Instant "unstable combustion"
 

His 911 made in 1972
The engine has been changed to a 1989 3.5L engine, before he buys

The situation of trouble
Idling is very stable
"Strong Blipping" causes misfires
I can run at slow acceleration
A little more acceleration will cause a misfire.
Light afterfire also occurs at the same time
Ignition time at idling is around 0 degrees
Fuel pressure at idling is 2.3 kgf / cm2
It rises to 2.6 kgf / cm2 when the throttle is opened


Checked
Change to new fuel ... No improvement
Plug cleaning ... No improvement
Replacement of ignition coil ... No improvement
Fuel filter replacement ... No improvement
MAF sensor replacement ... No improvement
O2 sensor replacement ... No improvement
Change to another DME unit ... No improvement
Replace 2 crank sensors with new ones ... No improvement

Next time I will try to replace the fuel pump

Fuel pressure too low?
What is wrong with the pressure regulator?


...found it now
The hose to the fuel pump was connected to the return side of the fuel tank.
...

http://forums.pelicanparts.com/uploa...1560311329.JPG
http://forums.pelicanparts.com/uploa...1560311329.JPG
http://forums.pelicanparts.com/uploa...1560311329.JPG
http://forums.pelicanparts.com/uploa...1560311877.JPG

Zero degrees ignition advance at idle is not correct for any internal combustion engine that I've ever seen (would result in terrible throttle response and poor running in general). Do you know what the factory advance specs are for your ignition system?

1989 would have been 3.2L has this engine been modified to 3.5?
Those fuel pressures appear to be correct for the 89 3.2L.

Is this a new issue? Has the engine been running correctly, and now not, or is this a fresh install?

I had the same idea.
However, it is also written in the manual.http://forums.pelicanparts.com/uploa...1560351128.JPG

It was a mistake of 3.2 L engine.

He went directly to the body shop after he bought it.
After the repair of the car body was completed, it was only a few moves that he drove.
It was because the car inspection had not been completed.

It is unknown how much the previous owner enjoyed the 911 performance.

The 1989 3.2L is not a 930/50. I believe it is 930/20 or 930/25, but I'm not sure where you're located or if it caries the same designation there. I believe you should be using the manual for the 1989 for trouble shooting and diagnostics, since the engine and engine management system are completely different that the original.

It was the failure of the DME unit.

The biggest thing I can add to the original troubleshooting:

- Don't Replace Parts. TEST!

Throwing parts that are easily testable at a problem makes no sense.

It could be something as simple as a bad distributor cap. Misfire test was a good enough for me.