vacuume secondaries not working
Thread Starter
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
vacuume secondaries not working
I recently installed a new Holley 770 Street Avenger carb on my 1971 car.
The original carb was a holley 780 so I feel the CFM is correct. It had a
670 from years ago and I wanted to change.
The problem is there is no 4 barrell sound when I run the car hard.
I have installed the lighter YELLOW spring but I still do not get full opening.
I just tried a test where I used a paper clip attached to the vacuume rod and then drove the car.
I then ran it up thru 6000 RPM in 3rd gear and then looked at the clip.
It had only moved 1/4"
But full opening of the throttle moves the rod 3/4"
Any suggestions on why the secondaries are not opening but 1/3 of the way?
Thanks
David
The original carb was a holley 780 so I feel the CFM is correct. It had a
670 from years ago and I wanted to change.
The problem is there is no 4 barrell sound when I run the car hard.
I have installed the lighter YELLOW spring but I still do not get full opening.
I just tried a test where I used a paper clip attached to the vacuume rod and then drove the car.
I then ran it up thru 6000 RPM in 3rd gear and then looked at the clip.
It had only moved 1/4"
But full opening of the throttle moves the rod 3/4"
Any suggestions on why the secondaries are not opening but 1/3 of the way?
Thanks
David
Last edited by dmayhew; Jul 29, 2008 at 08:26 PM.
Instructor
Joined: May 2008
Posts: 153
Likes: 0
I'm no expert, but if your throtlle linkage is opening the bottom butter flies on the secondaries, but not the top, then i wonder if you have to much cfm. I believe you staited that you changed to a lighter spring. I think you might have to give a little more info on the engine mods. I also think that different carbs act differently
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Even with the yellow spring, the Holley 770 vac sec carb is not going to fully open the secondaries on a 350 at any reasonable rpm. The closest you will get is with the white spring. This may get the secondaries about halfway open at WOT depending on your engine's volumetric efficiency and total airflow.
He has a Holley - there are no "top" and "bottom" butterflies, and the throttle linkage does not open the secondaries on a Holley vac sec. The secondary throttle is opened by venturi vacuum from the primary side acting against diaphragm spring pressure on the secondary throttle shaft.
Lars
Lars
Last edited by lars; Jul 29, 2008 at 09:19 PM.
Thread Starter
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
I never run it that hard.
There are no modificatiosn to the motor. It just had a 670 that was installed years ago and I wanted to install a carb. closer to the original factory CFM.
Chevrolet installed a 780cfm so I do not think a 770 is too much.
Thanks
David
Thread Starter
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
Even with the yellow spring, the Holley 770 vac sec carb is not going to fully open the secondaries on a 350 at any reasonable rpm. The closest you will get is with the white spring. This may get the secondaries about halfway open at WOT depending on your engine's volumetric efficiency and total airflow.
He has a Holley - there are no "top" and "bottom" butterflies, and the throttle linkage does not open the secondaries on a Holley vac sec. The secondary throttle is opened by venturi vacuum from the primary side acting against diaphragm spring pressure on the secondary throttle shaft.
Lars
He has a Holley - there are no "top" and "bottom" butterflies, and the throttle linkage does not open the secondaries on a Holley vac sec. The secondary throttle is opened by venturi vacuum from the primary side acting against diaphragm spring pressure on the secondary throttle shaft.
Lars
Thanks for the input. 1st. glad to hear from you and hope you are well
I will look into the white spring
David
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
David -
Thanks for the kind words. -
Keep in mind that GM was into the marketing thing... just because they put a 780 on the engine doesn't mean the engine was capably of taking full advantage of it. By using a vac sec carb, you can install a slightly over-sized carb without any performance or drivability penalty, yet get the "bragging rights" to the big "performance" carb. And GM did do this in their ads on that engine...
The 770 will run just great on your 350, but you may never see the vac secs fully open below 6500 rpm. The white spring should help out, but make sure you don't get a bog going into the secondaries with it.
A personal observation on this issue: I had a friend's 427 Vette on a chassis dyno with a vac sec 750 carb. With the thing at WOT and screaming at 5700 rpm, the secondaries were only about 1/2 open with the white spring. By manually forcing the secondaries wide open, we picked up 20 hp... After that, I installed an 850 mech sec carb on the engine and produced the best hp rating...
Thanks for the kind words. -
Keep in mind that GM was into the marketing thing... just because they put a 780 on the engine doesn't mean the engine was capably of taking full advantage of it. By using a vac sec carb, you can install a slightly over-sized carb without any performance or drivability penalty, yet get the "bragging rights" to the big "performance" carb. And GM did do this in their ads on that engine...
The 770 will run just great on your 350, but you may never see the vac secs fully open below 6500 rpm. The white spring should help out, but make sure you don't get a bog going into the secondaries with it.
