Secondary Choke Pull-Off Release Times

[Brian B.]

Hi. I have a mild 4000lb performance car; 12.9/105mph.  When street driving, I'm trying to completely eliminate a slight hesitation when I stomp on it while cruising at 40mph or so.  I've just about solved it making my own metered orifices providing a 2 second release time with 3/4 spring wind-up.  Is it possible to completely eliminate this while stomping on it.....and what sort of release times are some of you using for performance applications?  Thank you.

[Cliff Ruggles]

What is the carburetor part number and does it use the REAR pull-off to hold the secondaries closed?

If so, those are a huge PITA to time correctly.  They just hold TOO MUCH air inside them.  By the time you get a big enough hole in them to release without a slight delay they are too quick.

Even so they can be made to work, but it takes some effort to get them perfect........

[Brian B.]

My carb is #7044270 and is stock for my car.  It has not been drilled or messed with other than the pull-off, which controls the secondary air doors.  I did upgrade the accelerator pump using your high performance version.  The car runs perfect in every area except stomping on it at 40mph.(which is just too fun to stop). 60 foot times are 1.82(good for this car) with drag radials and problem free.  Thank you.

[Kenth]

I have found the biggest issue with non-responsive secondaries on a 7044270 unit is the air valves.
Replace the un-cut air valves with a set of cut air valves from any other 1967-74 Pontiac Q-jet to solve the issue.

[Brian B.]

Thank you.  It's very close now.  I will try 2 more orifices then try a set of flaps from another carb.

[Cliff Ruggles]

I'm running solid flaps on mine and have won the Stock Class at the Tri-Power Nat's 5 times with two trips to the King of the Hill Race (last man standing).

The key is a combination of things, fuel "sloshing" up the rods from a standing start is something often missed by folks tuning these carbs.

Pull-off release time, flap tension, hanger height, taper and tip length of the metering rods, POE set-up, etc also play a role here.

On the later units you can also get fuel "slosh" from the vent behind the choke flap and vent holes over the POE wells on some airhorns.

When set correctly you should feel nothing put strong accelleration, no noticable "kick", or transition of any type when things are set correctly.  That's the parts folks miss with these things, they expect to feel some sort of secondaries "kicking in", which happens with just about every type of aftermarket carb out there.  When you feel anything other than getting planted HARD in the seat and strong/smooth power from idle to the shift point......you need to keep tuning because you just aren't there yet.......FWIW....

[Brian B.]

Thanks for these tips.  I'm getting real close and trying to tune out any "kick". I put 100 mesh stainless steel screen under the bowl vent, installed an air horn baffle, and placed a small piece of electrical tape with "X" cut-outs on the air horn gasket to prevent slosh up through the secondary metering rod openings.  I'll get there.

[Cliff Ruggles]

I tune using high gear pulls from about 25-30 mph coasting (TH400 with the kick-down solenoid unhooked).  This simulates a full throttle start with traction, but takes "slosh" pretty much out of the equation.

Once well dialed in I do standing starts with good traction and IF you start having issues more times than not you are throwing some fuel out of the top of the carb over into the secondaries.

Most folks never run into this as their cars will not leave hard enough to give you a nosebleed!

Roasting or spinning tires doesn't test venting or seldom do those issues rear their ugly head.....FWIW.....

[Brian B.]

Here is a brief update.  I changed the release time of my choke- pull-off from 1.5 seconds to 2.5 seconds and it alleviated most of the issue for my car during street driving.  If I make future adjustments that significantly help, I will post them.  Thank you.

[Cliff Ruggles]

The true test is to use slicks or DOT's and perfect traction.  Any tire spin at all reduces transition time.  I've set up carbs on the street to be perfect then had to re-adjust things at the track with good traction. 

The Q-jet has a HUGE advantage over most other varieties because transition is FULLY adjustable.  The choke pull-off prevents stumble as it doesn't allow the flaps to "whip" open causing a momentary lean condition. 

POE plus a good strong pump shot from the accl pump also helps cover this up.  Another thing folks really miss when tuning secondaries is sorting thru an assortment of factory secondary metering rods.  You MUST use rods from the same cores, same tip length and same included angle leading to the tips.  If you are sorting thru factory rods you will find it mandatory to swap hangers around as well to effectively tune transition and fuel delivery across the rest of the loads/speed range.

It is actually possible to install rods with much smaller tips and go lean instead of rich IF the upper section is fatter and not the same transition to the tips.  For this reason I custom machine a set of rods from the same cores for serious racers so they can accurately predict metering when changing rods.

Not really necessary for the average street car.  For most set-ups it's just better to dial in one hanger and pair of rods and run with them vs splitting hairs to get the absolute most out of the combination at the track based on weather, DA and track conditions......FWIW.....