Carb Issue/Question
 

I'm running a mostly stock 455 with 4X heads in my '69 GTO. Intake is a stock '69 casting, and I'm running an HEI distributor. The carb is a Quadrajet that decodes as coming from a '73 Buick 455. The car ran horribly when I got it - hard to start, poor idle, poor drivability.

I had the carb professionally built by a local guy with a lot of experience. He said the carb was a mess with mis-matched components, leaks, and missing parts. He gave it a very thorough rebuild.

Now the car starts, idles, and cruises a lot better. But if I slam open the throttle at any speed, it bogs horribly for a second or two before it accelerates. My carb guys says that the accelerator pump is set to deliver the biggest squirt, and that the bog is a "lean" condition. He reasons that the 73 carb is probably set to run lean because of the emissions in place at that time. He says not to mess with the metering rods, but to go straight to the main jets and open them up to richen it across the board.

I am not doubting his expertise, but will fattening up the carb help to eliminate this horrible bog at WOT?

If it matters for this conversation, the engine tune seems spot on as far as timing, etc. The car starts easy, and runs fine on pump premium fuel.

TIA for your input and advice!

Sounds more like you need to tighten up the rear flaps.

There might be some pushback on this, but this is just something easy you can try that had an effect for me. Everyone likes to go round and round about what form of vacuum you should have your HEI connected to.

If you currently have your car setup so the vacuum advance is running on full manifold vacuum, try switching it to ported.

When I had my Qjet I had basically the same thing. It ran great, but if I had it connected to full manifold and slammed the gas it would fall on its face. Put it on ported vacuum and the problem went away.

Like I said, just something easy you can try that is easily reversible if it doesnt have an effect.

#7043240 uses CT .0774" secondary rods which are way too lean for a healthy Pontiac 455.
I would replace these with a set of .0527" CV, AU or CK rods.
Might even use .0440" DA or .0410" CE rods.

As for preload on air valve spring start with 1/2 turn from zero and adjust in small steps towards 3/4 until bog is gone.

Is the carb using a white plastic choke pull-off?

My fault for not specifying this in the original post: My carb guy/tuner has the vacuum advance completely disconnected. He says that with the way he has the timing set, it isn't necessary.

Yes, white choke pull off installed.

That will be the culprit unless it's one of mine. At least 10 years ago, probably closer to 15 they changed suppliers for those. They have WAY too much spring in them and not enough restriction, so open INSTANTLY and don't dampen the secondaries.

The result is a huge stumble/hesitation/bog going quickly to full throttle.

They also take 12-14" vacuum to apply them (factory spec is 5-6) so don't even do a good job unloading the choke on a cold start.

Here is a link to one that applies at 5" vacuum and restriction installed to set the release time to prevent issues going to full throttle:

https://cliffshighperformance.com/pr...pull-off-67-70

Cliff - thanks for the info. I just spent some time on your website. Looks GREAT! I'm very interested in your thoughts about fattening up my main jets or just going to a richer metering rod for my application, or is the choke pull-off the main culprit?

Start with the pull-off. It MUST have a timed release. A set of DA secondary metering rods wouldn't be a bad idea if the original rods are still in it. There still could be other issues. Worn/cracked secondary cam, tired secondary spring, hanger height, etc, but it will never work well with one of those newer white plastic pull-offs on it that open instantly......

Here is a pic of the pull-off. I used to do them in house but am currently sending them to an outside source to be "rebuilt" correctly.

They install the correct spring so the pull-off is fully applied at 5-6" vacuum and put a restriction in the inlet to time the release to prevent stumble/hesitation/bog going quickly to full throttle. That's why mine cost $30 and the POS stock ones are $10 shipped via Ebay.......
http://forums.maxperformanceinc.com/...1&d=1699617416

http://forums.maxperformanceinc.com/...1&d=1699617416

Do you know if the rebuilder replaced the secondary spring and cam?

Agree with Cliff on the CPO, but have seen some very worn cams and weak springs as well.

Jon

John, unlikely they did as no kits that I know of besides yours and mine come with them. I do NOT consider that an optional part of rebuilding the Q-jet. Even if the cam isn't worn down, cracked or melted, 99.9 percent of them are loose on the shaft from nearly 50 years of heating/cooling cycles, shrinkage, etc, and delay raising the hanger when the secondaries are called on. This results in the fat part of the secondary metering rods behing too far into the jet holes just long enough to produce a lean condition. It's one of the most difficult things to troubleshoot with one of these carbs because no matter what you do anyplace else it gets no better.........FWIW......

I would not trust a carb guy/tuner that disconnects the vacuum advance claiming it´s not necessary on a street driven vehicle.
Who know whats done to your carb? And ignition?

If part of the mod’s ever done to the carb are the drilling of 4 holes the tip in fuel pull over tubes as talked about in Doug Roe’s Q-jet book to be helpful, then that can be a source of a lean stumble during transition also!

This mod is one flat out fat big mistake to apply to the secondary side of a Q-jet!

Sorry I can only draw a picture of what the mod looks like since I am not home a access my book I speak of.

X2 on this statement

Here, this is what should not be done.
Solder them closed if you find this.

Cliff, I need a pull off fo a 70 RA III carb. Will that one work?

Yes...

Guy does not have a clue on tuning a engine much less a street driven vehicle.
JMO

Tom V.