`67 Bonneville 428 - Pontiac GTO Forum
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post #1 of 18 (permalink) Old 05-18-2016, 11:10 AM Thread Starter
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`67 Bonneville 428

Is the B-body 428 timed the same way as the 389 and 400? I have one here that is plumbed to manifold vac and falls on it's face only when the 4bbl opens. Should it be ported? All tune-up parts look good.
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post #2 of 18 (permalink) Old 05-18-2016, 06:58 PM
 
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Is the B-body 428 timed the same way as the 389 and 400? I have one here that is plumbed to manifold vac and falls on it's face only when the 4bbl opens. Should it be ported? All tune-up parts look good.
Should be, I don't see why not. Sounds more like a carb problem. Could be the accelerator pump ( Quadrajet Accelerator pump Page ) is not giving the carb a shot of fuel when you open it up. This will cause the car to go flat. Next, your secondaries may be opening too quickly, sucking in a big gulp of air before developing enough air velocity to draw in the needed gas to go with the big gulp of air. Does it fall flat and then eventually pick back up? Does it do better if you gradually ease into the carb as RPM's increase and open it fully up as opposed to slapping the gas pedal to the floor to pop the secondaries open?
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post #3 of 18 (permalink) Old 05-19-2016, 10:51 AM Thread Starter
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Thanks for the response. Acc pump is working fine. It runs great and pulls good right up to the point the secondaries open, then it starts to stumble and almost feels like it's missing or popping out the exhaust. Drop back to the primaries and it comes right out of it. If I stay in it, it doesn't recover. Either stabbing it or opening slowly.
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post #4 of 18 (permalink) Old 05-19-2016, 07:14 PM
 
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Thanks for the response. Acc pump is working fine. It runs great and pulls good right up to the point the secondaries open, then it starts to stumble and almost feels like it's missing or popping out the exhaust. Drop back to the primaries and it comes right out of it. If I stay in it, it doesn't recover. Either stabbing it or opening slowly.

OK, sounds like a carb problem. Since all seems OK on the primaries, I'd focus on the secondaries on this one. A) Fuel delivery problem going to secondaries- which may be internal B) Secondary air flap opening to quickly/soon and you are getting nothing but air.

First thing I would do is check to see that the "choke pull-off", what I call a vacuum diaphragm, is working. You will see it on the front/side of the carb and it has a linkage rod going to a slotted bracket on the secondary air flap shaft. When cruising, and at high vacuum, this diaphragm pulls the linkage tight, thus keeping the secondary air valve flap closed. When you stomp on the gas, vacuum drops, and the diaphragm releases to allow the secondary air flap to then open. The diaphragm also acts as a "regulator" in that it can control the speed at which the secondary air flap opens - fast or slow - just depends on the size of the orifice leading into the diaphragm; smaller tube, vacuum drop is slower (slower air flap release/opening) or larger tube (faster air flap release/opening).

As soon as you let off the gas from wide open, engine vacuum returns and the diaphragm snaps shut the secondary air flap cutting off air/fuel. However, I typically remove this AND adjust the tension on the secondary air flap - but you do not want to do this at this time.

If you push down on the secondary air flap to open it, does it open easily or does it have some tension to it? Does if lazily close or does it have a little snap to it when you allow it to close by hand?

What you do want is a little tension. If it seems to be "lazy" or doesn't have much tension in closing the air flap, you might need to add more tension. If not enough tension, when stomp the gas, the engine vacuum snaps the secondary flap open to quickly and you get that big gulp of air/stumble and flat spot. The flap requires some tension to slow the rate of the flap opening so it cracks open, letting the incoming air build velocity and suck gasoline through the fuel discharge nozzles (the big round tubes found under the air flap). This provides a smooth transition of power as the flap opens. You also don't want it too tight, or it may not fully open or it won't open until the engine hits higher RPM's.

Check these things out first, the diaphragm operation and then the tension on the air flap. If these seem to be good, then I am still leaning towards an internal fuel delivery problem. But, if the secondary flap seems to flop open (very little tension/resistance) when you push it open by hand, then a little adjustment might do the trick. Post your findings and I can go from there if you need to adjust the air flap tension.
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post #5 of 18 (permalink) Old 05-19-2016, 08:11 PM Thread Starter
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Everything seems fine as far as the secondaries and choke pull off goes. I'm going to check the #s on the carb tomorrow and if it's the right carb for the motor I'll order a kit for it and boil/rebuild it. I'll post up the #s and maybe you can help me out with them.
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post #6 of 18 (permalink) Old 05-21-2016, 01:04 PM Thread Starter
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The carb # is 7028262 with an M7 & VC under that.
I found a kit but it's special order and is going to take a couple days to get.
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post #7 of 18 (permalink) Old 05-21-2016, 09:48 PM
 
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The carb # is 7028262 with an M7 & VC under that.
I found a kit but it's special order and is going to take a couple days to get.

My reference book lists it for the full size 1968 400CI, 340 HP, OR the 350 HP engine.

The M7 is the date. If a Carter made Q-jet, the M=December and the 7 would indicate 1967. I believe the VC is the plant code, ie the Pontiac factory code used for ID just like the engine ID and was found on the factory build sheet during assembly.
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post #8 of 18 (permalink) Old 05-22-2016, 10:13 AM Thread Starter
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Originally Posted by PontiacJim View Post
My reference book lists it for the full size 1968 400CI, 340 HP, OR the 350 HP engine.

The M7 is the date. If a Carter made Q-jet, the M=December and the 7 would indicate 1967. I believe the VC is the plant code, ie the Pontiac factory code used for ID just like the engine ID and was found on the factory build sheet during assembly.
So, it's not correct for this engine but should work anyway. Right?
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post #9 of 18 (permalink) Old 05-22-2016, 11:38 AM
 
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So, it's not correct for this engine but should work anyway. Right?

I can't say it is or is not correct for your engine/car. The '67 intake has a different configuration under the carb - its a one year thing. The attached pic on the left is '67 & the pic on the right is '68. The Dec '67 date would put the carb as typically a 1968 model, but, Pontiac does not always follow the rules 100% - it could have been replaced at some point or it could have been a dealer replacement.

I do not know enough to tell you if the '67 carb is specific to the design of the intake or how to tell. I would PM geeteeohguy as he would know this. You may also have to make sure you use the correct under-the-carb gasket for your combo, again geeteeohguy will know.

Yes, it should work just fine.
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post #10 of 18 (permalink) Old 06-05-2016, 12:34 PM
 
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Has there been any progress made. I would be interested to know.
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