First real drive! Not tragic but not good. I think I need carb advice.

[Chris64]

Just to branch off another question - while inspecting the fuel lines - the left/right tank switching mechanism is higher than my current fuel level (about mid tank height but I only have about 5 gallons of gas on the right tank) - I did get a dribble of gas out of the tank (the other tank is more empty). I was going to try suctioning that line from there just to see if it draws from there, but needed to get back to work (I'm working). It seems weird that this line would be totally dry.

Anyway - it appears I'm missing a part under my truck. What's supposed to be here? There's one on both sides so I was thinking maybe a cross member.

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[Grit dog]

Can’t help with the mystery bolts without a larger pic and a peek under my K20, but on the fuel thing, IF the fuel pickup is where it should be from the factory and working properly (not plugged) it will use every bit of 19+ gal from the “20 gal” tank. Consistent on 2 trucks with 4 tanks.
5 gallons will pull fuel all day long and if more than 2 gal imo it should pull fuel consistently.
Remember you can check if the pickup is catching fuel by blowing into it, will here it gurgling air bubbles. If no bubbles noise, it’s not touching fuel. If you blow your cheeks out then it’s probably plugged.

 

[Chris64]

Those mystery bolt holes don't seem to exist on other frames I see. Here's a pretty clean frame - that big hole is the big bolt on mine (he removed one of his tanks apparently). I wonder what they had there. I think they had a camper on this so maybe one of those brackets it ties down to.

I'll sneak out tomorrow at lunch to do some more gas tank investigating.

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[Chris64]

Weekend Update: I switched out the fuel pump. I thought it went well but but then I got nothing out it (it was not primed). My dad came over and brought his knowledge of carbs with him. We started pouring a little gas in to the carb to get it to fire up and after 4 or 5 stalls it started drawing fuel from the tank (yay).

It still stalls as soon as you try to accelerate unless you do it very slowly. His hunch was maybe the throttle pump is making it too rich but that was just a guess.

Then we started getting a loud squeal because...of course. We can't just have things work! The easiest way to diagnose the problem was to disconnect the alternator belt and that did it. So I went back to my friendly "going out of business auto shop" with some good discounts. They had a really cool chrome 100amp for $100 or a generic 61amp for $30....so naturally I went with the bling (hopefully I won't regret that).

Anyway - sun was going down when I got it all back together. It seems like it's 1 step closer to switching the carb but now I'm getting a strange rattle (it may not be new, I just haven't been able to really listen to it idle).

This is my first attempt on a video so I don't know if it will translate. You can also see a moment where I push the throttle and the motor does a little bog but then picks up. It doesn't appear nearly as bad in this video as it is normally (and we figured out that if you finesse it, it doesn't die). The intermittent sound is what I'm asking about. Maybe it's valves but it seems to come and go. Sounds like rattling.
Around 18 seconds you can hear it.

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[Chris64]

Help. I'm trying to understand what this line here does.

It appears to go down the the little reservoir on the firewall radiator support where it then goes down to a hardline that goes all the way back to the gas tanks and hits a T-junction to each tank. The T is next to the fuel tank switch solenoid.

That all seems like a vacuum type setup but I can't figure out why the gas tanks would have vacuum lines running to them.

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[Chris64]

I'm really having no luck with this. After we got it running last week, now it's no longer primed. To make matters even more strange it doesn't seem like it's even trying to draw fuel. Almost the opposite. I will go through about half a cup of gas putting a little in the intake. It will start up and run for a few seconds and if I keep feeding it it will keep running. Eventually it dies and I'll go check the fuel line coming out of the tank and it's bone dry. I'll start a syphon and it will come pouring out. I'll reconnect it to the solenoid and try to prime it again and that line will be bone dry again. It's almost like the pump is pumping backwards but I don't think it is. It was working last weekend. The tank is about 5/8 full on gas. The other tank is almost empty though.

 

[Chris64]

Update: I machined a little tube to bypass the fuel tank selector. I think the driver (default) is nearly empty so I think it's draining the fuel line or something like that - OR maybe the switch itself has a leak. Either way, that seemed to do the trick for getting my fuel line to actually draw fuel.

I started the truck up and presto - it was running...sort of.

gas was emanating from the carb everywhere. It was bubbling out of the throttle pump. It was leaking out of the side. It was a mess.

That's it...I'm switching carbs.

I broke out the Edelbrock AVS2 I bought and decided to figure out how to install it.

