Slant Six Forum :: Stalling When Accelerating from Slow Speed

What I found today, was that when I disconnected the vacuum advance at the distributor, the engine speed didn’t change at idle. I hooked up my vacuum gauge to the carburetor port that is feeding this, and vacuum is almost nil until it comes off idle. At that point it’s at about 10. I also used my vacuum pump with the engine running to apply some vacuum to the distributor. At about 10, the engine would speed up.

I can’t find any specifications on how much the distributor port on the carburetor should be pulling in vacuum at idle. I do know if I connect to the carburetor port for the air cleaner, the engine speeds up and the timing advances.

The timing is currently at five after TDC.

When I connect the vacuum advance, the timing goes to 10° before TDC and idle speed increases. Simply revving the engine in neutral shows this to be really worse. Tonight, I put a vacuum gauge on the carburetor port that is supposed to feed the vacuum advance. At idle, it is showing about 13 inches of vacuum.

Author:	MattMPA [ Fri Jun 11, 2021 5:32 pm ]
Post subject:	Stalling When Accelerating from Slow Speed
Good day all! I have a 1972 Scamp, 225-908 that was bought new by my Grandmother. I restored it almost 20 years ago and at the time it mostly idled on and off the trailer. I'm done with the show stuff and am putting miles on the old girl. Since restoration, it's had a weird tendency to stall when accelerating from a stop or slow speed, like turning a tight corner. The carb was rebuilt by Holley. The car has a Petronix ignitor. She has A/C if it matters. 70,000 miles. I assumed it was an accelerator pump concern, so I had the carb gone over by a well known local shop. I saw it run just find on their test engine. It's dramatically better...but still a slight problem. Other than this, the car runs great. Timing is currently at TDC, no vacuum leaks could be found. Idles and runs fine. At speed, there's no problem if I need it to downshift, etc. Originally, if I "jiggled" the pedal I could get past this stalling issue. Or, VERY slow acceleration. I did do a search here, but didn't find a fix. A local mechanic friend suggests disconnecting the vacuum advance on the distributor and advancing 4 or degrees. Glad for your thoughts!

Author:	Doctor Dodge [ Fri Jun 11, 2021 8:48 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
A "flat-spot" or stalling can be caused by a number of different issues so good trouble shooting is needed to pin-point the real cause. A mis-adjusted or faulty carburetor is one cause. Slow or malfunctioning distributor advance is another possible area of concern. Low engine vacuum can also cause this problem even when the carb and distributor are in good working order. Get a vacuum gauge and check the engine's idle vacuum reading(s) and do a valve lash adjustment as a start, just to be sure you do not have any tight valve clearances. (tight valve lash hurts idle vacuum) Also use the vacuum gauge to see if adding a little extra initial timing advance increases the idle vacuum reading. (the carb and distributor both need a strong vacuum single to operate correctly) You can also use the vacuum gauge to see how the vacuum advance system is working and if it is causing the hesitation / stalling. (" T " into the vacuum advance hose and watch your vacuum gauge, as you drive) DD

Author:	volaredon [ Sat Jun 12, 2021 5:36 am ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Something your "mechanic friend" suggests always bugged me. Often while walking around shows cruise nights and such I see vac advance pods capped off. First, if unhooked this needs no cap. The vacuum comes from the other end, the source is the carb. Not the vac advance can. 2nd, idk why so many people unhook these to begin with, they must have stock in oil companies and not want max power at cruise speed. This isn't like the miles of added on vacuum lines arbitrarily (it seemed) piled on some of the cars back then. Vac advance actually is very helpful and needed but it has to be functional.

Author:	Jester [ Sat Jun 12, 2021 7:49 am ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
If it's a Holley 1920, my bet is Carb issue. Nothing fixed my carb, until I deep sixed it for a supersix intake and Carter bbd 2 barrel... No hesitation issues now.

Author:	'67 Dart 270 [ Mon Jun 14, 2021 5:35 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Holley 1920? Very likely a carb issue. Many slants have this issue. My '70 Valiant had the exact same issue. Start from a stoplight, it would pick up, quickly fall on its face, and then accelerate. These carbs are often misadjusted.

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Author:	MattMPA [ Mon Jun 14, 2021 5:41 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Thanks all! Sorry to be so late to respond. Was working over the weekend. I sill check these things out. It is dramatically better than before, but still not quite 100%. When it hesitates or stalls, it's usually from a very low speed or stop. On the road..all is good.

Author:	SlantSixDan [ Tue Jun 15, 2021 5:15 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. Tune-up parts and technique suggestions in this post. Also see this thread.

Author:	MattMPA [ Wed Jun 16, 2021 1:29 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
A bit of an update. I did find a substantial vacuum leak from the line starting at the intake manifold. It had disconnected from the diaphragm on the fender, but looked like it was still connected. The car now shows 20 vacuum, steady at idle. What I found today, was that when I disconnected the vacuum advance at the distributor, the engine speed didn’t change at idle. I hooked up my vacuum gauge to the carburetor port that is feeding this, and vacuum is almost nil until it comes off idle. At that point it’s at about 10. I also used my vacuum pump with the engine running to apply some vacuum to the distributor. At about 10, the engine would speed up. Everything else is running like it should except for this hesitation. I can’t find any specifications on how much the distributor port on the carburetor should be pulling in vacuum at idle. I do know if I connect to the carburetor port for the air cleaner, the engine speeds up and the timing advances.

