cold run issue

[Yellowbird RS]

After rebuilding my engine, and after 80 miles aprox., it starts up perfectly, but there is a little black smoke from the exhaust, which according Wayne's book is normal. The problem is when I drive and the engine is cold, it seems like it wouldn't have all the power and although I push the gas pedal all the way, I don't get more than 3,500 RPM and it feels a slight vibration. The strange thing is that after it warms up, it runs perfect, there is no smoke, I can get any range of RPM. I already checked the fuel pump psi and it is OK. My thermo switch is OK. The only thing I notice is that my WUR seems not to receive any current, although my Idl is correct at any moment.
Do you have any suggestions?
If anyone could also tell me where to connect the WUR cables (right now the brown one, which I think is ground, is attached to the coil base; and the red one, goes with an arnes to the alternator).
Help please!

[Yellowbird RS]

My car is a 1974 911 Carrera

[Jim Sims]

In a 1974 Carrera the WUR heater element is fed current through a relay mounted on the electrical chassis under the plastic cover on the driver's side of the engine compartment. Look for a round black relay and ensure it is working; try reseating it in it's socket. The relay coil has a blue wire connected to terminal 86 and two brown wires connected to terminal 85; the power for the WUR is fed by a red/white wire into terminal 30 of the relay and out to the WUR through relay terminal 87 to another red/white wire which goes through the 14-pin connector and into the engine wiring harness (the alternator wiring harness is part of the engine wiring harness). Check to make sure the 14-pin connector is tight and well seated. There is a reasonable wiring diagram for the 1974 Carrera in the Haynes "Porsche 911 1965 thru 1989 Coupe, Targa & Cabriolet Automotive Repair Manual." You may consider getting this manual as it is inexpensive. Cheers, Jim

[konish]

Just guessing here, but the current to the WUR sets control pressure from cold and will increase control press. as a result of both the internal heater (as far as current is concerned) and engine heat. Even without the internal heater, the WUR will eventually increase control pressure (obviously the time involved would depend on the outside air temp, etc) decreasing the amount of fuel metered into the pressure chambers inside the fuel distributor. Your problem sounds like your car is running REALLY lean, and as the engine heats up the mixture comes back into balance ala mother nature. Set your control pressure via the adjustable plug on the WUR if necessary. My SC had a horrible backfire when cold, and when I checked the control pressure it was REALLY high, but the car came from CA where it never saw temps below 50 degrees. I also suffered from an inoperable freg valve (now resolved) which would not affect you of course. Again just guessing and still learning...

R/
Dustin