Slant Six Forum :: Car stumbles taking off.

Stumbling on acceleration is usually caused by three things:

(1) vacuum leak
(2) misadjusted carb (choke or accelerator circuit not functioning correctly)
(3) ignition problem (timing wrong, coil failing)

My advice is to first check that you choke pulloff diaphragm is holding a vacuum and connected corectly. The choke hose should have full vacuum at all times.

Then make sure the distributor vacuum advance pod holds a vacuum and is properly connected. The hose to the distributor should have no suction when the engine is at idle, but should get full vacuum as soon as the throttle is opened even slightly.

Once these are verified to be working corectly, connect a vacuum guage and see what the vacuum is at idle. A well tuned stock slant with no vacuum leaks should get 17-20+ inches of vacuum at idle. If your vacuum reading fluctuates or it is low, you have a vacuum leak or misadjusted valves. Correct the problem as needed.

As stated above, make sure you are getting a good shot of fuel when the throttle is opened. With the car off, look down the throat of the carb and slowly move the throttle you should see a steady stream of gas.

Author:	Davids63Dart [ Wed Dec 10, 2008 2:49 pm ]
Post subject:	Car stumbles taking off.
When I leave from a stop, and give it gas, it stumbles for a second, then goes. Sometimes, it even causes the car to stall completely. I am not a carburetor person, so i need assistance on how to get this to stop. It seems like it's getting too much gas off the line. How do I adjust it? It is a Holley 1 barrel.

Author:	emsvitil [ Wed Dec 10, 2008 3:30 pm ]
Post subject:
More likely it's not getting enough gas off the line..... Check the accelerator pump and the pump linkage (try lubricating)

Author:	Wizard [ Wed Dec 10, 2008 3:54 pm ]
Post subject:
Check that you are getting vacuum advance. I had same issue except my computer wasn't knowing if throttle was closed or not because the electrical contact was not touching the throttle linkage. When I open throttle from idle, computer knows contact is broken and advances the timing just like vacuum to cut down on bucking/stalling. Fixed that and most of it went away. I still have bit of "hole" because computer is not advance quickly enough because of different carb I'm using (Weber, instead of bastardized holley 5220). Cheers, Wizard

Author:	wjajr [ Wed Dec 10, 2008 5:21 pm ]
Post subject:
I'm betting his car has no computer, just a guess. Remove the air cleaner, look down into the carb with the choke wide open and blip the throttle by hand. There should be a nice healthy squirt of gas shot into the center of the opening if the accelerator pump is operating properly. If there is just a dribble, or weak wimpy squirt of gas than your accelerator pump is not working as it should. It is time for a carb rebuild.

Author:	Charrlie_S [ Wed Dec 10, 2008 6:47 pm ]
Post subject:
Quote: Remove the air cleaner, look down into the carb with the choke wide open and blip the throttle by hand. There should be a nice healthy squirt of gas shot into the center of the opening if the accelerator pump is operating properly. Do not do this with the engine running. You just might have to get some replacement eyebrows.

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Author:	DusterIdiot [ Wed Dec 10, 2008 6:52 pm ]
Post subject:	Then again in slow-mo...
One thing I've noticed with the older carbs and if they are 'used' is when 'blipping' the gas (with engine off). Do the motion again slowly with even pressure... in a couple of cases I've been able to spot that from the point of 'presure on the pedal' to about an 1/8" of throttle motion there might not be a squirt, you can also do it again and watch the motion of the linkage to make sure something isn't twisting and binding or not communicating to get your 'shot' going... if you see some 'slop'/lack of action you might have to pull out the pliers and straighten things out a bit to get it all right again... Good luck, -D.Idiot

Author:	Davids63Dart [ Thu Dec 18, 2008 10:41 am ]
Post subject:
Not sure if it matters, but I just remembered that I don't have the vaccum advance hose hooked up to the dist. it's currently just plugged. Would that have any effect?

