This is my first post to the Forum, and I hope you guys can give me a hand.
I drive a '78 F-150 with a 400. I recently rebuilt the engine, and due to emission laws up here in Alaska (where I just moved) I had to put a 2bbl back on it well I got it idleing and runnig pretty decent, but when I go to shut it off it Diesels (runs on) on me pretty bad. I dont know if this is a timing issue or what. Sometimes even spits fuel back out the carb. I have the timing set at 8 degrees BTDC which to my knowlege is about right. I dont know if it could be the carb or not. Any help would be great.
Thanks

 

Timing is a possibility, but I would think the problem is that you don't have the anti-dieseling solenoid set up properly. Smog motors tend to run hotter, and Fords often had to have a higher idle to run smooth. The problem with that is, when you shut your engine off, the heat inside the cylinders can ignite the fuel and air, rather than the spark plugs (to which the current is off at this point). That's why the engine diesels. So, Ford added an electric solenoid. When your engine is running and there is current to the plugs, the solenoid holds the throttle open for the idle setting. When you turn off the current, the solenoid retracts, and lets the throttle plates rest on the idle speed screw.

On these particular engines, you set the idle by moving the solenoid forward or backward when the engine is running, then you disconnect the solenoid so it retracts and then set the idle much lower with the conventional idle speed screw.

If your engine is set up for this, make sure the solenoid is working, and then make sure it is set up like I mentioned. Some carbs use a dashpot instead of an electric solenoid, and that is a different procedure.

Hope this helps.

 

If you don't have one of those fast idle solenoids you can get them in a junkyard or from Edlebrock or Holley. A lot of times they are used with lerge cams that require a high idle. Other times the carb is not adjusted right and with proper adjustment they are not required.

The dieseling is caused by a wide throttle opening. Almost any engine will do it if the throttle is held open when the ignition is turned off.

 

I noticed that the "butterfly" on the carb isnt closing all the way as it should..which i figured was the case ...but i cant seem to get it adjusted to where it will, unless i adjust the choke all the way closed, would that solenoid help in this matter? The other thing that I was wondering about was the heat exchange tube, that goes from the choke to the manifold, well i do not have that...what will this effect? Just the choke operating correctly right?...the manifold is an Edlebrock Performer 400 and i have the port on the manifold blocked off, because I didnt have the tube anyway, but the nipple going to the choke is open with nothing attatched.

 

The choke plate, or butterfly on top the carb, is going to be open when you shut your engine off, because it's all warmed up. You close it again when you go back out to start your truck by pushing the pedal down.

Closing the choke all the way like you said will make your choke over-rich. Your engine won't get enough air as it warms up. The throttle positioning solenoid and the choke are two seperate linkages, so no a TPS will not close your choke.

Your engine diesels because the throttle plates (very bottom of the carb) are too far open when the engine is shut off, like Torque1st said. You may get lucky and be able to set the idle conventionally such that the plates are open enough so that the idle is smooth, but closed enough so that the engine won't diesel. But that's a pretty small window...in my opinion, the right way to do it is to use the throttle positioner.

As for the heat exchange tube, I'm surprised your truck runs good like you described without one. Its purpose is to take heat off the engine and warm up the thermostatic spring inside the choke, which makes it open. If you don't have it, your choke's not going to open and the carb will literally "choke" the engine of its air. Your choke may have an electric assist, but you really should have the heat running to it too.

Good luck!

 

What rpm do you have the truck idleling when warm. My 400 idles at 800 in park and 650 in gear and it idles nice and smooth but I have no emission equip. according the manual I have that what these engines are supposed to idle at. It could also be an elevation issue also if you've eliminated all other possibilities. Also make sure you dont have any vacuum leaks.

 

I may need to check for a few vaccum leaks, I recently had my heads re-done, and so the head gaskets and up is all new...possibly the intake gasket maybe leaking (front rubber looks like it is pushed out slightly), although I dont think so. I have no tach (my next investment) so i really dont know the RPMs.

 

Something's wrong. If you adjust the choke all the way closed, as you said, it should increase the idle by means of the fast idle cam. But you said the only way you can get the butterfly valves to close is to adjust the choke all the way closed.

I can't for the life of me figure out how the choke is working at all without the heat transfer tube.