I have a 83 GMC c/k Pickup. It has a 350 with a an edlebrock performer manifold, +1 cam and a brand new performer carb. 2 weeks after I installed the carb it started idleing to low at stops. It would run fine but when I came to a stop it would start chokeing down and quit. So this is the list of what I replaced:
1. Fuel Pump and filter
2. EGR valve
3. Spark plugs
4. Wires
5. Cap + Roter

It got a little better. It now runs ok but not back to 100% yet. It runs like ass at a full tank of gas and gets better after and lower than 75%. I have duel tanks and if I switch tanks with the toggle switch it instanly goes back to running bad. Even if you switch back to the first tank that was previously running good. It has to sit over night to start running good again. I have never used the same gas station twice to rule out bad gas.

So now I'm only useing one tank and putting $5 in at a time, keeping it around half a tank.

I took it to a "known good mechanic" and he listed all of the aftermarket parts on the truck and said it was to heavily modified to accurately fix it.

I don't know what else to do!:mad3: Please help me.

Originally posted by BFHCS
I have a 83 GMC c/k Pickup. It has a 350 with a an edlebrock performer manifold, +1 cam and a brand new performer carb. 2 weeks after I installed the carb it started idleing to low at stops. It would run fine but when I came to a stop it would start chokeing down and quit. So this is the list of what I replaced:
1. Fuel Pump and filter
2. EGR valve
3. Spark plugs
4. Wires
5. Cap + Roter

It got a little better. It now runs ok but not back to 100% yet. It runs like ass at a full tank of gas and gets better after and lower than 75%. I have duel tanks and if I switch tanks with the toggle switch it instanly goes back to running bad. Even if you switch back to the first tank that was previously running good. It has to sit over night to start running good again. I have never used the same gas station twice to rule out bad gas.

So now I'm only useing one tank and putting $5 in at a time, keeping it around half a tank.

I took it to a "known good mechanic" and he listed all of the aftermarket parts on the truck and said it was to heavily modified to accurately fix it.

I don't know what else to do!:mad3: Please help me.



you might try checking to see if there is any trash inside the carb.Even though it is new some may have gotten into and is causing a fuel flow problem. just a guess...and I would think it is fixable whether the engine is stock or modified it revolves around the same basic principles..good luck..

What model number carb do you have...?

I would pressurize the tank, and look for a pinhole leak in one of the fuel lines.

Did you adjust the carb? Just because it is new does not mean it is a bolt on and go type of install. When I worked at a Shop I loved that line from customers about a smog inspection, Me " Do you want a pre-test?" Them "No I just put a new carb on runs great" Me *snicker*Snicker* o.k. The carb needs to be dialed in.

When I unscrew the gas cap I can hear a rush of air entering the tank so I don't think air is leaking in anywhere. But maybe. Also the gasket between the carb and the manifold was the old seal. I'm going to swap that out for the new one that came with the carb.

Its a Edlebrock 650 CFM Performer with an electric choke


I have adjusted the idle and marked it. I first thought that the idle screw was turning by itself but that is not the case. Its stays where I put it. I didn't touch the 2 screws that are in the front of the carb. I assume they controll the air/fuel mix. Should I fiddle with them?

But why does run bad on a full tank or when I switch tanks?
anymore thoughts?

Originally posted by BFHCS
When I unscrew the gas cap I can hear a rush of air entering the tank so I don't think air is leaking in anywhere. But maybe. Also the gasket between the carb and the manifold was the old seal. I'm going to swap that out for the new one that came with the carb.

Its a Edlebrock 650 CFM Performer with an electric choke


I have adjusted the idle and marked it. I first thought that the idle screw was turning by itself but that is not the case. Its stays where I put it. I didn't touch the 2 screws that are in the front of the carb. I assume they controll the air/fuel mix. Should I fiddle with them?

But why does run bad on a full tank or when I switch tanks?
anymore thoughts?
I think you just answered your own question. Your pulling vacume on the tanks and it's causing a fuel flow problem at idle.
You either have the lines for the EVAP can routed wrong or there is a bad check valve in the system.
Quick way to check my guess is to drive it with the caps off the tank and see how it runs. If the idle pulls up real high then your pulling WAY to much vacume on the tanks.
On any of the pre FI trucks you should not hear a big "hiss" when you open the caps. It should pull a very light vacume to get the fumes pulled to the charcoal can hanging off the core suport. There is a check valve that keeps it from pulling to much.

