anyone know why my secondaries wont open. have a holley 600 cfm carb with vaccum secs. with offys 4 barrel. crane cam and lifters,dual headers and exhausts...i put the weakest spring in the secondary diaphram..not to much bog if any on take off...how do i produce MORE VACCUM?!?!?!?! to open the secondaries?!?!?!??!


I was thinking about taking the spring out and see how it runs then..but will it fall back shut....

help please

 

WTF? Was it really necessary to ask twice within 10 minutes? This must be important!

The term vacuum secondaries is a bit of a misnomer because they don't work on vacuum, they work on airflow. It is entirely possible that your 300 doesn't generate enough airflow to open them. Another possibility is that the diaphram is leaking in which case they will never open.

 

Silver Streak is right... it's not really vacuum... but you should be pulling enough air flow through there to open the secondaries. Which Offy intake do you have... single plane or dual? And are you running a manual or auto trans?

They're not going to open just because you mash the gas... you have to get the rpms up to get the air flow going. If your running an auto trans be sure your kickdown linkage is working and adjusted correctly. If it's a manual downshift or stay in it a bit longer, you'll hear a difference when the secondaries open.

 

The engine should be able to get them open, but sometimes you get a wierd one. I was dyno tuning a 383 in a Vette a few years ago that wouldn't open the secondaries on a Demon 750 to save its life. We finally got them to open and they were worth about 5 hp. The car was set up for roadracing and the engine definitely should have pulled them open, it just didn't need them.

On this truck most people run a 390 Holley or 500 Edelbrock and it's plenty. A 300 can probably make 250-275 hp on the 300 cfm offered by half of a 600, so it might not need the airflow.

 

Just came across this looking for 4 barrel info. It explains alot about how a 4 barrel works and how to tune it. May help you get yours running better.

http://www.bob2000.com/carb.htm

 

Check for the "usual" 4bbl problems like choke operation and linkage interference. If your choke isn't working properly the secondaries might not open. Most carbs require that the choke be fully open [aka engine warmed up] before the linkage will let the secondaries open. With the engine off and air cleaner removed, have a helper press the gas pedal to the floor and make sure the throttle cable is actually opening the carb fully. If the cable isn't installed/ adjusted properly you won't get full power. If that checks out open the throttle all the way by hand while looking down into the carb. The secondary plates should not open. they might "wiggle" a bit but should stay closed.

Now hold the choke plate all the way open and work the primary linkage again, the secondary plates should start to open when you get to roughly 3/4 throttle [the engine is off so no vacuum to hold them closed]. If they open now it's probably a choke issue, which could be its setting or linkage. If the plates still don't open with the choke manually held open then something is probably wrong in the linkage. That's usually mechanical binding or interference.

When I put a Holley 390 on my Offy intake I had to use a short spacer under the carb as the throttle linkage was hitting the intake and not opening fully. Problem #1 solved. Then the secondaries wouldn't open. A bit of the secondary linkage was hitting the adapter plate and not allowing them to open. Notched the adapter plate and problem solved.

 

http://www.holley.com/data/Products/...99R10219-3.pdf

Read through the Holley tech info on Vacuum Secondaries...may be that will help.