So i got a new engine put in the vette about 6 months ago and it has been running mostly well, except for poor gas mileage..(to be expected). One other thing it does is when i brake suddenly or sharply turn the wheel it starts to

putter like it is about to shut off. I just assumed it was running a littlerich and a little too much gas got pumped into the engine. But a few days ago it started running not so good. I was driving down the highway with no issues, i

get off the highway and start driving in the city. Everytime i let off thegas or came to a stop it started running really weak and puttering until i pressed the gas pedal. From a complete stop i would press the gas and it would do nothing for just a second and then just take off abruptly. This happened

again today and it sounded like it was about to die numerous times when i let off the gas to slow down to a stop. I parked it in the driveway and went to move it into the garage for the night and now it wont crank. Well it cranks

with a little gas pedal but as soon as i let off the pedal it dies. I tried for 15 mins to get it running and it just starts and putters out. I am super frustrated and not sure what do. PLEASE HELP!!

 

Check the choke- it kind of sounds like it's shut all the time.

 

I'm thinking a vacuum leak Tim, cuzz when he presses brake, he has issues

 

This sounds right, or a timing issue.

 

I would check fuel filter , and look for vac leaks.

 

This is from a older post I had written but I think it applies to your situation.

Become a student of tuning, you will not be dissappointed in your efforts...

What follows is with the assumption that you have a healthy igniton.
and
The choke flap on the carburetor is in the wide open (90deg Positon)

A. Vacuum advance is working properly

B. Mechanical Advance is working properly



To get rid of Fuel induced stumble first you have to pin point the cause.



A carburetor stumbles for one of two reasons

You must determine which one first?

1. Lack of Fuel

2. Too much Fuel

Both exhibit different symptomatic characteristics


A.Not enough Fuel; Is like some one just cut the key off for a split second and then the engine resumes accelleration.



B. Too much Fuel; Is an Engine Studder it never really cuts off like a lean condition, just hesitates but still tries to run and after the excess
fuel is burned the engine continues to accelerate.


so which one is it?


Well you need to understand how a carburetor works. ( I will ask for forgiveness explaining this, I am My Fathers Son) that is story for another time...



Carburetors for the most part work in this order, this description specifically describes the Q-Jets working order. For some carbs the idle and enrichemnt circuits have a different design.


1. Idle circuit

Does the car idle fine? Probably so or you would have said the engine would not run at Idle and youhave not indicated that?



2. Off idle Transition; This happens when the throttle is very slightly moved off of idle.
This circuit comes in to play after the idle circuits alone can no longer supply enough fuel and need a little help to cover up the transition of the ending of the idle circuit fuel supply and the beginnig of the accelerator pump fuel supply (When this happens as designed it is a seamless transition and generally happens before 1/8 throttle movement. (I am very doubtful is your problem)



If I were a betting man, the next described point is where your problem is, providing it happens in the period of throttle application that the accelerator pump is the circuit providing the additional fuel ...


3. Accelerator Pump; This is the Transitional fuel supply after the idle circuts and off idle circuit transitional fuel supply can no longer supply enough fuel to keep the engine running.


The accellerator pump delivers fuel immedeatly before fuel is deliverd from the primary boosters; this happens on the primary side of the carburetor (front Two Barrels) before the primay boosters deliver enough fuel to maintain a seamless transition of fuel maintaining your engines rate of acceleration that you have applied by the amount you have opened the throttle.



The next Step happens in conjunciton with the fuel delivery of the Accellerator Pump (primary side) and for trouble shooting purposes can be considerd part of the accellerator pump circuit (although they are not mechanically connected to each other their overall operation compliments the others function)



4. Power Piston; By this time in your carburetors operation the engines manifold vacuum has dropped enough to allow the springs expansive capability that is under the power piston to override the engines vacuum ability to keep the rods that are hung from the power pistons assembly that are inserted in the primary Jets to rise up out of their insertion in the primary jets. This allows the deliver of more fuel to the primary boosters through the primary jets.


So for clarity, lets cover the circuits that are now in operation (they are supplying the maximum amount of fuel they can at this time under the given engine load)

A. Idle Circuits
B. Off Idle Circuits
C. Accellerator Pump
D. Power Pision is not restricting fuel delivery because the engine is running at a rate that will not supply enough vacuum to keep the Power pistions needles seated in the primary jets


5. Secondary enrichment circuit; This circuit comes in to play the instant the secondaries start to open (under maximum actuation of the throttle or sufficient engine load). There are two holes one above each of the Secondary Air Door Flaps On a quadrajet. The motive force is not mechanical but is achieved when the air velocity is great enough moving past these holes that a differential pressure is created causing the fuel to be sucked out of its supply well inside the carburetor ( this is the principle that an eductor works on).

