Engine revs after shutoff/smoking carb

[84GMCK2500]

Howdy,

Well I bought the truck to learn DIY, and it seems I found a second problem to tackle. Any idea what could be causing it rev after turning the ignition off? Smoke coming from air filter (carb).

 

[chengny]

If it is just a short rev up (sometimes followed by a backfire) and then the engine shuts down, you may have your idle speed to high. This may be due to over advanced timing or simply having the carburetor idle speed too high.

Try dropping idle speed to spec - maybe 850 rpm?

If idle speed is normal, it's probably a "dieseling" condition- running on after the ignition is off. Often caused by carbon deposits in the upper combustion space. They get hot enough while the engine is running that they are actually glowing.

So, even when the key is turned off and the spark across the plug electrodes is eliminated, these hot spots remain as a source of ignition.

To blow out the carbon; you can try a heavy treatment of Seafoam, frequent WOT application and good long drives on the highway.

I would check timing first, then set idle speed at carburetor. If those don't help, look into carbon build up.

For temporary relief, try shutting down with the transmission in the drive position.

 

[Swims350]

smoke coming from the carb, remove the air cleaner and fire it up, shut it down and see if smoke comes out the carb.

if so, sounds like a classic leakdown issue, the fix, carb rebuild or replacement.

if it's a quadrajet factory carb, small flaps on the front, huge uns in the back, classic lower plug leakage issue that's fixed during rebuild with epoxy.

If it's aftermarket, then gasket issue most likely.

The dieseling issue is as described, idle too high, timing too far advanced, or other issues, most of the time, idle.