1974 GMC 4x4 carb issues

[SlackScotty]

I just picked up this 74 GMC 1500K pickup and plenty of issues with the carb. Put another one on it that came with the truck and running a bit better but still problematic. Hoping you guys can assist.

First... I know jack **** about this stuff but learning fast. Bear with me and please... speak to me like I know nothing. Because its true. lol.

I have the carb to the point where it fires up and runs well at idle but when placed in gear it struggles immediately to get out of the gate even at a super slow speed. Any attempts to apply throttle just results in it choking out and sometimes a backfire out the carb. I have seen flames twice now blow out. I have a suspicion about two things... some kind of vacuum issue or the fuel pump is causing too much delivery once its in gear and the truck is moving. Am I even close?

Here are a few pics of the carb. I see some kind of little device that can hold a hose and some wires on one end by the look of it but nothing connected to it. It has two hoses coming out of it on one side to the carb though. Not sure what this does or what should connect to it. Hopefully you pro's can spot things are are familiar with this type of carb and can get me rolling the right direction. What kind of carb is this btw? I ask because again... I'm learning as fast as I can.







Thanks for your time gang.

Scott

[donut]

1st off, it's a Quadrajet.
When was the last time a tune up was done? Usually when I hear backfire, I go to timing being off.
Many carb issues are really ignition issues. Course I'm no expert, but added info may help others.
When you swapped carbs, did you do anything to the new one or was it a bolt and go?

[motornut]

Hi looks like the temp wire is sitting on the new rubber hose
goes to the side of the block
vac leaks check that brake booster hose,clamps loosen up

[SlackScotty]

I had a hunch it was a quadrajet after watching a youtube video last night. A bit complicated to take apart and adjust by comparison to an Edelbrock that I seen in a different video. Is it a tough gig to go from one to the other? I wonder if it would be worth it to buy one. Anyways, for now this is the one I'm with. From what I understand, this carb was on it to begin with and the owner put the other one on it. He had it running great but it would die after the engine got hot every single time so we decided to put this one back on and see whats up. You are probably right about the tune up. The idea was to get this running so we could drive it to the shop to give it one but so far I cant drive it out of my neighbourhood unless I go 10mph. Any attempts to apply fuel and go faster kills it. Whats involved in a tune up? Is this something I can play with on my own with help from others or far more complex? Looking fwd to continue learning this stuff. Last thing I want is to be off road camping and not able to fix things so no better time than now to dive in the deep end.

As for vacuum leaks, I watched a few video's on how to check for them. What is your recommendation to do that?

[gchemist]

Welcome to the site. I always freak out when I see blue engines. It's cool.

Read these articles on how to rebuild and tune a Q-jet.
http://www.73-87.com/7387garage/garage_drivetrain.htm
Best carb ever for a daily driver. GM did their homework on it.
If you want to replace it: checkout this company http://www.nationalcarburetors.com/

[SlackScotty]

Thanks gchemist. Will check those out.

[SSC's76]

You vacuum lines look ok you are missing one for the anti diesel solenoid but that's no big deal. Just for grins pull a few spark plugs and check the comdition. The plugs may be fouled out and a load on the engine may cause it to miss like you describe. Also make sure 5 and 7 plug wires aren't crossed. That will cause an issue just like you describe as well. I don't think it's a Low fuel issue usually won't backfire at low speeds and low fuel will just kinda Peter out. Let us know what you find
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[Corts60]

Before all the nay-sayers chime in, there is nothin wrong quadrajet carbs. Only people who don't understand them hate them.

Having said that, they are more complicated than an edlebrock or Holley to overhaul and things can easily get bent or misplace if you're not paying attention.

First off, in the last pic, the silver round can that has an arm pushing on your throttle and a vacuum line going to it is your dash pot. That was used to bump the idle up when using a/c and (unless having a vacuum leak) will not be a source of your problem.

Next, I would buy a vacuum gauge and a timing light. You will need these to set base timing and idle/air mixture.

Let us know if you know how to set timing or need us to explain it and we'll move on from there.
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[hotstuff]

It would help to know the carb # , it runs vertical on the carb body behind the accelerator linkage . Do you get a strong squirt of gas in the carb , when you rack the linkage?

[SlackScotty]

I'm going to start with the plugs. Easiest so will start there.

Do you have a map of what the plugs should be from the engine to the coil? I can go from there to see if crossed once I know how they should be connected.

