Spit, Sputter, Spit, Sputter

[k5 beast]

I have an all original 79 w/ a 400 that spits and sputters when you turn off the ignition, other than that it runs great. Starts right up, idles good, good power. It seems to wont to keep running. If I leave it in reverse when I turn off the ignition it seems to peter out quicker. If I put it in park and turn it off, I can get out shut the door and walk away with the truck still fumblin and bumblin. Any ideas?

 

[HarryH3]

Most engines of that era have a solenoid that sets the idle speed of the carb. The solenoid is only on when the rig is running. When the key is switched off, the solenoid retracts and the throttle blades close all the way, to prevent run-on. If the throttle speed screw on the carb has been adjusted to prevent the solenoid from letting the throttle close, then you'll get exactly the behavior that you described.

 

[k5 beast]

Where might the throttle speed screw be located on the carberator? To be honest I've never had the air filter off this thing.

 

[guido666]

This is a common problem with engines from the era when they just started putting on emissions controls ( ). It is referred to as "dieseling". Your '79 probably doesn't have the feedback (electronic) carb on it with the solenoid. I forget the some of the troubleshooting procedures to correct this, although I bet it has to do mostly with vacuum (like lines, vacuum canisters, etc.). If you have a repair manual there should be listings for "dieseling", or you could search Google.

 

[diesel4me]

"Runs so good,it don't wanna stop!"..

Harry is right about the "Idle Stop Solenoid"..most older rigs no longer have one that works,and folks disable it and crank the idle speed screw up to compensate..its high idle speed,or the fact the throttle butterflies no longer close fully without the solenoid working that causes "dieseling"..overadvanced timing can do it too,all it takes is a hot peice of carbon to light the mixture off,even when there is no spark at the plugs with the ignition off..

90% of the time,adjusting the idle to a slower speed in "drive",so it just keeps running without stalling,will cure most dieseling problems..lean mixtures tend to aggravate this problem..so will hooking the vacuum advance to manifold vacuum instead of ported vacuum on some motors..

I'd definately do something about it!--it can cause the timimg chain to jump,especially if its got over 100,000 miles on it.. doesn't do the rest of the motor any good either..

 

[k5 beast]

Were in buisness now, I took a look at the carb yesterday and I do have the electronic solenoid, the piston apeared to be retracted to the point where it wasn't interfering with the throttle coming back, although I couldn't really tell. I pushed it in a little and adjusted the throttle srew to the point where it barely idles while in drive. I also sprayed some carb cleaner in there at high RPMS. Then I added a fuel additive form chevron that is supposed to help with the carbon deposits. Although I already had a full tank of gas, so I hope it mixes well. When I turned it off at work today it shut down properly. Thanks alot I believe that has got it and if not I'll try more fuel additive.

 

[HarryH3]

Watch the solenoid while someone turns the ignition switch on. It should extend. If not, then check to see that it's getting 12 volts. You're supposed to set the idle using the solenoid. The end of it is hex shaped so that you can adjust it in and out with a wrench.

 

[diesel4me]

Harry's right..

That solenoid works like Harry said..if its not getting 12V,there is a fuse for it in the fusebox..see if its blown..if not,check the wire itself..

If the solenoid just plain croaked,(and many do after 20 years!)..you can usually get away with setting the idle speed with the speed screw on the carb to a slow idle..usually the motor won't diesel if the idle speed is slow enough..I've installed many Edelbrock and Holly carbs that had no idle solenoid,and they never needed one..stock GM carbs used during the "emission years" seem to be the ones most likely to make the engine diesel...