time for carb rebuild? cold start stall

[72jrfan88]

Hello all. Maybe someone has some advice. On cold starts the car fires right up seems to hit fast idle but then shuts off. After starting it again I can usually baby the throttle to get it to warm up but it doesnt seem to want to hit fast idle again when I do this. Once warm and idling properly it runs and drives great. Is there any common issues that may be going on here? It does not do this every time. Only sometimes. I know I am supposed to pump the gas pedal a few times but ive been pumping it more prior to starting thinking more fuel the better. Or could I be flooding it out and that is why it stalls out. Any help would be great. Ive done new plugs and set timing and dwell to spec late last fall. Thank you in advance!

[Murphy]

Which carb do you have? What kind of choke?

[72jrfan88]

Hello. I have the original carb. It is a quadrajet. It says "manufactured by carter" on it . Im assuming that is the hot air type choke with the coil spring on the manifold? I know its not electric. Whatever came stock. Thank you

[Murphy]

Engine off, Remove the air cleaner, does the choke linkage work freely? Move the arm from the choke coil up & down which moves the choke plate open & closed. Almost sounds as if the choke plate is suck open when its cold........

[72jrfan88]

The linkage appears to be working properly. I moved the arm and it compresses the coil. I didnt take the whole air cleaner off but I could still access it on passenger side. It is connected to a rod that goes into a metal rectangular piece that sits on manifold. This also appears to move up and down freely. Im not sure the proper name for this. Maybe the choke pull off or thermostat that could be bad.

[Murphy]

You need to remove the air cleaner.

The light blue arrow points to a choke linkage rod which controls the choke plate (yellow arrow)When you move the choke coil rod up & down as you've mentioned these 2 parts indicated need to move as a result. I've seen the clip come off the rod that controls this plate before. the linkage all works but the plate doesn't move..........

[Mo's70MCs]

How about the choke pull off diaphragm? It might have a small tear in it an not holding vacuum? (working)

 

 

On cold startup the choke flap is fully engaged/closed for maximum richness to allow easy cold starting. Once the engine starts, engine vacuum will be applied to the choke pulloff and its plunger will be retracted, opening the choke a little for a less rich mixture to keep engine running properly.

 

The pulloff plunger also controls the secondary air valve opening rate. When wide open throttle (WOT) is applied, the engines manifold vacuum is reduced significantly and the pulloff begins a controlled release of its plunger. This controlled release is what partially controls the opening rate of the secondary air valves.

[stangeba]

There is a white plastic vacuum choke pull off in the picture on the passenger side. This should hold vacuum when tested and helps to pull the choke open slightly so it doesn't run too rich. If it's leaking the choke doesn't open enough at first, runs too rich and needs to be replaced. This also works with the secondaries and if leaking, or not hooked up, will cause a bog on hard acceleration.

Nothing like computer controlled fuel injection, like we have had for several decades, for good cold drive-ability.

Bruce

[Patrick Sheean]

Hi Eric, my Monte did the same thing when I bought it, I rebuilt the carb but it still did it. Turned out to be carbon build-up in the intake manifold. The center of the intake manifold has openings to a plenum which heats up and releases the choke, (at least on my '71 it is like that) and it would do exactly what yours does on cold mornings, in the afternoon if it was warmer it would not do it. So off with the intake and got it tanked and cleaned and painted since it was off and Monte runs perfectly now. Just my .02 worth.

[wallaby]

I'm trying to figure why it doesn't do the fast idle the second time around. If the intake was plugged and not heating the coil, you'd think the coil would respond as if the engine were cold and set the choke/fast idle and be reluctant to let the choke OPEN.

 

Then there's the possibility that it used to do just that, and someone relaxed the tension on the choke coil so it would open quickly. I've even seen the tension set the opposite way so the manifold coil pushes the choke full open all the time. Hard to start that way, but works fine after you get it warmed up.

[Patrick Sheean]

I'm trying to figure why it doesn't do the fast idle the second time around. If the intake was plugged and not heating the coil, you'd think the coil would respond as if the engine were cold and set the choke/fast idle and be reluctant to let the choke OPEN.

 

Then there's the possibility that it used to do just that, and someone relaxed the tension on the choke coil so it would open quickly. I've even seen the tension set the opposite way so the manifold coil pushes the choke full open all the time. Hard to start that way, but works fine after you get it warmed up.

 

All I can say is mine did just that, second time around choke would stay open. I'm only guessing, but my Monte did exactly what Eric's did until I removed the carbon build up.

 

After I rebuilt the carb. it was better, but my problem was not fully resolved until the intake was clean.

[72jrfan88]

Thank You all so much for your help thus far, and thank your Murray for those pictures. Here is what I had found so far. I took off the air cleaner like described above. All the linkages work properly and operate the choke flap properly, so that connecting rod did not fall off. I tested the choke pull off by pushing the plunger down and putting my finger on the port hole. the plunger stays depressed, once i take finger off it slowly raises, put finger back on and it stops on the dime. From what i understand this is working. I did notice that the small vacumn hose going from the pull off to the carb was dry rotted and VERY loose. i could spin it with one finger on both sides of the fitting. it also seemed pretty short to begin with like it was cut down at some point.(pictured below is new piece of hose) i put a new hose on started car, following proper procedure with about two pumps on gas pedal and it it fired up perfect. Went into fast idle and came off on its own eventually. I finally did not have to baby the throttle!!!! I did notice however that i did not engage throttle for quite some time to get it to go into low base idle, because of this i saw coolant coming out of overflow in that short of time. Is this because the engine was in fast idle to long and over worked? And should i be concerned about this? I took it for a drive and it ran great, great throttle response which I seemed to be missing at times. basic Hard acceleration is great (i.e punching gas to pass a car while already moving at a staeady speed. But Ive always had a bog I cant get rid of that is intermittent. It is only when i put the pedal to the floor.Usually from a dead stop or very slow roll up to 10 mph then punch it to floor (very hard accelearation) It accelarates good for a second but wants to pause, once it catches the car takes off great. I dont really push the pedal down to floor anyway as I dont think this this car was meant for that. Is this common for a quadrajet or possibly a normal function? I realize this carb is over 40 years old and im sure its due for a rebuild soon,I just dont want to make things worse when it cruises so good on normal acceleration. Ive attached some pictures. Please let me know if this loose hose could have caused those cold start issues. Also if you see anything else in the pictures that seems out of whack to you, I would love the advice. That you guys so far you are all great on here.

