I need some help before I shoot this truck. I have been working on this carb for over two months and it is about to go out in the trash unless I have another problem that I am not aware of. I have a 400 motor with an edelbrock 400 perfomer non egr intake with a 4160 four barrel holley carb. The truck ran great when I bought it last fall and then started flooding out. I rebuilt the carb and it hasn't run the same since and I have taken it apart and checked/cleaned everything about six times. It will usually run and idle fine for a day or two after I clean and readjust everything. However, lately it won't idle smoothly at all and runs rough and spits a little black smoke once in a while. Yesterday I could not get it to run or idle without disconnecting the PCV line from the carb and my idle screws have no effect now and it back fired a couple of times which it has never done before. I have replaced the power valve twice without any noticeable improvement as well. I have all new vaccum lines and new in line fuel filter. I have had it idling and running as smooth as a babies butt a few times but then one day it will start acting up again. Is this just an old worn out carb which seems to be as old as the truck or is there something else going on here? I have been thinking about getting the edelbrock 1406 as a replacement. Any help would be appreciated.
Thanks, Dett

 

When you get it running, the first thing to check is the float levels. Pull the front and rear site plugs. There should be no fuel running out. You should be able to rock the truck a little, and a little bit of fuel sloshes out of the hole.

If you disconnected your PCV line, and let the carb suck air through that huge hole, then that's why your idle screws have no effect.

Have you noticed if the problem is only in the morning when you first start up? And then later on after it has sit for awhile, the problem goes away for the rest of the day? If so, it may be you have a carb icing problem. Ice will build up inside the carb and make it run rich. Later on, heat from the motor will reach it's way up and melt it away, and it will run ok. You will notice this type of problem on cool rainy or snowy days when the humidity is high.

 

It sounds like you have a vacuum leak. E'Brock non-EGR manifolds are known for having a very narrow mounting flange which makes it hard to seal the carb to the manifold. I'd suggest you get a can of carb cleaner and while the engine is running squirt a little around the base plate of the carb. It will briefly stop the leak (if it's small enough) and smooth the idle out. You might check the area where the manifold mounts to the heads as well.

If you really feel like spending a few more bucks get yourself a vacuum guage and hook it up and test the intake system to verify the diagnosis. However even if it does indicate a vacuum leak you still have to hunt it down.

 

also, it probibly backfired the last time because the PCV was disconnected.

 

I do have the floats set correctly. Yesterday it would not idle at all even after the truck was warmed up. I live in a relatively warm climate 50-60 degrees in the winter so I don't think it is icing up. I did hear some hissing sounds like a vaccum leak some where but not sure where it is coming from. It kept stalling every time I tried to reconnect the PCV line. It would stall instantly. It did run better when I advanced the secondary throttle a bit but still died when I hooked up the PCV line. The idle screws were doing nothing before I disconnected the PCV line. Some times the idle screws will work and sometimes not.
Thanks for the help so far,
Dett

 

The idle screws not working is a big clue that you do have a vacuum leak somewhere. The idle air is supposed to go through the front of the carb, where it can be mixed with the idle circuit fuel with the idle adjustment screws. If you have alot of air coming in somewhere else, then it's not going to mix with the fuel in the carb, rendering the idle adustment useless.

If you can't find a leak on the intake manifold anywhere, a place where air can come in, and it not be obvious, is the secondaries of the carb. I have had this happen to me for several different reasons.

1. I rebuilt the carb, fiddled with the secondary stop adjustment screw, and got it set too far open.

2. I rebuilt the carb, and for some reason the secondaries would stick, and not shut all the way. I loosened the phillips head screw that hooked to the vacuum actuator, and then re-tightened it, and it freed up.

3. I had a carb that had been setting around, and the secondary throttle shaft was a little gummed up. Sometimes it would run rough, sometimes not. And it depended whether I had gotten down on the gas pedal or not. A little wd40 on the secondary throttle shaft fixed that one.

 

Any possiblity of the power brake diaphram is leaking?