Author:	Reed [ Thu Dec 18, 2008 10:51 am ]
Post subject:
Quote: Not sure if it matters, but I just remembered that I don't have the vaccum advance hose hooked up to the dist. it's currently just plugged. Would that have any effect? YES. The vacuum advance is supposed to advance the timing as soon as the engine comes off of idle. It absolutely should be hooked up on a street driven car. Stumbling on acceleration is usually caused by three things: (1) vacuum leak (2) misadjusted carb (choke or accelerator circuit not functioning correctly) (3) ignition problem (timing wrong, coil failing) My advice is to first check that you choke pulloff diaphragm is holding a vacuum and connected corectly. The choke hose should have full vacuum at all times. Then make sure the distributor vacuum advance pod holds a vacuum and is properly connected. The hose to the distributor should have no suction when the engine is at idle, but should get full vacuum as soon as the throttle is opened even slightly. Once these are verified to be working corectly, connect a vacuum guage and see what the vacuum is at idle. A well tuned stock slant with no vacuum leaks should get 17-20+ inches of vacuum at idle. If your vacuum reading fluctuates or it is low, you have a vacuum leak or misadjusted valves. Correct the problem as needed. As stated above, make sure you are getting a good shot of fuel when the throttle is opened. With the car off, look down the throat of the carb and slowly move the throttle you should see a steady stream of gas.

Author:	starion88esir [ Fri Dec 26, 2008 9:23 pm ]
Post subject:
Quote: Stumbling on acceleration is usually caused by three things: (1) vacuum leak (2) misadjusted carb (choke or accelerator circuit not functioning correctly) (3) ignition problem (timing wrong, coil failing) My advice is to first check that you choke pulloff diaphragm is holding a vacuum and connected corectly. The choke hose should have full vacuum at all times. Then make sure the distributor vacuum advance pod holds a vacuum and is properly connected. The hose to the distributor should have no suction when the engine is at idle, but should get full vacuum as soon as the throttle is opened even slightly. Once these are verified to be working corectly, connect a vacuum guage and see what the vacuum is at idle. A well tuned stock slant with no vacuum leaks should get 17-20+ inches of vacuum at idle. If your vacuum reading fluctuates or it is low, you have a vacuum leak or misadjusted valves. Correct the problem as needed. As stated above, make sure you are getting a good shot of fuel when the throttle is opened. With the car off, look down the throat of the carb and slowly move the throttle you should see a steady stream of gas. Can you tell me how to check these out to make sure they're both working as they should?

Author:	Reed [ Fri Dec 26, 2008 10:14 pm ]
Post subject:
Choke pulloff- (1) remove the air cleaner (2) locate the choke pulloff diaphragm on the side of the carb (3) locat ethe hose that runs from the choke pulloff diaphragm to the carb and detach the hose from the carb (4) depress the plunger on the diaphragm manually and put a finger over the end of the hose. The plunger should stay depressed and not move. Any movement in the plunger means that the diaphragm is ruptured and will (a) cause a vacuum leak and (b) cause your choke to stay on too long. Keep in mind that some plungers are of a "two stage" design with an inner and outer plunger. If you have one of these, the inner plunger may pop back out but the outer plunger should stay depressed. Vacuum advance diaphragm- (1) Locate the vacuum advance pod on the side of the distributor. (2) locate the hose that runs to the vacuum advance pod. If there is no hose, you will need to get a 3.5 foot section of 1/4 inch vacuum line (or maybe 3/16, its been awhile) (3) the hose should run from the pod on the side of the dist up to the ported nipple on the carb. What is a ported nipple you ask? It is not a seaside prostitute, it is a vacuum fitting on the carb that gives a vacuum sugnal only when the throttle is off-idle. To check, start the car and put a piece of latex glove over the port on the carb. If the glove isn't slightly sucked into the hole at idle, try opening the throttle. If the glove is sucked in at idle, then that isn;t the distributor fitting. If the glove is not sucked in at idle but is sucked in off idle, then that port will work for the distributor. Using a vacuum guage instead of a rubber glove works better. (4) once you have identified the correct port, make a note of it. (5) attach the hose to the distributor and suck on the end with your mouth. You should feel resistance. If you are able to suck anything at all then your diaphragm is ruptured. (6) test/replace the diaphragm as needed, and run the hose to the correct port on the carb. Hopefully that all makes sense.

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