1972 MFI woes

[motomiata]

I am at the end of a long restauration / recommissioning of a '72 T with MFI. 48.000 miles and engine has never been rebuild. The engine did not run properly before I started restoring the car. The car was driveable but running very rich.

The engine starts and runs at high idle but stutters and backfires when revved. Lowering the hand throttle to around 1000 rpm stalls the engine and it takes some cranking to get started again. Car is not driveable

Work done to the engine:
Cam shafts timed and double checked
Valves adjusted and double checked
Compression shows all cylinders close to 140 psi
Throttle bodies cleaned and checked and bench calibrated
New throttle links, correlation checked and double checked
New air and fuel filter
New soft fuel lines front and rear
Fuel tank cleaned and POR15 internally, new pick up screen filter
Fuel pump cleaned and checked
MFI pump rebuild and calibrated at UK specialist, pump timed with new drive belt
Injectors ultrasonic cleaned at Bosch service center
New SSI's. Intake and exhaust ports looked even and tidy, no sign of oil seeping from worn valve guides, no oil smoke at start up
New plugs, cords, points, coil, rotor, dizzy cap, dizzy rebuild and checked
Ignition timing and dwell adjusted and double checked
Swopping known good dizzy and CDI unit to no avail

Only clue is a very sooty #6 plug.

The problem has been more or less the same throughout above procedures.

It seems to me the only thing left on the plate is checking my injectors for proper function and spray pattern. Would a single bad injector on #6 coarse above symptoms?

Any suggestions are highly appreciated, thanks.

Hans

[kwikt 911]

Silly question but are your fuel lines correctly connected at the MFI pump. Also try dissconnecting the throttle micro switch, had one that was shorted out once and caused me some grief.

[Flieger]

Thermostat working?

[kltarga72]

Make sure fuel lines are connected properly from tank to fuel pump and then check fuel filter hoses are also connected correctly.

I purchased a new Bosch fuel pump when I had my MFI pump professionally rebuilt, new injectors, plugs etc. I then fought an intermittent rough running engine thinking I had an electrical problem. The problem was the new Bosch fuel pump. I have heard others have had issues with Bosch fuel pumps.

Lash

[P9elfer]

Be certain the lines at the pump are connected correctly. There are three connections. The engine will run as you describe if the fuel return is directed to the engine instead of back to the fuel tank.

[2.4te]

I suggest a read of the Porsche Mechanical Fuel Injection Check Measure Adjust document. Perhaps someone can provide a link to it, otherwise I have a pdf if it helps.

[motomiata]

Thanks for the replies and suggestions. I do have the CMA and have read anything I have been able to dig out concerning the MFI.

I will double check my fuel lines from the console to both pumps.

Would it be possible to reverse the polarity on the fuel pump so it runs backwards ?

[Porboynz]

Double check the fuel line routing from the fuel filter console to the MFI pump, there is a rogue diagram circulating with reversed fuel line connections. I would have thought it impossible to route the hard injector lines incorrectly. Make sure the MFI pump is timed correctly to the engine, maybe you have it 180 degrees out. Yes it is possible to reverse the pump connections, but unlikely unless rewired poorly, they are labelled + - on the plastic terminal housing so easy to check. Check the fuel pump fuel line connections. Good luck.

[tobluforu]

How many clicks have you gone, left or right on the rack, does it do anything in regards to idle, etc.?