backfire

[marandall]

Ok I have a 1979 chevy k20 with a 350 in it. today I went for a test drive since I am about done with the truck rebuild. Engine was running fine with no problems. came back from the test drive and installed the hood onto it and now it back fires and throws black smoke  from the carb and spits and sputters when at idle and backfires from time to time when you give it throttle. don't understand what could be the problem since it was running fine with the hood off before I out it on. even with the hood up and a fan blowing in front of the truck ( used the fan because  I thought it wasn't getting enough air ) still no difference. Don't understand what the problem could. If anyone has ran into the before please any suggestion would be helpful. thank you 

[bd]

1) Check whether the distributor is loose and spun a few degrees then check the timing.

2) With the engine running and warm, carefully look down the carb throat for fuel dumping from the main nozzles at idle.

[raf2330]

I'm assuming it still has a Q-jet on it? It's been me experience that when I leave these older trucks sitting for a while that somehow garbage works its way into power valve/piston area of the carb and can cause the symptoms you describe, or even not wanting to run at all. I've had the same thing happen as you described even after just driving some after sitting. I've just pulled the top off and cleaned it out and away she goes. But then again maybe you have all new equip. on there.?

[454Man]

I ran into the same thing with mine when I put my engine in. I ended up leaning the idle mixture some.

What's under the hood??? If you have to ask maybe we shouldn't race...

[marandall]

it has a q-jet on it. ok I let the engine sit for about 2 hours and I just went outside to start it again to see if I could find the problem before the sun was completely gone. it started right up and ran fine until it got to operating temp which it likes to sit around 160 to 170 degrees and it started to act up again. So I wandering if the choke is not operating  like it should since the temp is colder then when it was when I put it on over the summer. oh and the q jet it brand new also

[rich weyand]

Check that the choke wires have 12 volts across them whenever the ignition is on.  Pull them from the choke and hook up the voltmeter and just turn on the ignition.  No need to start the engine.  The choke wires should show 13.2 volts or thereabouts, the voltage of the battery.