Next thing I will do is pick up a fuel pump. Pretty cheap I understand and not overly difficult to swap. If this doesnt work out, no biggie as I will have a spare. We had pulled the fuel line off the carb and turned the engine over once and it pumped fuel out but it was just a flow as opposed to a spurt. A guy today told me that under 5-9lbs (If I recall in my reading) it should practically shoot right across the engine compartment. Your thoughts on that? I read today in a link above to pull the hot wire off the coil before turning the engine. If you guys agree... I will do this again but curious about how much fuel pressure I can expect. I will put a bottle on the end of the thing this time. Should it flow or spray?

Should none of these work out... I will pick up the timing tool and vacuum tester and take your lead on this. I know nothing of this stuff.

I will get the number off the carb for you tonight after work. When at idle the carb runs like a dream. I can throttle the gas manually as much as I like and it rev's up dreamily. No hesitations at all. Only under load which makes me think that during the course of these headaches the plugs may have been fouled. This makes sense to me.

[SlackScotty]

I have another request. If any of you guys have the tools and the time... take those pics of mine and write on them some numbers and then let me know what all those hoses and diaphrams are and what they do? No worries if you cant but hoping maybe one of you can. Easy way for me to learn this way.

Thanks in advance

Scott

[Corts60]

If You really want to change the fuel pump, go for it and don't waste your time checking your old. The only proper way to check a fuel pump is with a fuel pressure tester. Looking at the fuel coming out of the pump tells you nothing. Pumps create flow, not pressure. It is the resistance to flow that creates pressure and when your line is disconnected you have no resistance and it only lets you know you have flow.
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[Psycho_SWB]

Like people stated, check your spark plugs first, follow the wires and make sure the firing order is correct. Then check your timing. Your timing looks like it may be retarded by looking at the direction of your distributor cap.

[motornut]

Check the little metal tubes sticking out of the carb ,they can pull out,
as we wiggle and tug the hose on and off.....
try a wiggle on each see if loose ,a slight tap reseats them,
I've heard of blasting air in those can clean gunk out, not something i've done
hoses crack and split,get oily and stretch


using a can of WD40 and spraying around the intake /carb might show engine rev change

the vac advance was sticky on mine,it got better with cleaning,but I replaced the dizzy

the weights advance was frozen,worn oblong ish too

[SlackScotty]

Okay, picked up some gas line antifreeze to put in the tank in case there was moisture. Hasn't made any difference.

I also replaced each plug with some top of the line (so they said) and I'm a bit concerned. Each plug is covered in oil. 3 were cracked and broke the moment I put the socket on them. Only one plug had a relatively clean end. Should I be freaking out about how much oil is on these plugs? See photo. After these changes it actually ran worse. It now chokes and stutters a bit at idle which it didn't do before. Next thing I would like to trouble shoot is the lines to the spark plugs.

How do I tell if they are connected correctly? What is the firing order?

Thanks

[Corts60]

The firing order is 18436572. #1 spark plug is the forward most on the driver side. Follow that wire back to the distributor. That is #1 on the cap. Now just go in a clockwise order and check that all the wires are going to the right plug. 1,3,5,7 on the driver side and 2,4,6,8 on the pass side.

Also, the most important thing about troubleshooting is that you do not change too many things at once. You'll end up chasing your tail all day. Confirm one component at a time and move on methodically.

[SlackScotty]

Thanks. So to verify order...

Passenger Driver
8 7
6 5
4 3
2 1
radiator


Thats the block. How about the distributor?

[Corts60]

Yes, you're right.

The distributer is clockwise looking down on it, 18436572. Like I said, follow #1 plug wire back from the plug to the distributor, then start going clockwise and following the wires.

Just by looking at your pics I can tell that #1 is the wire closest to the carb on the driver side.

[SlackScotty]

I think I got it. My distributor should look like this then?

Pass Driver
6 5
3 7
4 2
8 1
Carb

[Corts60]

Does this help you?

[Corts60]

No matter which way you cut it, you are going to need to time this thing so you have a baseline. Go buy yourself a light.

[SlackScotty]

Pretty much tapped out this paycheck so next one perhaps the timing light. Will see what comes up between now and then.

Thanks for your help Corts60 (loving your wagon btw... great ride. Was just checking it out) and thanks to all you others as well.

Cheers

[motornut]

the part "unsure of funtion "looks the the control for the heat riser
does still have a rod running to the manifold?
seized or moving?

[SlackScotty]

Hey Motornut... not sure which unit you are referring to with reference to a rod. That "unknown" item is a diaghram from what I can tell and has two hoses on it. One goes to the manifold cover. Not sure what it does either actually.

here are some pics of the engine from the side with the cover on. Maybe draw on a pic about which piece you are referring to?

[motornut]

sure
in my 79 you see the rod better
holds this butterfly closed ,helping with warm up
seen these rust siezed closed (worse)or open
i just remove them when i can