[Mezzo]

I have '72 T/A 455 HO. Just had the carburetor rebuilt professionally, new timing chain, new fuel pump, new wires (HEI), new plugs, etc. However, I have this unusual hesitation that occurs at about 1,500 RPM. It like a flat spot. On heavy acceleration it stumbles or sometimes back fires. Engine is original unrestored and never been apart. Engine runs very strong and the hesitation diminishes after running a while. I was thinking that maybe the fuel tank pick up (sock) is causing the problem. Fuel pressure is good @ 7-9 PSI, and well as vacuum @ 16". Any suggestins on what to do next?

[70RAlll]

Originally Posted by Mezzo

I have '72 T/A 455 HO. Just had the carburetor rebuilt professionally, new timing chain, new fuel pump, new wires (HEI), new plugs, etc. However, I have this unusual hesitation that occurs at about 1,500 RPM. It like a flat spot. On heavy acceleration it stumbles or sometimes back fires. Engine is original unrestored and never been apart. Engine runs very strong and the hesitation diminishes after running a while. I was thinking that maybe the fuel tank pick up (sock) is causing the problem. Fuel pressure is good @ 7-9 PSI, and well as vacuum @ 16". Any suggestins on what to do next?

Sounds like your carb. You must remember, most carb restorers rebuild Chevy,Pontiac or any engines carb. Pontiacs breath differently than the more numerous Chevy. I will bet it is one of the following:
1. The 2ndary hold off spring is too loose. It is right under the passenger side of the top secondary plate rod-where your choke pull off connects with a rod. You will see a flat head screw that comes out facing the pass side right below the end of the 2ndry rod on pass side.. Below that screw you will see the head of a small allen head screw. When you loosen that allen head, only should take a 1/4-1/2 turn, the flat head piece will turn- that is the spring. So when you start to loosen it, have a small screw driver on the flat head. Loosen the allen, and turn the flat head a full 1/2 turn. That will be too much, but then drive and see if the flat spot is still there.If yes, return back the 1/2 turn you adjusted , this is not your problem. If the flat spot is gone, start adjusting back in 1/4 turn increments until the flat returns, then back 1/8 turn.

2. If not the spring, then your 2ndary rods are too thin. Unscrew the hanger that is right between the two 2ndary air plates(valves), lift out and you will see the two rods. Measure the tips with a dial caliper. You will need to find some thicker rods. People seem to think thinner(more fuel) rods are better, actually had best performance with thick rods. Anyway, the thinnest is .030, my favorite ones are .55 and have a BU stamped on them.

One of these two adjustments will most likely fix your problem. Since the engine is rebuilt,and performs fine except for stomping on it at 1500 RPM, sounds to me like the 2ndary's adjustment. There are a couple other adjustments on secondarys but these two are the easiest..

Hope this helps you..

[Mezzo]

70RAIII:

Thanks for the reply. I will try these steps over the weekend. As added info, the hesitation does occur even on light acceleration as is passes the 1,500 RPM range. Stomping on it at the lower RPM causes it to back fire. There after it is smooth and pulls strong. I'll let you know the outcome.

[johnta1]

When you loosen that allen head, only should take a 1/4-1/2 turn, the flat head piece will turn- that is the spring. So when you start to loosen it, have a small screw driver on the flat head. Loosen the allen, and turn the flat head a full 1/2 turn.

The words in bold are important.
The spring will unwind if the screwdriver is not there.
Then you'll be at zero starting from scratch.

Turn it clockwise to tighten the spring tension.

Good luck.

[Formulajones]

On a side note, quadrajets don't really like much more than 4-5 psi fuel pressure. Becomes a fine line of keeping that tiny float bowl full so it doesn't run dry at WOT yet you don't want to blow fuel passed the needle and seat either.

I would recommend checking with a wideband so you can see exactly what is going on during the hesitation phase. Wideband is really the only accurate way to dial in a carb, otherwise you are just guessing at jets and metering rods.
99% of the time I find that hesitations or stumbles are actually a lean condition rather than an overly rich condition. Lazy timing curves or an inop vacuum advance also do funny things like this as well.

[Mezzo]

Thanks for all the helpful tips. I will return with results. Regarding the fuel pressure, I will look at it again. All I remember was it is to factory specs on the high end. Also, the exhaust smells slightly rich so I do not thing it is a lean issue. I did have the HEI rebuilt and set to a HO advance curve. The carb was rebuilt by www.customrebuiltcarbs.com by Phil Cancilla.

