Rebuilt Quadrajet

Mason Lawley · Dec 30, 2021

Re-installed the quadrajet after doing my first carburetor rebuild. The first video is of the truck running without a foot on the pedal at all. Seems to idle high for awhile, and then eventually dies.
Any help to point me in the right direction is much appreciated. Pictures of the carb at the end of the post.

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Second video there is a foot on the pedal applying gas, and still dies out the same.

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Here are some pictures of the carburetor as well, had the vacuum gauge plugged into the rear manifold vacuum and was getting right at 20 in. -Hg during both videos.

1978 Chevy C20 Stock quadrajet and 350

Tank6x2 · Dec 31, 2021

Nice job!

fast 99 · Dec 31, 2021

I am new here but have been rebuilding those carbs for decades. Why did you rebuild it in the first place? It is hard to say but here are some ideas. If the motor is cold choke plate should be only slightly open. Does the front choke pull off hold vacuum? Easy way to check that is compress the linkage and hold your finger over the vacuum port. Idle screws should be about where they were on disassembly. If you don't know that, start at about 3 turns out. It doesn't sound rich in the video so check for vacuum leaks. If you can safely do it dribble fuel in the top. if lean the motor should increase RPM. If rich it will slow down.

Mason Lawley · Dec 31, 2021

Thank you for the reply and help!

Well I had been using it as my daily for about a year. Drove it from central California to Reno, Nevada and back and forth a few times. The latest trip on my way back from Reno, we were just about home and the truck made a strange noise as if something fell off. So we pulled off the road to take a look. Didn't notice anything out of the ordinary, so we started it back up to take off, and the truck went full throttle and was backfiring. Checked the throttle linkage to see if it was stuck open, and didn't notice anything.

Had it towed back to the house and tried to do some research, ending up coming to the conclusion that the carb needed rebuild.

When tearing the carb apart, i noticed that on the back of the float, that small piece that hangs was not hanging, it had dropped to the bottom of it's slot. And the small metal hook was nowhere to be seen.

I will check the choke pull off for vacuum on Monday, when I can get back out to the truck. I had the vacuum gauge plugged into the rear manifold vacuum port, and it read 20 in.-Hg during the video. I will try and trickle fuel in on Monday as well.

mtbadbob · Dec 31, 2021

Does it start right back up after it dies? It almost sounds like it runs out of fuel, like the needle is hanging up on the inlet valve. The needle is what hangs off the back of the float on the little clip, seats against the inlet valve to control the fuel inlet into the bowl.(in case you didn't already know the terminology) Also, make sure that choke plate is open a bit on cold start, like around 1/16th-1/8", and the idle will be fast if the auto choke was set prior to cranking, then it should kick down with the blip of the accelerator pedal after a minute or so of fast idle.

Paladin · Dec 31, 2021

Get a Holley!

mtbadbob · Dec 31, 2021

Paladin said:
Get a Holley!

Then, find the nearest dumpster & toss it in!

Anyway, I hope you bought a new BRASS float and installed it when you did the rebuild.

Mason Lawley · Dec 31, 2021

If I remember correctly, it started right up after the first video but only ran for a couple seconds. I had to refill the bowl with gasoline after that, so it does make sense that it is running out of fuel.

In regards to the float and the needle/inlet valve. In which configuration is the most fuel let into the bowl.... with the float all the way up and the needle all the way down... or the float all the way down and needle all the way up...

And I'll be sure to open up the choke plate a bit. I had to take the choke thermostat off to tighten up that screw behind it as the adjustments weren't doing anything but turning. Now they move the choke plate as they should, and I closed it a bit too much.

Thank you for all the help! I've got a plastic float currently in it from a rebuild kit I got at O'reillys, but if i need to take it apart again to fix this problem I will definitely replace with a brass one!

Mason Lawley · Jan 1, 2022

Scratch that float and needle question. I found some diagrams that helped me to understand. I will take the carb off next week and rebuild it again and take note of it.
Thanks again for the help!

fast 99 · Jan 1, 2022

mtbadbob said:
Then, find the nearest dumpster & toss it in! Anyway, I hope you bought a new BRASS float and installed it when you did the rebuild.

I respectfully disagree. Problems with QJ are usually a rich condition and can easily be repaired. A stock Chevrolet will run better on a QJ than any other carb. One thing I have found out doing emission infrared repairs is the brass float doesn't work the same as the stock fiber one. The problem with a fiber float is after being handled during rebuild it will likely sink shortly afterward.

