Engine bogging down - Pontiac GTO Forum
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post #1 of 12 (permalink) Old 03-27-2016, 06:27 PM Thread Starter
 
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Engine bogging down

Hello

Forgive me if some of the topic may be covered elsewhere, but I'm pretty new to the forum and am just starting to work on my 69 GTO.

I'm having an issue with the engine really bogging down and almost shutting down under hard throttle. It's a 69 GTO with a 400 that I believe was rebuilt with some RA 3 components such as the cam. So not a RA3, but likely more than a stock 400.

Car starts and idles fine...healthy cam I suspect based on idle.

Under hard throttle, the car really bogs down and has trouble getting over 4000 rpm. Very sluggish.
The carb was just rebuilt and was supposedly run on a dyno, so likely not the problem....but can't rule it out.

I'm trying to get some suggestions on how I can diagnose the problem.
Carb, vacuum advance, fuel pump, etc. ??

Where would be the best place to start?

Thanks for any feedback !!
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post #2 of 12 (permalink) Old 03-27-2016, 07:18 PM
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What carb do you have?
Does it wind up if you feed the gas to it slowly or does it just bog when you mash it?
Fuel filter clean?
What is the timing set at?
Plug wires in good shape and seated?
Need more info, stock distributor or aftermarket?
If it starts and runs good at idle I would suspect a fuel problem, sounds like it is either starving or flooding.
Is the choke sticking?
Remove the air cleaner and check the carb, look down in it and make sure the accelerator pump is working and that there is a good flow of gas in the primaries and secondaries.
Symptoms you describe sound like a plugged fuel filter so I would start there.
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post #3 of 12 (permalink) Old 03-27-2016, 07:27 PM Thread Starter
 
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Some answers below

What carb do you have? Stock quadrajet

Does it wind up if you feed the gas to it slowly or does it just bog when you mash it? If I feed it slowly, it's better...but still bogs around 3500 rpm

Fuel filter clean? I'll check that.

What is the timing set at? About 16 degrees at idle.

Plug wires in good shape and seated? New and seated fine.

Need more info, stock distributor or aftermarket? Stock

If it starts and runs good at idle I would suspect a fuel problem, sounds like it is either starving or flooding.
Is the choke sticking? The choke seems fine.
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post #4 of 12 (permalink) Old 03-27-2016, 07:54 PM
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Sounds like it is running out of fuel, plugged filter or float adjustment.
These old tanks have a lot of crap in them, my filter was clogging up so fast I ended up replacing the tank, sender and lines.

There are others here with a lot more knowledge about the Poncho engines than I do and they will help you to figure this out.
Welcome to the forum.

Also Cliff sells a really good book on rebuilding the QJet, I bought it and did the mods on my '67 and that fixed my midrange sputter.

Cliffs High Performance Quadrajets :: Qjet Carburetor Rebuild Kits, Parts, Quadrajet Rebuilding, Quadrajet Parts, Bushing Kits, Carb Tuning
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post #5 of 12 (permalink) Old 03-27-2016, 07:59 PM Thread Starter
 
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Thanks for the input. !!
I'll post any progress I have.
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post #6 of 12 (permalink) Old 03-27-2016, 08:09 PM
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Originally Posted by Osubucks999 View Post
Thanks for the input. !!
I'll post any progress I have.
You're welcome.
If you go to Cliff's site and click on testimonials the first one discusses the problem you are having.
I have a 2nd 703 I bought and rebuilt using his book, I sent my original to him to be replated and rebuilt and it is on its way back to me now and should arrive the first of next week.
Keep us updated.
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post #7 of 12 (permalink) Old 03-27-2016, 10:03 PM
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In addition to Goat Roper's advice I would also check the mechanical advance, if it is sticking you will lose your vacuum advance at about 3500 rpm which may cause the problems you are describing. Good luck and welcome to the forum!

Randy


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post #8 of 12 (permalink) Old 03-28-2016, 08:20 PM
 
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Garage
"If I feed it slowly, it's better...but still bogs around 3500 rpm"

If your secondary flap opening "pops" open too quickly, it will gulp air before creating the needed airflow to draw in the gas. There is a small spring on the side of the carb that is held in place by a small allen screw on the underside of the spring. You will also see a very small screw on the side which is turned to make this adjustment. You insert a screw driver into the screw slot, then loosen the allen screw just enough so you can turn the adjusting screw. Turn the screw, I believe clockwise, to tighten up on the spring, but only go about 1/4 turn from your starting point. Then snug down the allen screw (DON'T OVERTIGHTEN or it could strip out, its small) to lock it back into place.

What this does is tighten the rate of opening on the secondary air flap. This will slow down down the opening of the secondary flap because the flap will encounter more resistance and should in turn allow the air going into the secondaries to develop more velocity and pull the gas from the secondary wells to get the gas flowing BEFORE the secondary air flap pops wide open - you won't be sucking in a big gulp of air without the gas and then waiting for the velocity to catch up and get the gas flow going through the secondaries.

IF this shows and improvement in any way, then you know where your bog is coming from. If not, then you have eliminated one possible cause that I have experienced on a number of occasions in years past. The early Q-jets have a cut-out/square notch in the secondary flap while the later (beginning in the early 1970's) did not and are solid. Pontiac found that the flaps with the notches allow too much air through before the gas flow began - bog.

Could also be the secondary metering rods/hanger are not the correct ones and you need to "fatten" up the secondary flow of gas - your engine needs more gas when you go wide open throttle.

So check all the obvious first as suggested previously because it could be a fuel delivery problem or an electrical issue.
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post #9 of 12 (permalink) Old 03-29-2016, 05:28 PM
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I've had your symptoms with my '67 GTO with a Q-jet a couple of times, and each time it was fuel related, but not the carb. The first time was a collapsed filter sock on the pickup line inside the fuel tank. That one was a real duesy. Awhile later, same problem. Turned out to be a cracked fuel hose from the tank to the steel line at the rear of the car. It wasn't leaking, but the pump was sucking air. Replaced the short section of hose, and problem solved. Another thing that helped me with today's lousy alcohol-laced fuel was to re-connect my fuel vapor return system. Even on the hottest days, my carb gets nice cold fuel, and there is no vapor lock at all. When I was running a bypassed system due to ignorance, I experienced vapor lock at low speeds on hot days.
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post #10 of 12 (permalink) Old 03-29-2016, 06:02 PM
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Originally Posted by geeteeohguy View Post
I've had your symptoms with my '67 GTO with a Q-jet a couple of times, and each time it was fuel related, but not the carb. The first time was a collapsed filter sock on the pickup line inside the fuel tank. That one was a real duesy. Awhile later, same problem. Turned out to be a cracked fuel hose from the tank to the steel line at the rear of the car. It wasn't leaking, but the pump was sucking air. Replaced the short section of hose, and problem solved. Another thing that helped me with today's lousy alcohol-laced fuel was to re-connect my fuel vapor return system. Even on the hottest days, my carb gets nice cold fuel, and there is no vapor lock at all. When I was running a bypassed system due to ignorance, I experienced vapor lock at low speeds on hot days.
Yup, same thing on my goat, I used air to blow the sock but used too much and blew it off.
Instantly plugged my filter.
New tank, sender, air gap loop and hoses.
Start with the delivery system before messing with the carb or you can create more problems.
I found out what GOAT means Get Out Another Thousand.
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