Weak Secondary Opening
Thread Starter
Instructor
Joined: Apr 2009
Posts: 114
Likes: 1
From: Bradenton, FL
Weak Secondary Opening
Need help with a secondary issue on my carb. I have read many of trhe previous threads on this issue and I have followed all the directions but I still cannot get my secondaries to open very much. I have adjusted the timing correctly. I replaced the 750 Holley with a 670 Holley Street Avenger and I installed the weakest spring. I also replaced the throttle cable with a new Lokar cable to ensure WOT. Below are a couple of pictures. One shows the paper clip and the amount of secondary action and the second shows the Lokar install. Any suggestions would be greatly appreciated. Thanks!!!
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
When you press the pedal to the floor, are the primary throttle blades opening all the way? Have someone hold the pedal down and open the secondary plates by hand and make sure they're not binding or hanging on something. It's doesn't take much drag to stop them- that little vacuum pot has very little pull.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
Air cleaners can affect the opening point and rate, because the diaphragm is vented to the atmosphere and it "sees" the vacuum drop accross the air cleaner. It's a delicate balance of forces. The secondaries are opened by engine demand.
On the other hand, a too large vacuum-operated secondaries carburetor can often be used without spolling drivability, but the secondaries may not open entirely. Your engine is probably not using the maximum cfm capacity of your carb.
On the other hand, a too large vacuum-operated secondaries carburetor can often be used without spolling drivability, but the secondaries may not open entirely. Your engine is probably not using the maximum cfm capacity of your carb.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
If you have a relatively thick gasket under your carb, and/or too much tightening torque on it, it can slightly bend the secondary shaft, preventing the vacuum diaphragm to operate correctly.
Last edited by 73StreetRace; Jul 29, 2009 at 05:36 AM.
Thread Starter
Instructor
Joined: Apr 2009
Posts: 114
Likes: 1
From: Bradenton, FL
Thanks for all the input. I will check out some of the suggestions to see if they help. The vacuum diaphragm is in good condition. I replaced it on the 750 I took off and I checked this one as well. If you all think of any more let me know. If I can't get this to work maybe I'll switch to a QJ.
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
thats a lot of effort just to find out that if your engine didnt have enough vacuum to open a holley it probably wont open a qjet either.
qjets only open as far as the engine demands. that is why they are so versatile and the same cfm was used on a large range of engine. id wager that on some of the smaller engines it was put on the back two barrels hardley ever got cracked open
ps
also check the diaphram to make sure the check ball is in it. then make sure that there is a good seal between the diaphram and the carb body. sometimes that little cork O ring comes out
qjets only open as far as the engine demands. that is why they are so versatile and the same cfm was used on a large range of engine. id wager that on some of the smaller engines it was put on the back two barrels hardley ever got cracked open
ps
also check the diaphram to make sure the check ball is in it. then make sure that there is a good seal between the diaphram and the carb body. sometimes that little cork O ring comes out
Last edited by baxsom; Jul 30, 2009 at 07:37 AM.
