off-idle stumble has me stumped

I've had an off-idle stumble with my 4MV quadrajet since I bought the car a few years ago: at a stop light / stop sign, pressing the throttle more than a little bit will cause the engine to stall. By feathering the throttle I can increase RPMs a bit, and then the throttle will work fine without stalling. Once it's off idle, it runs like a champ.

Recently the carb got into a bad enough state that I couldn't even start the car -- it would flood instantly (it had a LOT of carbon build up, so I think something finally just got clogged), so I decided it was time to finally rebuild it. I did that, and it started right up, so I gave it a basic tune-up and took it for a test drive. Once it warmed all the way up, the off-idle stumble came right back.

So I'm scratching my head a bit here. I fixed up the carb basically as good as I know how to, and it had no effect on the off-idle stumble. What else could cause that? o No:

Just to be thorough, here's all the things I have done so far:

rebuilt carb:

• replaced all gaskets
• replaced in-line fuel filter
• replaced accelerator pump, which was what I *thought* was the cause of the problem...
• cleaned all metal parts by soaking in solvent and brushing with toothbrush, then drying with alcohol and compressed air
• I did NOT replace the float; I put it back exactly as it was before.

verified that the choke:

• sets the fast idle cam correctly when cold
• opens up as the engine warms and drop the fast idle cam down to slow idle
• is wide open once hot
• stays open when the throttle is pressed

I set the idle mixture by doing the following:

• attach external tach (I used a timing light that clips on to a spark plug wire)
• start the engine and let it warm up on fast idle
• wait for choke to open and switch down to slow idle
• adjust idle mixture screws so that the RPMs are as high as it'll go
• adjust idle speed back down to about 750 or 800 RPMs (I forget which I used). Anything below this and it will idle like crap when put in gear; I have no idea how the factory-spec 620 RPMs is achieved...

So my questions:
1. Did I use the right procedure to set the idle mixture?
2. Could a vacuum leak cause this? How do I actually check if I have a vacuum leak or not?
3. Could it be a problem with the timing or timing advance?
4. Could it be something else?

Again, it idles pretty well (though a little faster than it probably should), and it runs great once it's throttled up, but getting between the two states is the problem.

I'm not an expert mechanic by any means, so be patient with me if I'm asking dumb questions. I just want to figure out how to do this right

 

Hi Bill and welcome to V8 Buick.
What is the Quadrajet code number? should be a 7029240 or similar.
If you got a parts store kit, those are typically junk and many times the accelerator pumps fail. Todays fuel can cause them to swell up and the the cup will fold up in the bore. And then the stumble returns.

You set the idle screws correctly for highest vacuum or idle speed.
Timing can also be critical for your performance.
I have a quadrajet tuning sheet I would be glad to send to you if you provide your email address.

 

Yep, the engine is bone stock. QJet number is 7029240. Don't know about the distributor, I'll check that and get back. I'll also check the vaccuum (I've got a gauge on the way), but I can tell you right now it's at least enough to pull the vacuum break on the choke and keep it there throughout operation. I haven't done squat with the timing, so I'll verify that the advance is working.

Rebuild kit was from The Carburetor Doctor, so hopefully that's a bit more legit than a parts store one. Seemed pretty good quality to me. It also came with a massive PDF of manuals and diagrams that I couldn't manage to find anywhere else, which was helpful to me as I'm pretty much a beginner at this. The original accelerator pump was pretty shriveled and inflexible when it came out.

 

Verify accelerator pump shot, you should have 2 strong streams of fuel when throttle is opened slightly.
There are different accelerator pump designs, some work a lot better than others.
See link below to see differences in Q-Jet pumps:
http://quadrajetparts.com/quadrajet-accelerator-pumps-t-11.html

 

How easy do the secondary air valves open? They need to be set So that only when needed they open, as the air draws them open against torsion spring tension. If they easily flap open with your finger with the engine warmed up and off (not running) then you must reset the spring tension. Let us know what you find out

 

A 4 MV has a adjustment screw on the air horn rite in front ware the air cleaner round gasget goes . You can use a slimed down mix adj tool usually flex able to turn it . I would go 1/8 turn up , turning left. This sets the max needle lift or height when u give gas and vacume drops. The spring under the piston lift it up. If you get lost on turning usually down all the way and then up two an half turns is good . That may not be it but it's one thing it may be.

 

Which hole in the accelerator pump lever did you put the linkage in? There are 2 holes. The inner hole gives the most pump shot. Like Greg G stated, look for 2 strong streams of fuel being squirted into the primary bore when opening the throttle.

 

My vacuum gauge finally came in and I checked it. It reads 20 inches.

As for the timing & distributor, I wasn't able to locate a number on the distributor, but based on its level of wear relative to everything else in the engine, and based on the fact that nothing else in the engine has been replaced, I'm going to assume it's stock.

With the vacuum advance disconnected and plugged, with the engine fully warmed up and idling in neutral, it appears to be *AFTER* TDC. As in, no amount of advance dialed in to my light would make the lines line up. So I think I've located at least one of the problems

Heh, I can't see a damn thing inside the carb while it's mounted on the engine. If I yank the accelerator linkage though, I do see a nice cloud of gasoline mist rise out of the carb if the choke is open though.

With the engine off, they don't flap open too easily. However, it's very hard to open them while the engine is running. The vacuum brake on the choke holds it closed. Maybe I'm missing something, but I don't see how just air pressure is supposed to open them with that strong vacuum holding it closed.

It's in the outer hole, as per specifications for this engine. I guess it couldn't hurt to try using the inner one and see if it makes a positive difference.



EDIT: for purposes of correcting the timing, which one of the marks on the balancer do I use?



Does the scale start from the mark I've labeled 1, or 2?

 

Yes, move your pump rod to inner hole.
The vacuum break holds closed at idle vacuum, at wide open throttle, there is no vacuum and the break releases, so the secondaries can open.

I think line 2 on your picture is top dead center. You will have a line on your balancer also.

 

So I wonder how all the stumbles are turning out ? Hopefully cured