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Secondary Stumble

Houle #382

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Posting for a buddy of mine, 67 Charger with the original factory installed 440 with automatic. Solid lifter cam nothing crazy Holley street dominator intake, 850 Holley carb, stock exhaust manifolds with 3:91 gears. The car runs strong but has developed a secondary stumble with the 850. Tried bigger jetting and gets worse. Put on a Edelbrock 650 AVS-2 great throttle response but still has secondary stumble. Put the 850 back on tried different rods and springs same issue. Put an older 625 Carter on and same issue secondary stumble, tried jetting up the 625 and stumble got worse just like the 850. So obviously not carburation has to be ignition, it does have the Chrysler electronic ignition and we did try a different orange box but no change, Timing is 12 degrees initial and 35 total so where do we look from here.
 
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Check the fuel pump and routing of the fuel line... maybe change out the fuel filter ???
 
Not sure what you mean by "secondary stumble", I presume stumbles as secondary's open? Sounds like vacuum leak or just more carb tuning needed. With a Holley 850 double pump I would be tuning the secondary accelerator pump not the jets, presuming no vacuum leak. Larger secondary shooter or more aggressive pump cam, or cam position. Sure could be fuel filter or float level.
 
Not sure what you mean by "secondary stumble", I presume stumbles as secondary's open? Sounds like vacuum leak or just more carb tuning needed. With a Holley 850 double pump I would be tuning the secondary accelerator pump not the jets, presuming no vacuum leak. Larger secondary shooter or more aggressive pump cam, or cam position. Sure could be fuel filter or float level.
Thanks for the info yes stumbles when secondary's open but we tried 3 different carbs and all 3 produced the same issue. When he punches it she will stumble then takes right off.
 
Maybe you are chasing the convertor? What stall?
 
Thanks for the info yes stumbles when secondary's open but we tried 3 different carbs and all 3 produced the same issue. When he punches it she will stumble then takes right off.

Don't take this the wrong way, but "three different carbs" doesn't prove anything other than you've tried three carbs none of which are optimized for that condition on that engine. I'd look at the accelerator pump/cam/squirter area. Hook up your vacuum gauge so you can see it while driving, and see what the vacuum does around when the problem occurs; it may be as simple as too small/late a power valve. That thing should rock with an 850 holley. Whatever it is, it's not in the jetting if it's happening at the first crack of the secondaries. Wait.....is this a vacuum secondary or double pumper carb??
 
beanhead, good point, I misread the OP as a double pumper. If those are all vacuum secondary carbs, the secondary's are opening too quickly. The power valve sure could be part of it too.
 
Don't take this the wrong way, but "three different carbs" doesn't prove anything other than you've tried three carbs none of which are optimized for that condition on that engine. I'd look at the accelerator pump/cam/squirter area. Hook up your vacuum gauge so you can see it while driving, and see what the vacuum does around when the problem occurs; it may be as simple as too small/late a power valve. That thing should rock with an 850 holley. Whatever it is, it's not in the jetting if it's happening at the first crack of the secondaries. Wait.....is this a vacuum secondary or double pumper carb??

Double pumper, he has tried different power valves and jetting with no change in stumble.
 
With an 850 DP a person can spend a bit of time sorting out the secondary accelerator pump. I'd check to see how the secondary accel pump actually works, just watching it, the play with the shooter size & accel pump cams. They make BIG differences. Also make sure the needle check under the shooter is moving properly.
 
Have you tried a bigger accelerator pump nozzle on rear squirter?. Mine bogged at take off, put bigger ones in front and back, cured problem,. easy fix., or test for you anyway.
 
The car runs strong but has developed a secondary stumble with the 850.

So the car used to run fine with the 850, but now it does not, is that correct? How long did the car run fine? When did the stumble appear? How does the owner know that the stumble coincides exclusively with the secondaries opening? If it is the secondaries, the symptom should present differently for the DP verses the vacuum carbs.

The ignition system does not know when the secondaries open.
 
So the car used to run fine with the 850, but now it does not, is that correct? ( Correct ) How long did the car run fine ( couple of years ) When did the stumble appear? ( with in the last couple of weeks ) How does the owner know that the stumble coincides exclusively with the secondaries opening? ( no matter what speed he is going when he hits it it will stumble then take off ) If it is the secondaries, the symptom should present differently for the DP verses the vacuum carbs. ( True but it does not as he has tried 2 other carbs that have vacuum secondaries and has the same issue as his double pumper )
 
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Might sound far out there but.... What else happens when you mash it? The engine torques sideways, might look at wiring, plug wires, etc, anything that could be getting tugged on when the motor moves on acceleration.
 
I too think your problem will be in the carb, or in the fuel, unless the timing is too much for the condition at the stumble.
However, I never start a diagnostic without first doing, if applicable, a valve adjustment, and then a compression test. And making sure the plugs are not running too hot.

If your primaries are too lean, the plugs will get hot. The car will cruise just fine set like that. But when you mash the gas, those hot plugs might protest, and one symptom is a stumble.
What can actually happen, is called pre-ignition. Under the heat of compression, the mixture is instantly ready to explode. It could happen that the hot-running plug causes the mixture to ignite ..... before the spark gets there. If this happens, before TDC, then the piston will be driven violently down, on the wrong side of TDC, effectively trying to turn the engine backwards. This lasts only a few milliseconds until the plugs cool off. Or whatever else is hot enough in the chamber to cause preignition. You, the operator interpret that lurch, as a stumble.
The proof is easy to find, just retard the timing. If it goes away, then you know so has the preignition. After that, you have to figure out what is causing it.
It could be hot sparkplugs. It could be insufficient octane. It could be running too lean directly prior to mashing it, or it could be something else in the chamber, glowing hot enough to start things up.
If it turns out to be pre-ignition, you have to fix this before it destroys your rod bearings or worse.
Happy Hunting.

Ps, I'm not saying yur problem is this. It could be as simple as a lean tip-in lol, as others are pointing to.
 
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