Carburetor secondaries not opening
Thread Starter
Heel & Toe
Joined: Aug 2015
Posts: 17
Likes: 0
Carburetor secondaries not opening
Hey folks,
I have an 86 Corvette carbureted motor(Holley 670) Today I realize the secondaries are not opening. It's so happens that the power break booster is bad and I know there is a vacuum line from the back of the carburetor to the brake booster. Could this be the problem? And a diaphragm feels fine.Thanks!
I have an 86 Corvette carbureted motor(Holley 670) Today I realize the secondaries are not opening. It's so happens that the power break booster is bad and I know there is a vacuum line from the back of the carburetor to the brake booster. Could this be the problem? And a diaphragm feels fine.Thanks!
Last edited by Charlieg0357; Nov 18, 2015 at 05:34 PM.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
I disagree w/the vid...if you hold the thing WOT, the secondaries should start opening (a little) before you hit your red line.
Other than that, you'd have to put it on a chassis dyno and watch the secondaries under a load.
Or put a Go Pro under the hood.
Or take the hood off.
Other than that, you'd have to put it on a chassis dyno and watch the secondaries under a load.
Or put a Go Pro under the hood.
Or take the hood off.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
^Not very well. The vacuum source for the 2ndaries on most if not all Holley carbs is the primaries. When the velocity becomes great enough through the primaries, the vacuum in/around the venturi becomes great enough to overcome the spring and pull the 2ndaries open. I don't think a manifold vacuum leak would have much impact on that, unless it was an enormous leak.
Last edited by Tom400CFI; Nov 18, 2015 at 07:03 PM.
Melting Slicks
Joined: Aug 2008
Posts: 2,082
Likes: 50
From: Palmdale CA
I like the Go-pro idea.. but ya when you wrap the engine the secondary Butterfly should open some.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Melting Slicks
Joined: Sep 2008
Posts: 2,376
Likes: 111
From: Cherokee National Forest TN
Change when the secondarys open with a diaphragm spring change.
http://www.summitracing.com/parts/hly-20-13/overview/
http://www.summitracing.com/parts/hly-20-13/overview/
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Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Hey folks,
I have an 86 Corvette carbureted motor(Holley 670) Today I realize the secondaries are not opening. It's so happens that the power break booster is bad and I know there is a vacuum line from the back of the carburetor to the brake booster. Could this be the problem? And a diaphragm feels fine.Thanks!
I have an 86 Corvette carbureted motor(Holley 670) Today I realize the secondaries are not opening. It's so happens that the power break booster is bad and I know there is a vacuum line from the back of the carburetor to the brake booster. Could this be the problem? And a diaphragm feels fine.Thanks!
If it has vacuum secondaries they'll never open by just gunning the engine in your driveway because the air velocity thru the primaries will never get high enough. To verify they are opening stick a bobby pin onto the top of the secondary lift rod (from the diaphragm) and take it out for a spin. If they are opening (and they probably are) the bobby pin will get shoved down lower on the rod.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
A simple check and suggestion.
As said a lot of carburetors work on vacuum which comes from the manifold. On certain carburetors that vacuum comes thru the gasket on the bottom of the carburetor which there can be holes or passages that are parallel to the gasket. When the holes become clogged (from carbon) either in the gasket or hole at mating surface of the carb to the gasket, then the vacuum can't reach the carburetor to activate it's proper operation.
I would check the bottom surface to see if all is clean and aligned properly.
As said a lot of carburetors work on vacuum which comes from the manifold. On certain carburetors that vacuum comes thru the gasket on the bottom of the carburetor which there can be holes or passages that are parallel to the gasket. When the holes become clogged (from carbon) either in the gasket or hole at mating surface of the carb to the gasket, then the vacuum can't reach the carburetor to activate it's proper operation.
I would check the bottom surface to see if all is clean and aligned properly.