Pontiac GTO Forum - View Single Post - `67 Bonneville 428
View Single Post
post #4 of (permalink) Old 05-19-2016, 07:14 PM
PontiacJim's Avatar
Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Posts: 3,738
Mentioned: 1 Post(s)
Tagged: 1 Thread(s)
Quoted: 404 Post(s)
Originally Posted by Rukee View Post
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.
PontiacJim is offline  
For the best viewing experience please update your browser to Google Chrome