A personal observation on this issue: I had a friend's 427 Vette on a chassis dyno with a vac sec 750 carb. With the thing at WOT and screaming at 5700 rpm, the secondaries were only about 1/2 open with the white spring. By manually forcing the secondaries wide open, we picked up 20 hp... After that, I installed an 850 mech sec carb on the engine and produced the best hp rating...
Drifting
Joined: Oct 2004
Posts: 1,496
Likes: 1
From: Lafayette Louisiana
That's an easy fix, but a hard spell.
First clear your throat and go WHOOAAAAH!!! until you let off the go pedal
With a little practice you can even add tire chirps on the shifts.
Sorry, I can't sleep tonight.
First clear your throat and go WHOOAAAAH!!! until you let off the go pedal
With a little practice you can even add tire chirps on the shifts.
Sorry, I can't sleep tonight.
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Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
the correct formula for carb sizing a holley is (cubic inchesXmax rpms)/3456*volumetric efficiency.
do a simple google search on carb forumula and that same formula comes up over and over again.
from what i understand VE is normally 80-85%
so (350Xsay 6000 rpms)/3456*.85=516cfm carb
do a simple google search on carb forumula and that same formula comes up over and over again.
from what i understand VE is normally 80-85%
so (350Xsay 6000 rpms)/3456*.85=516cfm carb
Thread Starter
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
David -
Thanks for the kind words. -
Keep in mind that GM was into the marketing thing... just because they put a 780 on the engine doesn't mean the engine was capably of taking full advantage of it. By using a vac sec carb, you can install a slightly over-sized carb without any performance or drivability penalty, yet get the "bragging rights" to the big "performance" carb. And GM did do this in their ads on that engine...
The 770 will run just great on your 350, but you may never see the vac secs fully open below 6500 rpm. The white spring should help out, but make sure you don't get a bog going into the secondaries with it.
A personal observation on this issue: I had a friend's 427 Vette on a chassis dyno with a vac sec 750 carb. With the thing at WOT and screaming at 5700 rpm, the secondaries were only about 1/2 open with the white spring. By manually forcing the secondaries wide open, we picked up 20 hp... After that, I installed an 850 mech sec carb on the engine and produced the best hp rating...
Thanks for the kind words. -
Keep in mind that GM was into the marketing thing... just because they put a 780 on the engine doesn't mean the engine was capably of taking full advantage of it. By using a vac sec carb, you can install a slightly over-sized carb without any performance or drivability penalty, yet get the "bragging rights" to the big "performance" carb. And GM did do this in their ads on that engine...
The 770 will run just great on your 350, but you may never see the vac secs fully open below 6500 rpm. The white spring should help out, but make sure you don't get a bog going into the secondaries with it.
A personal observation on this issue: I had a friend's 427 Vette on a chassis dyno with a vac sec 750 carb. With the thing at WOT and screaming at 5700 rpm, the secondaries were only about 1/2 open with the white spring. By manually forcing the secondaries wide open, we picked up 20 hp... After that, I installed an 850 mech sec carb on the engine and produced the best hp rating...
LArs , I understand your point on the marketing angle from GM. I can report that on low end performance the cars does run better.
I have purchased the spring set today and will try it tonight..
Just a FYI , I called Holley and your's and their advice are the same.
The forum is lucky to have you.
David
Thread Starter
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
I am going to follow the advice my mother gave me years ago and not say anything here.
David
Thread Starter
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
the correct formula for carb sizing a holley is (cubic inchesXmax rpms)/3456*volumetric efficiency.
do a simple google search on carb forumula and that same formula comes up over and over again.
from what i understand VE is normally 80-85%
so (350Xsay 6000 rpms)/3456*.85=516cfm carb
do a simple google search on carb forumula and that same formula comes up over and over again.
from what i understand VE is normally 80-85%
so (350Xsay 6000 rpms)/3456*.85=516cfm carb
Thanks Baxon
The Holley charts do show 770 as correct on a 350 @ 6500 RPM.
But the formula you site does diasgree with that.
I think Lars has me on a good path to a little better top end and then I will just need to accept what I have or put the 670 back on.
The bottom end performance with the 770's #72 jets over the older #65 primary jets on the 670 have made a big difference from a standing take off.
Thanks again
David
Thread Starter
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
Lars
I installed the white spring and test drove the car.
There seemed to be a definite improvement in performance.
It also clearly seemed louder than yesterday.
The paper clip test showed 7/16" movement. This up from the 1/4" yesterday.
So 3/16" more travel or about. 75%
There was also no dead spots or bogging down.
Thanks for the advice. I guess I learned this lesson the hard way.
David
I installed the white spring and test drove the car.
There seemed to be a definite improvement in performance.
It also clearly seemed louder than yesterday.
The paper clip test showed 7/16" movement. This up from the 1/4" yesterday.
So 3/16" more travel or about. 75%
There was also no dead spots or bogging down.
Thanks for the advice. I guess I learned this lesson the hard way.
David