 

[montefrazer]

What was the engine temp when you took the pictures? The choke isn't closed enough for a cold engine or not opened enough for a warmed up engine.

[Leghome]

I had a 68 Nova that acted the same way sometimes. Mash the accelerator and it would fall flat on its face intermittently. I never did get that problem figured out. I always blamed it on the smog pump and the quadrajet combo.

[Mo's70MCs]

Are we certain that it's even the carburetor? Lots of people tend to pass judgement right away. Although, throwing a kit on it and dipping it in some carb cleaner will do it wonders. Have you put a vacuum gauge on it?

[stangeba]

Great pictures! just pointing out a bit of information about these carbs, if you look on pic 2 on the rear or secondaries, there is a choke lock out (short green shaft) that prevents the secondaries from opening till the choke is full open. There is also a small spring not visible in these pictures, that can be adjusted to prevent the secondaries from dumping open too soon and causing a bog. The secondaries (top portion) will only open as far as needed determined by the air flow into the engine or RPM.

Bruce

[72jrfan88]

What was the engine temp when you took the pictures? The choke isn't closed enough for a cold engine or not opened enough for a warmed up engine.

 

That was in the morning after it sat all night. But I played with the levers a bit initially. Should that flap be closed more?

[wallaby]

stone cold, that flap should fully close when the throttle is pressed. When the throttle moves it lets the choke close and the fast idle becomes engaged.

 

If that photo was taken before you touched the throttle, it looks about normal.

[72jrfan88]

Ok thank you. In my second picture I believe its the choke thermostat attached to manifold. I was told last year that this was bad because the car would stall out when it was put in drive unless you let it warm up for 10 mins. Turned out this problem was not the case. My timing and everything was way off. Did a full tune up and along with timing and dwell adjustment. It no longer stalls out when putting it in gear. What other symptoms are caused from a bad choke thermostat? Could that result in any bogging ect?

[montefrazer]

But I played with the levers a bit initially. Should that flap be closed more?

 

Was this before or after you took the picture? If you moved the throttle linkage, part the gas pedal linkage is connected to, before you took the picture, it needs to close more. When cold, the choke should snap closed when that linkage is moved and be a bit hard to move when you push on it with your finger. Should close again when you stop pushing. When the engine is at operating temp, it should be fully open.

[wallaby]

The choke operation is important on the Q-jet. It should fully close when the throttle is applied before starting; that's part of the starting ritual. (engine stone cold).

The choke coil on the manifold pulls the choke closed. Once you hit the throttle prior to starting, the choke should snap closed and the fast idle cam will rotate into position to raise the idle speed. As soon as the engine starts, engine vacuum is applied to the "choke pull-off" vacuum can and the can will try to open the choke. At this point, the choke coil is trying to CLOSE the choke and the pull-off is trying to OPEN the choke. These two items have something of a balanced tug of war until the choke coil warms up and relaxes its pull...as this happens the pull-off slowly gains ground and begins opening up the choke. Eventually the coil gets warm enough and stops trying to close the choke, and the pull-off holds it fully open.

The choke pull-off also holds the secondary air valves closed. Those are the big plates over the rear barrels of the carb. When you apply full throttle, engine vacuum drops to zero and the pull-off looses suction and can't hold those plates closed any more. As the pull-off slowly releases its pull, the plates begin to open depending on engine demand. (you need a load on the engine to do this, it can't be seen by revving the engine)

 

The Q-jet also has a lock-out mechanism to prevent the secondaries from opening until the engine is fully warm (to prevent backfiring, stalling, etc) and that lock-out is tied to the choke mechanism. The choke must be FULLY open to move the lock-out and let the secondaries function.

Here is the lock-out (passenger side, rear of carb)

The picture shows the lockout in the unlocked position. In the locked position the plate where the pencil tip is would rotate clockwise and interfere with the cross pin seen on the left. It jams against the cross pin and prevents the secondary shaft from rotating.

 

So, yea, the choke is pretty important. It can make the car hard to start and also affect performance. If it doesn't open fully, it can give terrible gas mileage and lack of power. That choke pull-off gizmo is the key to making the whole carb work, not just when the engine is cold.

 

I know I tend to ramble and get long-winded on some of this technical crap, but I hope someone can glean some useful info from it.

[72jrfan88]

Thats great information. The choke does snap all the way closed on a cold engine when throttle is applied which is good. I dont experience back firing. I see that green shaft with the pin in one of my pictures. Is it supposed to be adjusted or does it look right. I believe it is locked. ?

[wallaby]

Yes, your photo shows it in the locked position. That's normal until the choke reaches the full open position.

[stangeba]

Quadrajet's are really very good carbs till people mess them up.

 

My primary shaft was so loose from wear I couldn't believe it! I had MY carb rebuilt (about 15 years ago now!) and the primaries bushed and all is well.

Enjoy!