Holley Vacuum Secondaries Won't Open

[ernie d]

Recently installed a 750 Holley 750 DP on my 454. Carb is list number 80508-S. Only problem is vacuum secondaries are NOT opening. I know because I hard strapped the secondary throttle shaft to stay closed, drove it to WOT. Then removed the strap (to allow secondaries to open, redrove it, no difference in accel performance. So I know the secondaries are not opening.

I did get a quick change kit and am currently running the lightest secondary diaphragm spring (which should be the earliest to open secondaries).

Any ideas on how to correct?

Thanks!

Ernie

 

[AKRAY4PLAY]

simple things first. have you hooked a vacuum guage up and drove down the road?? it is possible that the motor is not making enough vauum, or there is a vac leak, preventing things from working right. vac guages are cool tuning tools, highly suggest getting one if you don't already. double check your linkage to see if everything is operating smoothly. Brian

 

[vette-dude]

If you have vacuum then the most likely culprit is the diaphram in the vacuum cannister on the side of the carb. the other thing is that it could have too strong of a spring for the vacuum to pull in the secondaries. You can get a kit that has various strength springs in it to fix that problem. The diaphram is a simple fix also.


Randy

 

[JohnZ]

First, there's no such thing as a vacuum secondary double-pumper; you can have either one, but not both. Second, manifold vacuum has nothing at all to do with opening the secondaries on a vacuum secondary carb; the vacuum signal that opens the secondaries is "venturi vacuum", created by airflow velocity through the primary venturi past a pickup port, that's connected via an internal passage to the diaphragm chamber that operates the secondary throttle shaft.

The most common cause of secondaries not opening is a missing tiny cork gasket for that vacuum passage between the diaphragm housing and the carb body, especially if the housing has been removed to change springs. Next most common cause is a ruptured diaphragm - remove the housing, pull a vacuum on the port into the housing to pull up the rod, and put your finger over the port; if you see any movement of the rod, the diaphragm is bad, usually caused by it being torn by the cover attaching screws.

 

[vette-dude]

First, there's no such thing as a vacuum secondary double-pumper; you can have either one, but not both. Second, manifold vacuum has nothing at all to do with opening the secondaries on a vacuum secondary carb; the vacuum signal that opens the secondaries is "venturi vacuum", created by airflow velocity through the primary venturi past a pickup port, that's connected via an internal passage to the diaphragm chamber that operates the secondary throttle shaft.

The most common cause of secondaries not opening is a missing tiny cork gasket for that vacuum passage between the diaphragm housing and the carb body, especially if the housing has been removed to change springs. Next most common cause is a ruptured diaphragm - remove the housing, pull a vacuum on the port into the housing to pull up the rod, and put your finger over the port; if you see any movement of the rod, the diaphragm is bad, usually caused by it being torn by the cover attaching screws.

Your correct JohnZ. I forgot about that pesky oring gskt. Also missed the DP in the first sentence. No such animal for sure. I would bet the oring is gone. Glad you caught the incorrect diagnosis and chimed in. Haven't worked on a Holley for a while and memory ain't what it use to be. Hope it helps the guy with all this info.

Randy