Quadrajet Secondary issue?

[Tyler22474]

Hi forums! I'm new, wanted to get some advice on an issue i'm having.
Recently acquired my grandpa's old plow truck. We're up in the heart of the salt country, so it's obviously completely shot in the body department. But other than that, the truck has only 57k original miles, and is mostly complete except for a tailgate and some bits and pieces.
Anyway, I just got it running and driving. (Not fully roadworthy yet), and i'm having an issue with my engine performance. Anywhere else in the throttle besides barely tapping, it begins to bog pretty badly. If i just keep my foot barely pressing, it will run really well and accelerate slightly. But as soon as i push it any more, it begins to bog down and won't accelerate until i floor it to kick it down a gear. I've changed the fuel filter in the quadrajet, and put a new fuel line from the fuel pump to the carb. (Old one rusted off trying to remove it to flush old gas through.) Anyway, it idles about perfectly, and the accelerator pump is visibly working. Any ideas on what it may be? Right now my thoughts are either the fuel sock is completely covered in rust and varnish, or maybe my secondaries are somehow not getting fuel. But advice is much appreciated. Looking forward to reviving her to her former glory over the next few years. Thanks, Tyler

 

[75gmck25]

Secondaries rely on air flow and vacuum changes to open up, so you are not even close to the point where they take effect.

You checked your accelerator pump and it apparently works. However ethanol fuel does screw up the pump seal, and newer pumps use different seal material to fix that issue. It may help to buy a new accelerator pump.

Quadrajets also rely on a spring and vacuum operated piston, and a thin rod in the jets to manage the transition from idle to cruise, to power mode.
- With full idle vacuum the thick section of the rod is pulled down against the spring, into the jet (partially closing it off). Now you should be operating mostly on the idle circuit and your mixture screws are used to adjust the idle.
- As vacuum drops the spring pushes to overcome vacuum, and the rod begins to rise up, so the thin part of the rod is in the jet (allowing more flow). The thin part of the rod is the power mode section.

The size and taper of the rod, combined with the spring strength, will affect the off idle transition. You want the rod to move up slowly and flow more fuel through the main circuits, so it then adds to the fuel coming from the idle circuit. This is the transition.

Since you probably have a stock setup, you might want to check the transfer slot exposure of the throttle blades, adjust the idle mixture carefully, and maybe even richen the idle mixture a little bit. You want to bridge that transition from idle to cruise with a little more fuel coming in quicker. It may also help to raise the float level a little and replace the filter in the fuel line inlet.

With Edelbrock’s it’s very common to use a higher vacuum-rated spring set to allow the rods to transition to power mode sooner. If you use a lighter 8” vacuum spring it transitions sooner than a 5” spring, so you get richer mixture sooner. However, I don’t see many suggestions to try the same solution with a Quadrajet, so maybe it’s not as critical.

 

[fast 99]

Above suggestions are all on point. Might add need to start with basics, fuel pressure and volume plus basic ignition checks. If that checks out move on to the carb. With a small screwdriver find the large vent hole in the top carb housing. Carefully insert the screwdriver, should be able to feel the primary metering rod linkage. If stationary it's stuck, will run very lean. Also if the secondary air valve wind up spring is not properly adjusted will allow too much air too fast causing a similar issue.

 

[Matt69olds]

Start by a detailed and careful inspection of the fuel system. Check the lines, especially the rubber hoses at the tank. Any cracks or pinholes means the pump is sucking air instead of fuel.

More than likely the rubber components have deteriorated with today’s nasty fuels. If you decide to tear into the carb, get a rebuild kit from Cliffs Q-Jets. He has high quality parts that are compatible with today’s gas, and the tech tips to make it work like new. Be VERY careful when removing the power piston and metering rods, do not pry are use excessive force. It’s very easy to bend the metering rod hangers, which screws up the fuel distribution. Then people wonder why it runs worse than it did before “rebuilding” it.

Most of the symptoms of the reputation of the “Quadrabog” are from the idiots who attempt to work on them without a clue of what they are doing.

 

[Sad Sack]

First thing I usually do is look for wet spots around carb gaskets, good filter, check vac and fuel lines, make sure all screws-nuts bolts of carb are tight, choke pull is functioning correctly using the push in plunger then plug vac hose orifice and see if the pull off lever moves out immediately or comes out an 1/8" in like 30 seconds, ...the usual stuff.

 

[Johnny Atomic]

Hi forums! I'm new, wanted to get some advice on an issue i'm having.
Recently acquired my grandpa's old plow truck. We're up in the heart of the salt country, so it's obviously completely shot in the body department. But other than that, the truck has only 57k original miles, and is mostly complete except for a tailgate and some bits and pieces.
Anyway, I just got it running and driving. (Not fully roadworthy yet), and i'm having an issue with my engine performance. Anywhere else in the throttle besides barely tapping, it begins to bog pretty badly. If i just keep my foot barely pressing, it will run really well and accelerate slightly. But as soon as i push it any more, it begins to bog down and won't accelerate until i floor it to kick it down a gear. I've changed the fuel filter in the quadrajet, and put a new fuel line from the fuel pump to the carb. (Old one rusted off trying to remove it to flush old gas through.) Anyway, it idles about perfectly, and the accelerator pump is visibly working. Any ideas on what it may be? Right now my thoughts are either the fuel sock is completely covered in rust and varnish, or maybe my secondaries are somehow not getting fuel. But advice is much appreciated. Looking forward to reviving her to her former glory over the next few years. Thanks, Tyler

mine also was my Grandpas and also 57k orig miles.

 

[Edelbrock]

Everyone has different methods and different procedures that they like. Some would do it this way, some that way. Your best bet is to read through it all and goes the direction that makes sense to you.

I didn't read all the other posts, so these things might already be mentioned.
I would would get it running, then shut it off and take the fuel line off the carb and put it in a bucket.
Then I would restart the engine and looks to see how fell the fuel is flowing out of the fuel line.
Don't need much pressure, don't need a lot of volume, but you do need some of both.

If that checks out, I would look at where the fuel line goes into the carb to see if there is a little screen there that might be obstructed.

If that all checks out, I would hook the fuel line back up and start the engine. Then I would have someone sit in the cab and the slowly press the throttle further and farther while you spray more and more starting fluid into the carb to see if you can get the RPMS way up.

This is all to see whats going on with the fuel system.

My next step would be to remove the carb and install a known good used one. They can bad had on Craigs or FB Marketplace for maybe $50 and only takes a few minutes to swap out.

Carbs are all different, and swapping one out for another can sometimes make a HUGE difference in engine performance. Changing brands, or even replacing with the exact same make model can make a big difference. Lots o little ports that can get plugged, various adjustments, a wide range of different internal parts etc.


Good luck on your repair.