I have a question on tuning the idle in my LT1 cammed 350 tbi. If I unplug the IAC with the aldl terminals jumped and the key on, and then unplug the jumper and start the engine my idle is at 400 rpm. It is very erratic and lopes until it eventually stalls. With the IAC plugged in, the engine is smooth as can be at 525 rpm but the IAC hisses badly and the exhaust volume is huge. If I bump the minimum idle up to 500 or so it will smooth out a little but not much. The big change is when I plug the IAC back in. I have tried varying the timing in the chip and the fuel. I have my BLMs sitting at 125ish at idle with 21 in/hg on the vacuum gauge. What is the deal? Any help would be appreciated.

The other thing it does is surges for several seconds on startup cold or hot. When the engine is hot, it blows black smoke for about 1/2 second with a very rich smell. Any suggestions except for leaning the crank air/fuel mix. It fires right off but the surge has me worried.

 

The IAC's are loud by their nature. The larger the opening (greater steps) the louder they get. Adjust the idle blade screw until the IAC counts are about 10 steps. Do this warmed up and in closed loop. Before starting the engine crank the screw in some (to open the blades too far). Then once in closed loop monitor the IAC counts and turn the screw out until 10 counts are set.

For crank to run AFR blend, try adjusting the table at $401 'COLD START CRANK CORR vs %RPM'. As the %RPM increases taper out the fuel.

Surging is typically caused by a lean condition.

RBob.

 

400 rpm is very low. The min speed for the engine to be running is 400 rpms. Id target 550-600 rpm.

21 inHg of vac at idle is alot. Even with a smooth idle i usually only see around 18 inHg. This will definatly make the IAC noisy as there is a large pressure difference between the manifold and the outside. Check the IAC counts in your datalogs to make sure it isnt excessivly high, although if its pulling good vac, I wouldnt sweat it too much. Like RBob said, increasing the min air adj. at the tbi can reduce the ammount of air that must be provided by the IAC. Itll also improve fuel dist.

As far as the momentary issues after startup, you might want to look at the choke AFR adjustments as well. There are several tables of AFR values and decay multipliers that richen the mixture right after startup to provide a choke function.