MFI too Rich then Too lean

[cmcfaul]

Trying to get MFI dialed in, post engine rebuild.

Removed the plugs and they are all uniform in color and very black.

When cold, which is almost all the time now, it runs very rich (about 10 on the AFR meter). If I can get it hot, after at least half an hour of driving, it runs lean (about 15 on the AFR meter) at part throttle.

I went way lean on the idle screw and get 12.5 at idle when warm.

Running a DC 40 cam (E cam with more lift) on an otherwise stock, higher compression, engine. I know the mapping will never be perfect but something is screwy.

Chris
73 911 E

[cmcfaul]

you could be right. When hot it may leak air from the throttle body bushings. They are 40 years old. But could they leak enough air to cause such a large change in the AFR? Putting my finger on the bushings does not change anything so I cant detect any significant leakage. Thinking it has to be something with the thermostat on the MFI. All parts of the MFI and intake are connected as stock. Specifically, the hot air hose from the heat exchangers is connected to the MFI thermostat.

Chris

73 911 E

[Dodge Man]

After springing for the motor work, rebuilding up the butterflies is a lot of more $$$. I have had a lot of small vacuum leaks give me problems. Back in the day we used to spray carb cleaner on the intake gasket locations(at idle/high intake vacuum) and listen for idle changes to find the leaks. IMHO Go simple and wiggle the shafts with the butterflies about 1/2 open to get a feel for shaft wear. Best of Luck.