How can you tell if an off-idle stumble is caused by a lean or a rich condition? Carb

[Kapdin]

spent the last hr researching q-jets. im trying to forget everything ive learned to see if im just dumb but so far ive found wat jims talking about

Tech Tip #3
How can you tell if an off-idle stumble is caused by a lean or a rich condition?

A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. To narrow it down,
tap the roll pin out of the accelerator pump lever by using a small pin punch or a small finish nail. I actually use a small, broken drill
bit that’s just the right size. Using a hammer, gently tap the roll pin in towards the choke air horn wall. Don’t jam the pin right up
against the wall: Leave just a little bit of a gap so you can get a screwdriver blade in between the wall and the pin to pry it back again.

With the pin tapped out, remove the accelerator pump lever. I like to do this with the engine running so I won’t have any trouble
starting the engine without the accelerator pump. Now, rev the engine a little with the throttle. Notice if the engine seems quicker and
more responsive, or if the hesitation & stumble is worse. If the engine actually feels more responsive with the accelerator pump
disconnected, you have a rich condition. If the hesitation is worse than before, you have a lean condition. If there is no change
whatsoever, you have a non-functional accelerator pump.

To verify a suspected lean condition after this test, simply hold your cupped hand lightly over the choke air horn area with the engine
running at idle, restricting the air flow. If the idle speed and idle quality momentarily increases, you have a verified lean condition.
You need to select a jet/rod combination that will give you a little more Cruise Metering Area. Make these changes in less than 10%
increments using the Figures provided in this paper.

 

[HotRodPC]

The only thing I don't like about this type of test is, it doesn't say what to expect if you have it right as is. Unless of course you metering rods to go the wrong way, then you back up to where it was previously.

I think most of already know, it's very common for QJets to be rich. For mpg purposes, it doesn't hurt to be on the lean side. Just don't go so far as to cause a lean misfire or you will hurt your mpg.

That's my .02 cents to add.

 

[MadOgre]

A carburetor mounted to a dual plane manifold with no crossover between primary bores from one side to the other can cause an off idle hesitation.

 

[crazy4offroad]

Lean typically backfires if very lean, rich typically soots up the plugs.

 

[Tobe's84Jimmy]

i’m probably at least the third owner of my 84 jimmy and my off idle stumble was because my distributor was a tooth off. after getting it on the right tooth, problem was gone. good luck, hope you sort it out

 

[GMFORLIFE]

Never want to run on the lean side.
Ive purchased my last 2 carbs from allstate carb.
Holley 3310's
They are flow tested before shipping.
They look so nice.
They run even better.
Best ,$300 you could possibly spend on your ride!

 

[Maxgforce]

Lean typically backfires if very lean, rich typically soots up the plugs.

That is correct

 

[broketiredugly]

easier than driving out the roll pin, and potentially damaging things, is to just slowly push the accel pump down with your finger, hold it there until engine smooths, then try the rev test. the needles on the quadrajunk have been a problem area since before most of yall were born. we would polish them right where the wear marks were, which is the area that caused the lean stumble. and remember, float height is critical, and they dont like intakes as referenced above. i never removed the roll pin after the first one i ever rebuilt. you remove the bottom first, fold it over as you unhook the linkage to the choke and accel pump.

 

[Ricko1966]

Make sure your throttle plates are closed and idling on the idle circuit.Lots of people run the the throttle stop screw in to far and are idling on the transition circuit.Now that your using the transition circuit to idle, the transition circuit doesn't have the fuel to transition from idle to open throttle.