A few notes
1) there was no place to put the kick-down line so I did a little customization to put it where the spring return can go (there are multiple options).

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I should've taken a better side pic of my handiwork.

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2) The bracket that holds the throttle cable needed a few mods. I knocked out the factory pin and bored out the hole for mounting as it lined up better. I just opened up the mount hole on the other side so it's like a "C." It should do the trick. I'll probably pull it off and weld a washer to make it a little less whoopty.
3) I'm still unclear on why a vacuum line goes back to the gas tanks. For now I left it disconnected because I'll need to make a step-down hose to fit the AVS2.

It fired up instantly. It revved great. Idle might be a smidge high but it's hard to say until the motor is warmed up.

Needless to say, I'm very happy. I'll be finishing up the mounting and wiring over the holidays.

And my old carb was indeed a Quadrajet (I wasn't entirely sure before). I plan to either try to do a rebuild or send it out to someone to get it rebuilt.

Looks bad still, but a different kind of bad

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[legopnuematic]

The line from the carb to the fuel tank should be a vapor line.

On my 76, goes from the carb, to the charcoal canister, a hose from the charcoal canister goes to a hard-line on the frame back to the tanks.

 

[Chris64]

The line from the carb to the fuel tank should be a vapor line.

On my 76, goes from the carb, to the charcoal canister, a hose from the charcoal canister goes to a hard-line on the frame back to the tanks.

Is that what that tank is? OK - so I'm fine to just connect it to an open vacuum line on the carb, right?

 

[CountKrunk]

Buy the manual and see how the fuel lines are supposed to be.

Rock auto sells a solid one. I have it for mine. Good reference tool.

 

[Chris64]

My latest change. I didn't like the spring going forward and I also didn't like that I didn't have redundancy on the spring. I made a little bracket that has a little ear on it to keep it from rotating up. This puts the springs in a mostly stock position. It doesn't touch anything or rub on anything. I may make another one now that I know it works. It bolts on to the kick-down knob.

Now you can all tell me that there's an easier way to do this.

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[75gmck25]

I had the same Quadrajet on my ‘75 GMC, and that extra hose is definitely the gas tank vent and vapor recovery.

The original carburetor is a unique design that uses a port on the carburetor to control gas tank venting without having any other external control valve for vapor recovery. In most other years the vapor canister has at least 3 ports (your should only have 2) and the third port is a control vacuum line.

I have the canister vent line just running open, since newer carburetors don’t have a control port where I could connect that line. Seems to vent fine, and I don’t get a gas smell in the truck. I’ve also tried connecting the vent line to a T in the PCV line, but that seems to create a vacuum leak and lean out the carburetor.

 

[Ricko1966]

I had the same Quadrajet on my ‘75 GMC, and that extra hose is definitely the gas tank vent and vapor recovery.

The original carburetor is a unique design that uses a port on the carburetor to control gas tank venting without having any other external control valve for vapor recovery. In most other years the vapor canister has at least 3 ports (your should only have 2) and the third port is a control vacuum line.

I have the canister vent line just running open, since newer carburetors don’t have a control port where I could connect that line. Seems to vent fine, and I don’t get a gas smell in the truck. I’ve also tried connecting the vent line to a T in the PCV line, but that seems to create a vacuum leak and lean out the carburetor.

Put a plastic fuel filter in the end of the vapor line,it can still vent,but keeps dirt,bugs,etc out of it. If the throttle return spring pulls straight forward on the throttle cable,it is easier on the throttle shaft. With the spring pulling towards the rear and the cable pulling to the rear it is a constant rubbing under pressure on the rear of the throttle shaft when you open and close the throttle. If the spring is pulling the cable in a straight line,and the cable is pulling the spring all the pressure is on the spring and the cable not on the throttle shaft.

 

[Chris64]

Put a plastic fuel filter in the end of the vapor line,it can still vent,but keeps dirt,bugs,etc out of it. If the throttle return spring pulls straight forward on the throttle cable,it is easier on the throttle shaft. With the spring pulling towards the rear and the cable pulling to the rear it is a constant rubbing under pressure on the rear of the throttle shaft when you open and close the throttle. If the spring is pulling the cable in a straight line,and the cable is pulling the spring all the pressure is on the spring and the cable not on the throttle shaft.

I see what you're saying. I liked it being like stock and tucked under the air-cleaner. Back to the drawing board.