Author:	Jase [ Wed Jun 16, 2021 3:52 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Quote: What I found today, was that when I disconnected the vacuum advance at the distributor, the engine speed didn’t change at idle. I hooked up my vacuum gauge to the carburetor port that is feeding this, and vacuum is almost nil until it comes off idle. At that point it’s at about 10. I also used my vacuum pump with the engine running to apply some vacuum to the distributor. At about 10, the engine would speed up. Your making good progress... Sounds like your distributor is connected to "ported" vacuum, which is not supposed to send any vacuum signal to your distributor's vacuum advance until the throttle is opened. I suspect that is how the factory intended it to be and it is working as intended.

Author:	DusterIdiot [ Wed Jun 16, 2021 5:47 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
This is all normal for a mopar... the vacuum pod will show advance from 20" - 10" on a vacc gauge (if you were running a hand pump and a dial back timing light and mapping the advance curve...), you will see max advance around 20-15 or 16" hg and the advance will taper off until about 10" (you don't want a lot of extra advance under heavy load or under lots of go pedal...) If you hand pump the vacc advance and it loses reading, you either has a lose hose or the pod is blown and needs replacement. Quote: I can’t find any specifications on how much the distributor port on the carburetor should be pulling in vacuum at idle. I do know if I connect to the carburetor port for the air cleaner, the engine speeds up and the timing advances. Port will show zero at idle, until the throttle plate in the carb sweeps just past the port in the throat, exposing it to the vacuum under the plate. At full throttle under load the port will go back to close to zero if the carb is not undersized for the build. The other port shows full manifold vacuum which will advance the timing and increase the engine revs... Make sure you don't have any other vacc hoses that are leaking, and check the mechanical advance by timing light to see that it advances smoothly, hopefully the valve lash is correct... Good Luck.

Author:	MattMPA [ Thu Jun 17, 2021 7:45 am ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Thanks once again, guys! Yet another update. I've spoken with the carb shop, who suggests that it needs bigger jets and offered a way to check. This requires two people, so I tried a couple other things this AM. (Frankly, their method seemed a bit suspect....) First, I've advanced the timing to about 5BTC. Didn't notice much, if any change. The accelerator pump lever was in the middle hole. Tried the other two only in my driveway. Much worse in both cases. Disconnected the vacuum advance hose and plugged it's feed line. Surprisingly, an improvement. Not 100%...but better.

Author:	MattMPA [ Thu May 08, 2025 4:30 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Good day everyone revisiting this situation. Life got in the way and now with spring I want to get the car on the road. It was suggested that I try a different carburetor shop, which I did. A friend of mine is a local technician at an independent shop and he asked me to let him look at the distributor. There were some issues with the return springs. He worked on the distributor, removed the Pertronix igniter and put it back to points. Once it installed, the car idles better and the timing is more regular. Previously, the timing was varying 5° at idle. However, the stalling, off idle hesitation remains. I disconnected the vacuum advance, plug the line, and set the idle as Chrysler suggests. The timing is currently at five after TDC. When I connect the vacuum advance, the timing goes to 10° before TDC and idle speed increases. Simply revving the engine in neutral shows this to be really worse. Tonight, I put a vacuum gauge on the carburetor port that is supposed to feed the vacuum advance. At idle, it is showing about 13 inches of vacuum. Glad for any thoughts.

Author:	SlantSixDan [ Thu May 08, 2025 5:30 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Welcome back. Quote: The timing is currently at five after TDC. It's never going to run well with that setting. Put it to 5° Before, with the engine idling at 700 rpm and the vacuum advance disconnected. Quote: When I connect the vacuum advance, the timing goes to 10° before TDC and idle speed increases. Simply revving the engine in neutral shows this to be really worse. Tonight, I put a vacuum gauge on the carburetor port that is supposed to feed the vacuum advance. At idle, it is showing about 13 inches of vacuum. Carb's messed up. At curb idle (~700 rpm), there should be zero to minimal vacuum at the advance port. So, let's please have some more info on this carb. Pics from multiple angles (with the air cleaner off so we can see the carb itself and its surroundings) would be helpful. But even without those: has this carb been taken apart and/or modified, that you know of? What-all has been done? And, is this a California-spec car, or a 49-state/Canada unit?

Author:	MattMPA [ Thu May 08, 2025 6:46 pm ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
First...thanks for the thoughts...and the welcome. I will gladly get some photos in the AM. Originally, the carb was done by Holley when I restored the car many years ago. I sent it back once. Then a few years ago, took it to a local shop that didn't work out despite at least 2 tries. Most recently, it went to a well respected performance shop locally. My understanding is that the carb is "stock". Can was built in Canada, sold new in PA, doesn't have any of the California stuff. All 49 state. One thing I wonder about...could this have anything to do with the carb-manifold gasket? I don't recall what was used...the carb was last done almost 2 years ago. My thinking too about vacuum....

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Author:	MattMPA [ Fri May 09, 2025 7:34 am ]
Post subject:	Re: Stalling When Accelerating from Slow Speed
Have the photos ready but had issues uploading..used third party. Hope these help. Will gladly take photos of whatever you'd like to see. In one image, the missing hose on the side of the carb (driver's side) is for the valve on the air cleaner. Removed with air cleaner housing for photos..usually attached. Also, some leakage trails are visible...this was corrected by tightening the bowl. (Couldn't see it before sticking the camera down there!)