/lean compared to the others. if the plugs are real old, replace and drive it then check them again.
check heat range of plugs too.
use copper top plugs, nothing fancy or expensive.

assuming all air leaks are fixed, i would next look at the injectors. check spray pattern and compare fuel volume of the injectors. you can do this by weight or measuring. i would try several tests. test with the sensor plate lifted partially, as in your low rpm problem, and test at WOT.

when i checked my volume, i used 6 containers to catch the fuel, then poured each into one common container to weigh it on a postal scale. that eliminates variations of container weight.
you could even do the same to measure it. using the jar from #1, mark the level, pour the fuel put, then pour each of the others into the jar to compare the level to #1. in my case, i had one that put out about half of the others.

if you still have the problem, i would look into more air leaks and recheck timing, along with vac retard and advance if you have them connected.
then i would recheck the mixture.

[Scott R]

5% on CO before the CAT? That's actually high for you car.

[Peter Zimmermann]

Maybe a typo...0.5%?

Pull your spark plugs. If you have one dark one, replace that injector. This is a pretty classic symptom, but there is always a chance that the problem could be something else.