There are many things I do during a rebuild to avoid a comeback. Here's a few suggestions. ALWAYS toss the float and replacing with a fiber one, new pull offs, sealing the well plugs, removing the top housing metering rod adjustment plug [if equipped] for later adjustment, tapping in the emulsion tubes, making sure the float hinge pin isn't collapsed, new filter, checking the secondary air valve, and if loose re-bush the front throttle shaft. One other thing, if the float well has ever had water standing in it the probably of a that carb ever working correctly is nearly 0.

Other missed items are the vacuum and centrifugal advances not working and if it has a divorced choke mounted on the intake, crossover heat with a working heat riser is mandatory.

mtbadbob · Jan 5, 2022

fast 99 said:
I respectfully disagree. Problems with QJ are usually a rich condition and can easily be repaired. A stock Chevrolet will run better on a QJ than any other carb. One thing I have found out doing emission infrared repairs is the brass float doesn't work the same as the stock fiber one. The problem with a fiber float is after being handled during rebuild it will likely sink shortly afterward.

There are many things I do during a rebuild to avoid a comeback. Here's a few suggestions. ALWAYS toss the float and replacing with a fiber one, new pull offs, sealing the well plugs, removing the top housing metering rod adjustment plug [if equipped] for later adjustment, tapping in the emulsion tubes, making sure the float hinge pin isn't collapsed, new filter, checking the secondary air valve, and if loose re-bush the front throttle shaft. One other thing, if the float well has ever had water standing in it the probably of a that carb ever working correctly is nearly 0.

Other missed items are the vacuum and centrifugal advances not working and if it has a divorced choke mounted on the intake, crossover heat with a working heat riser is mandatory.

Yeah, I agree with you, I was referring to his "buy a Holley" comment! I know the brass floats tend to be a bit heavier, but they will not saturate like the fiber one's tend to. I'm not going to argue with you, sounds like you've done far more rebuilds than me! I'm just shade-tree mechanic, do my own things.

Bextreme04 · Jan 5, 2022

mtbadbob said:
Yeah, I agree with you, I was referring to his "buy a Holley" comment! I know the brass floats tend to be a bit heavier, but they will not saturate like the fiber one's tend to. I'm not going to argue with you, sounds like you've done far more rebuilds than me! I'm just shade-tree mechanic, do my own things.

The brass floats are good but require a different than stock float level adjustment. In general the brass ones tend to have less buoyancy and require the float level to be set lower than the stock level to get the same level in the bowl. If you set the brass float level to the same level as stock they tend to be too high and run super rich.

fast 99 · Jan 5, 2022

mtbadbob said:
Yeah, I agree with you, I was referring to his "buy a Holley" comment! I know the brass floats tend to be a bit heavier, but they will not saturate like the fiber one's tend to. I'm not going to argue with you, sounds like you've done far more rebuilds than me! I'm just shade-tree mechanic, do my own things.

mtbadbob said:

Yeah, I agree with you, I was referring to his "buy a Holley" comment! I know the brass floats tend to be a bit heavier, but they will not saturate like the fiber one's tend to. I'm not going to argue with you, sounds like you've done far more rebuilds than me! I'm just shade-tree mechanic, do my own things.

Click to expand...

Ok I agree with you. Didn't mean to start anything. I just shake my head every time I hear toss a Q-jet out. If anyone wants to toss them my way? Although Hollys are decent carbs they are prone to leaking. On vehicles that came stock with Q- jets they are no better. Actually, today I like Carters better than Hollys for a conversion. The jetting is very close right out of the box. However, as we all know Carters are different and can be harder to jet.

mtbadbob · Jan 5, 2022

I'm with you, always have defended the Quadrajets as the best option for any GM. I've always had good luck with getting them tuned right for my vehicles. I have a $2,500+ one on my '70 W-30, factory carb's for them are bringing stupid money!

gotyourgoat · Jan 5, 2022

fast 99 said:
Ok I agree with you. Didn't mean to start anything. I just shake my head every time I hear toss a Q-jet out.

That is a lot of shaking. You are going to get whiplash if you hang around here too long. Probably mentioned every 16-20hrs.

I'd never kick a quad out of bed though. Hell, I ran one for quite some time after National carb literally laughed at it. :favorites13:

Rebuilt Quadrajet

Mason Lawley

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