I'm having a little trouble with my truck. Its an 86 Sierra Classic with a 305 4bbl 700R4. A few months back I decided to take the carter AFB off the truck and put the original rochester set up back on it. I used a good running parts truck and swapped everything over and used my buddy's truck as reference to make sure all vacuum lines etc where proper. The problem I'm having is that the truck now will diesel "forever" or at least until I pull the coil wire from the HEI distributor cap out, which is generally foolowed by an ugly ping and a backlash of gas from the carb. I've tried lots of different things like retarding the timing, changing all the vacuum lines with new ones, lowering the idle (which sometimes no matter what the timing is or how far backed off the idle screw is the truck still will run at the same idle)changing the Rochester with other ones and even putting the Carter setup back on, which now even with the Carter back on the truck still diesels once it is warm. I was wondering what the culprit is- timing chain, distributor or some type of wiring problem. When the truck warms up it will generally rev at a couple hundred RPMS higher than normal...is this sufficient to have a neverending diesel? My truck also has some type of computer behind the glove box that is wired into the distributor- could there be problem there? The truck was running fine before I messed with it. I'm sorry to bother you guys but I don't want my Chevelle to become my winter driver and be subjected to our beautiful salty roads. Any advice would be much appreciated.

Could it be built up carbon in the combustion chamber glowing red hot and igniting the mixture after the ignition is turned off?? I have seen this in the past. I have also cleaned carbon from the combustion chambers by: slowly misting water down the throat of the carb while the engine is running, being careful not to stall it. I have also done the same thing with ATF, but after pouring 1/2 quart down the carb slowly, I pour the rest in the engine and let it stall, then let it sit for a couple hours, then restart it. There is also products designed just for this that work well, like GM's top end cleaner and Marvel Mystery Oil. Make sure the engine is at operating temp. before trying this, also when using ATF there will be alot of smoke coming from the exhaust, this is to be expected.
Cleaning the combustion chambers this way has worked to stop dieseling engines, as well as pinging on some engines. Its cheap and easy to do too!!

Is there an idle solenoid on this engine. If so, then this is what controls idle speed & will/should drop off when the key is turned off. Just a thought. good luck