[Formulajones]

A rich smell is deceiving and won't be a sure sign of a problem since pump gas doesn't smell that great anyway.

I can have a car idling at a 14:1 AFR and still smells bad. They actually smell a tad better when I have a richer 13:1 AFR, but each engine will vary with what works best.

A stumble at 14-1500 rpm is likely where the carb is transitioning from the idle circuit to the main circuit, and will require enough pump shot to cover the hole. This is 9 times out of 10 a lean condition in that area. Just an observation from what I see here all the time, but it's hard to diagnose over the internet.

On fuel pressure, it might be fine with the pressures you are seeing, so long as it's not blowing fuel past the needle and seat. You'll have to stick your head over the carb with a flash light and closely watch the boosters. If there is any fuel dripping from them while the car is idling, then your needle/seat is leaking fuel (either bad needle/seat, dirt, or fuel pressure is overwhelming the needle/seat).
Quadrajets work best with lower fuel pressure (which also means more volume). The higher pressure you report also means less volume. Volume is important with a quadrajet since the float bowls are extremely small, especially on a thirsty 455.

There are tricks to compensate, such as larger diameter needle and seat, smaller brass float so it doesn't disperse as much fuel etc.....

[70RAlll]

Before you start trying too many adjustments, try the two I suggested first. johnta1 is exactly correct on holding the flat head screw driver. the second you get the allen head loose, you will feel the tension on the flat head. look good before you start and mentally mark where it is . . as far as fuel pressure, with the FB having a return line, I don't see 7-9 psi being much of a problem. the seat should hold that pressure ,sending excess fuel through the return line. let me know if you need a set of larger 2ndary rods.
I really think you will find a solution in these adjustments. May not, but think you will.good luck with this. PM me if you need a set of larger 2ndry rods. I could get some out tomorrow.should have by Saturday via US Priority mail.

[Mezzo]

Well I spoke to Phil Cancilla at Custom Rebuitd Carbs in NJ where I had the carb rebuilt in April and he seems to think it is the accelerator pump. The bump seal likely did shrink from gas containing ethanol. Very plausible since you cannot find ethanol free gas in the Chicago area. Funny thin is I had no hesitation right after the installation but somehow it has crept back as described earlier. When the driving season ends, Phil will take a look at it again.

[70RAlll]

I have not seen a rebuild/seal kit for a Quadrajet in years that did not have an accelerator pump seal made of nitrile material. They are designed to work fine in ethanol.Plus, a commercial rebuilder would definitely use an accelerator pump seal that was impervious to ethanol, it is their job to understand 90% of all fuel in the US now has ethanol.

Try this :

With the engine turned off, look down the primary airhorn while pulling by hand the throttle cable back. You should see a stream of fuel directed into each primary valve.If you see this stream, then the accelerator pump seal is working fine. If it shrunk from ethanol exposure, it will not seal enough to move fuel- you should have problems at take off from a dead stop if the accelerator pump is not sealing ..

[Mezzo]

Well my hesitation problem is still persists. I had the 7042270 carb rebuilt for a second time by Custom Carbs in NJ and I am now beginning to believe that the carb is operating properly and something else is causing dead spot. The hesitation still exists at 1500 +/- RPM albeit briefly. I am now thinking that the ignition module may be bad. I have done the following to date

1. Rebuild carb
2. Installed new M&H hire harness (HEI)
3. Replace intake gaskets and cleaned intake manifold
4. New HEI wired, cap, rotor, coil.
5. HEI distributor tested 1 year ago

Any suggestions would be helpful on where to look next.

[70RAlll]

It is your carb, not your other components. Quit wasting money chasing down these problems.. Your 2ndy rods need to be fattened up and the 2ndary spring neeeds to be tightened.. I don't care how good you believe your carb rebuilder is, that hesitation is a carb issue. One of the more difficult tunings on a quadrajet and why very few can really tune a quadrajet properly. quite easy to clean and rebuild a quadrajet, most difficult to tune one..

[Mezzo]

Originally Posted by 70RAlll

It is your carb, not your other components. Quit wasting money chasing down these problems.. Your 2ndy rods need to be fattened up and the 2ndary spring neeeds to be tightened.. I don't care how good you believe your carb rebuilder is, that hesitation is a carb issue. One of the more difficult tunings on a quadrajet and why very few can really tune a quadrajet properly. quite easy to clean and rebuild a quadrajet, most difficult to tune one..