This circuits operation can be witnessed when you see that a light mist of fuel is dispensed onto the top of the secondary air door flaps (remember only vapor burns) .

6. Wide OPEN THROTTLE; If you have gotten this far you with no stumble you are past your problem...
Wide open throttle tuning is a different story. I can go into tuning this some other time and give some ball park figures.
I have a solid starting place but but all circuits in the carburetor must be calibrated to acheive the best results and it is hard to get perfect without a DYNO



Ok now that you know what all of the circuits do lets find the problem...



1.
After the car has sat for lets say about 2 hours. With the ignition off pull the air cleaner off and actuate the throttle a quarter of the way. When you very first pick the throttle up you should get a squirt of gas from the Accellerator pump into the primary side of the carburetor.

If you do not get a shot of fuel there is an issue with the accellerator pump circuit the following could be the possible causes provided it was assembled correctly during rebuild:


A. The Check ball in the Acclellerator pump circuit is not checking and keeping the accellerator pump well full. The accelerator pump should work as described above at all times.

A.(1 If the the accellerator pump does not deliver fuel after the engine has been sitting for a while this could mean that the check ball is leaking by. The fix for this is to dissassemble the carburetor and replace the check ball when you do this make sure and use the old check ball and a small punch to make a new seat for the check ball by lightly tapping on top of the check ball while its seated in its bore.

B. The Rubber cup on the accelerator pump is not sealing against the bore of the accellertor pump cavity (self explanatory)

c. The linkage is not actuating the pump (unlikely unless it is missing)


If this is working as it should (delivers fuel as described above)



2. Check the Power Pistons operation; with the engine OFF stick a straw or SMALL screw driver down the vent tube. You will have to hold it down at a slight angle (top of the screw driver leaning toward the back of the car) in order to reach the power piston. With the engine off you will be able to lightly depress the the power piston and collapse the spring under it.

Note: The power pistons will stick of time and is not uncommon, the above procedure should help identify this.also verify correct fuel pressure and supply.


If you can depress the spring then crank the engine and let it idle and repeat the above procedure.


If you cannot depress the Power Piston with the engine at idle.


It means that the manifold vacuum has over come the spring and is acting as it should.


Note: There are ways to tune the sping tension and release rate of this, That's for antother time.




3. Disconnect the Accelerator pump rod from the linkage on the top of the carb by driving the roll pin out. Do not drive the roll pin all the way toward the center of the carb towared the primary air horn because you won't be able to get anything behind it to force it back into the installed postion if you bottom it out against the primary side air horn. (make sure the disconnected rod cannont hang on anything when you actuate the throttle. This is beacuse you more than likely cannot get it out of the base plate throttle assembley with out removing the carb to perform this test.

Start the engine and quickly actuate the throttle to one quarter of the throttles travel.


If the symptom get worse YOU ARE NOT GETTING ENOUGH FUEL


If it get better YOU ARE GETTING TOO MUCH FUEL!!!


If you have gotten this far with NO STUMBLE, your problem is happening in between the time the Primary side can no longer provide enough fuel and before the secondary enrichment circuit(s) supplies the fuel that covers up the transition between the primaries abilty to supply enough fuel to maintain seamless engine operation and secondary circuits operation to supply enough fuel to achieve maximum engine RPM.

The cause of this can only be one thing but may have several different fixes


Cause: The Secondary Enrichment circuit is not supplying fuel when it should be.


Possible causes


4. Secondary Enrichment Circuit not operating properly

A. The spring is wound too tight on the secondary air door (when adjusting, the springs first contact on the pin in the secondary air door shaft + 7/8 to one full revolution of the adjustment screw is about right)

B. The vacuum pot that controls the release time on the secondary air door is not releasing fast enough (should release fully between 1 to 1.5 seconds) You can cheat a slow release of the vacuum pot by bending the connecting liknkage where, when fully retracted still leaves slack where the air door can open about 1/8"'

This is a ball park figure and can vary between engines for best performance

Check these thing first and if not set correctly adjust now per the above recomendations.


5. First adjust springs air door tension (As described above)


6. Adjusting the secondary vacuum pots release rate is not as easy and is another post but can be done provided you have some taps and a small grinder this will also include a trip to the hardware store.


Just for the record you can troubleshoot any carburetor as described above if you take the time to identify the circuits on the carburetor you are working one at the time no matter which make or model.

This is just a carburetor primer, there is alot more to this but above are the basics.

When I get time I will write an ignition troubleshooting guide.


Hope this helps, Jim