Originally posted by BFHCS
When I unscrew the gas cap I can hear a rush of air entering the tank so I don't think air is leaking in anywhere. But maybe. Also the gasket between the carb and the manifold was the old seal. I'm going to swap that out for the new one that came with the carb.

But why does run bad on a full tank or when I switch tanks?
anymore thoughts?

You need to check your "vent" lines sounds like they are clogged. I have this same problem after I dropped a 355 in my 93 Toy. I used the "Toy" tank and about 1/2 tank truck ran like crap....I heard the hiss noise 1 day when I was working on it. I drilled a "vent" hole in the gas cap and never had this problem again.........been fine for 2 years now........:beer: :D

Now the charcoal filter can is not hooked up anymore! So where is that check valve or maybe I don't have one anymore. I going to try the gas cap off and one with a hole in it. Basicly what I have is both lines come out the tanks on each side and somewhere merge together in a switch box. Then run a single line into the fuel pump then straight up into my carb with a filter inbetween. The charcoal can sits lonely off to the side not hooked up. Thanks for all your help so far.

With my '83 Toyota with the chevy 383, for the Edelbrock Pro-Flow injection I have a return line to the tank. I put a vented gas cap on because there is no way for air to get back into the tank after the pump takes it out.

I went and filled the right side tank all the way. I knew it was going to run bad. It did. So I stopped in a parking lot and took the gas cap off and drove around a little. It didn't do anything. The truck still choked down and stalled out. I started traceing the lines out. Both tanks have 3 lines comeing out and meet on this pump. Then 3 lines leave the pump and head towards the engine. 1 is the fuel line and I haven't traced out the other 2 but I think one might be the return line. I really think the problem is in that pump. Anythoughts on why 3 lines and not just 1. Why does each tank have 3 lines going to the pump?

Thanks

I think you mentioned driving around and it gets better. Thats the water and gas mixing as you drive. Plus the engine heating up helps hide the problem. Just a thought. Maybe you filled up at a station and got a whole bunch of water. As you use the gas you start getting to the water in the bottom. Just a thought. Draining your tank can be costly but not that much when you get down to it.

ok I just re read your post and what I said doesn't make sense. Your tank is almost 20 years old. Could be rust or something clogging stuff up. Sorry I can't help anymore.

I know that there is a whole lot to read so here are the main facts:
1. It idles bad at a full tank and when I switch tanks
2. It all started when I filled up both tanks at the same time
3. Removing the gas cap does not make a difference
4. It is not bad gas
5. I don't think its the carb because it runs fine at less than 75% tank of gas
6. I replaced a lot of parts with no luck (see 1st post)

I think the only thing left to do is::nuke:

i would drain the tanks completely; sure sounds like you might have some water in there. you can also install a water seperating fuel filter, very common on marine applications. have you replaced the fuel filter? what was in it?
good luck.
also, have a proffesional take a look at your rig and get that carb dialed in. they hardly ever work right out of the box.
oh yeah, i almost forgot :flipoff2: fawking newbie!

Originally posted by BFHCS
I know that there is a whole lot to read so here are the main facts:
1. It idles bad at a full tank and when I switch tanks
2. It all started when I filled up both tanks at the same time
3. Removing the gas cap does not make a difference
4. It is not bad gas
5. I don't think its the carb because it runs fine at less than 75% tank of gas
6. I replaced a lot of parts with no luck (see 1st post)

I think the only thing left to do is::nuke:

Pull your saddle tank down and take the fuel sender out. On the fuel sender there is a suction line. This suction line is where your fuel pump gets its fuel. On that line there is a metal like filter sock. Cut it off. Sometimes they will bend over and it will cut the fuel flow off. It will run still but not have much power. This could be your problem.

Originally posted by BFHCS

It got a little better. It now runs ok but not back to 100% yet. It runs like ass at a full tank of gas and gets better after and lower than 75%. I have duel tanks and if I switch tanks with the toggle switch it instanly goes back to running bad. Even if you switch back to the first tank that was previously running good. It has to sit over night to start running good again. I have never used the same gas station twice to rule out bad gas.


Could it be your selector valve or what ever it is called that switch your tanks for side to side. Maybe the valve is sticking or something after you try to switch tanks. You said it gets worse when you try to switch tanks. Just a thought.