The hesitation occurs off the idle circuit. Once engine speed under load gets past 1500 RPM, it runs fine. Any suggestions on who in the Chicago area would be a good reference to tune it?

[Judge273]

John at Tripower comes to mind. He is the north suburbs too. 847.816.3222

[Mezzo]

Originally Posted by Judge273

John at Tripower comes to mind. He is the north suburbs too.

I have met John. I will be contacting him. Thanks.

[Formulabruce]

70RAIII is correct.
I will add that as time went on in the 70's Rochester improved on the Q~Jet design by adding a fully adjustable Idle circuit. This is a huge help with alcohol in the gas, however small mods and adjustment can make yours perform correctly. ( '77-80 Firebird carbs being the Ideal for maximum adjustability )
When I worked at the dealership, there was a huge consensus that stumbling off idle was the accelerator pump, or a "ridge" in the pump bore.While this DID happen on some carbs it was NOT usually the problem.
When a stock 72 engine reaches about 1500, your vac advance is getting done and mechanical is taking over. Youll want to make SURE you get the correct weights and springs on that distributor. Your "delay" could also partly be the timing switch to mechanical advance being too slow, or weights to heavy, or springs too stiff.
The carb tune to engine and the Dizzy dialed in will make it run well.
Just because its new or rebuilt doesn't make the distributor " all set"
Good luck!!

[njsteve]

Originally Posted by Mezzo

Well I spoke to Phil Cancilla at Custom Rebuitd Carbs in NJ where I had the carb rebuilt in April and he seems to think it is the accelerator pump. The bump seal likely did shrink from gas containing ethanol. Very plausible since you cannot find ethanol free gas in the Chicago area. Funny thin is I had no hesitation right after the installation but somehow it has crept back as described earlier. When the driving season ends, Phil will take a look at it again.

I had a couple Pontiac carbs done there and I think he got a bad batch of accellerator pumps a while back. He also uses the pump diaphragms that do not have a coiled skirt spring. So once they get mushy and fold over, they are ruined for life.

Once I replaced my accel pump with one from Cliff that has the internal retating coil spring, it worked much better.

http://www.cliffshighperformance.com/acc-pump.html

You also need to run the jetting/metering rods 10% richer to offset the enthanol in today's fuel.

[Mezzo]

Originally Posted by Formulabruce

70RAIII is correct.
I will add that as time went on in the 70's Rochester improved on the Q~Jet design by adding a fully adjustable Idle circuit. This is a huge help with alcohol in the gas, however small mods and adjustment can make yours perform correctly. ( '77-80 Firebird carbs being the Ideal for maximum adjustability )
When I worked at the dealership, there was a huge consensus that stumbling off idle was the accelerator pump, or a "ridge" in the pump bore.While this DID happen on some carbs it was NOT usually the problem.
When a stock 72 engine reaches about 1500, your vac advance is getting done and mechanical is taking over. Youll want to make SURE you get the correct weights and springs on that distributor. Your "delay" could also partly be the timing switch to mechanical advance being too slow, or weights to heavy, or springs too stiff.
The carb tune to engine and the Dizzy dialed in will make it run well.
Just because its new or rebuilt doesn't make the distributor " all set"
Good luck!!

FB, you might be onto something on the distributor weights and springs. Would it make sense to take the weights and springs off my old unitized distributor and place them on the HEI unit. Any other suggestions?

[Formulabruce]

1. No dont steal weights from the unitized, get a recurve kit for an HEI and read the directions that come with them on vacuum and what spring, weight etc.
http://www.ebay.com/itm/Moroso-72300...1fb858&vxp=mtr
2. NJ Steve has an excellent point, the pump MUST have a spring in the skirt...in the carb.

[Mezzo]

Originally Posted by Formulabruce

1. No dont steal weights from the unitized, get a recurve kit for an HEI and read the directions that come with them on vacuum and what spring, weight etc.
http://www.ebay.com/itm/Moroso-72300...1fb858&vxp=mtr
2. NJ Steve has an excellent point, the pump MUST have a spring in the skirt...in the carb.

FB, I read an article last night on fine tuning the Rochester Q-jet 4MV. In there they talk about adjustable part throttle (APT) setting. In there it describes a an "off-idle stumble" and the APT screw can be turned up a 1/4 turn at a time until it is eliminated. These adjustment is made after the idle mixture screws are adjusted. Any experience with this on a 7042270?