Early Quadrajet off idle stumble/hesitation

[Cgoldhill]

Recently professionally rebuilt 7042210 casting quadrajet that came stock on my 72 c10 350 4 speed.

When i first got it back, I put it on, set the idle mixture screws to highest vacuum reading and made sure my choke was opening all the way after it was warm. Truck drove awesome, nothing super fly but good low end torque.

Ive had the carb on for about a solid month and all of a sudden Ive gotten a crappy off idle hesitation that could sometimes turn into a pop through the intake if I blipped the throttle enough in neutral. While driving the issue translates into a initial bucking/hesitation as you go through the gears.

One day the truck was fine, the next it started happening.

Hei ignition, new plugs and newish wires, 12 degrees initial timing with vacuum advance hooked to manifold vacuum.

From what i see this early casting doesnt offer any kind of external APT (adjustable part throttle).

The issue is most pronounced anytime you blip or stab the throttle in neutral, whether its 1/2 or WOT.

Ive done a search and can only find info for later apt carbs. Others talking about rejetting but this is a stock 350, and the carb run just fine with current. Just need to rule anything out before i have to go be that guy asking for free stuff from my carb rebuilder.

Any ideas on what to check, clean, adjust?

This does not to seem to be any kind of a secondary air door gap opening issue.

[brown7373]

First thing I would check would be the accelerator pump. Some rebuild kits are better than others. If the accelerator pump cup is not made of a rubber compound that is impervious to the additives in todays gas, it can swell or distort and leak. The pump is actuated EVERY time you increase the throttle opening. Some kits sold by parts stores are complete junk. I would ask the professional which kit he used.

The only kits I use come from www.cliffshighperformance.com . There are other good kits, but I have had no problems with Cliffs kits.

[geezer#99]

The apt won't give you those symptoms. You might get a little tip in stumble but no backfiring.
After you check the accelerator pump, reach down in the air vent and make sure the metering rods aren't stuck down. With the motor off you should feel them push down and pop back up.
Could be just some gunk stuck in one jet around the rod.
Here's a link showing the location of the apt in your carb.
http://www.crankshaftcoalition.com/w...4MV_carburetor

[Cgoldhill]

i checked the accelerator pump, looked down in the carb and didnt see two solid streams of fuel, didnt see anything. Pulled the carb top cover off and checked the accelerator pump. Plunger was in good shape and i could get fuel to move when operating the pump by hand. Put the carb back together and still same symptoms.

I ordered a rebuild kit. Will be replacing the top cover gasket and accelerator pump. Not looking to do a full rebuild but anything else i shoukd check while i have the top cover off?

[Cgoldhill]

Looking at accelerator pump assemblies Im realizing mine was missing the garter spring which sits inside the plunger. Inititally when removing the pump i noticed the cup wasnt secure to the plunger shaft and assumed it came loose during removal.

Definitely need this spring for good operation right? I see some kits come with the spring and others dont. I also dont see a numbered part in the quadrajet parts diagram, just an accelerator pump assembly.

[special-K]

Could be too much fuel, rich mix

[brown7373]

I had a 69 GP that a week after a carb rebuild did what you describe. The pump looked good, but it had swelled and would stick in the bore and pull off the pump shaft. It was made from rubber that was NOT impervious to the additives and ethanol in today's gas. I used to buy NAPA kits, and order a special accelerator pump designed for the additives. That helped some, but then the formulation or change from 5% to 10% ethanol changed that. The kits at Cliffs , or just the pump, are more expensive, but they will work properly for years. Maybe NAPAA and other parts stores have updated and improved their kits, but I'll stick with what I KNOW works.

[geezer#99]

Try this first before you rebuild it.
Use a small screwdriver and push the plunger down without moving the linkage. You might see fuel discharged lower in the plunger stroke. If you do, just adjust the pump link.
FWIW I've seen pumps with and without the garter spring. Depends on what the pump cup is made from.

[Coley]

.....hmmm, something about popping back through the carb leads me to think this might have something to do with the distributor....vacuum advance issues? or?
As for the carb, the first I do with all the quadrajets I use is remove them, partially disassemble and fully clean them top to bottom with the carb cleaner. Then blow compressed air through everything front to back. This has always 'bumped' the operation of the carb for me.
....now about that 'spark' or possible timing issue?
Coley