School me

Cobain03 · Aug 2, 2006

i want to know EVERYTHING there is to know abuot how vaccum secondaries work. how to check them, test them, checking the floats, amount of RPM needed to open, everything.

my four barrell is not kicking in.

DJCarbine · Aug 2, 2006

How have you come to the conclusion that your secondaries are not kicking in?

ForceFed70 · Aug 2, 2006

DJCarbine said:
How have you come to the conclusion that your secondaries are not kicking in?

My question exactly.

Unless there is a mechanical obstruction keeping the secondary throttle plate from opening your engine would run very lean when you put your foor into it. It would bog badly and probably stall untill you back off on the pedal.

Cobain03 · Aug 2, 2006

you can go WOT untill about 4k rpms (engine dont rev high) and bring it back down and look at the vents inside the carb on the secondaries side and see that they are dry

DJCarbine · Aug 2, 2006

Fuel evaporates pretty quick.. especially in an environment like a carb where its being sucked in very tiny like. Just because the butterflies are dry in the secondaries, doesnt mean they arnt working. Carbs suck in fuel on demand... depending on how long you stayed in the secondaries range, the butterflies dont have to be soaking with fuel after you are done driving. Heck.. if my primaries were soaking with fuel, that would mean my carb is flooding. Dont ask me how I know

ForceFed70 · Aug 2, 2006

Cobain03 said:
you can go WOT untill about 4k rpms (engine dont rev high) and bring it back down and look at the vents inside the carb on the secondaries side and see that they are dry

Not a proper test.

Easiest is to look down carb with engine off. Is the throttle plate/butterfly open? If so, then the plate is opening properly (start engine and let idle if you want to confirm that plate closes) If the plate is opening properly, you would know if fuel is not flowing... car would bog badly and barely run once you put your foot into it.

Cobain03 · Aug 2, 2006

might not be "proper" but if they have had fuel in them, then they should be wet, or at least a little slick.

ForceFed70 · Aug 2, 2006

Cobain03 said:
might not be "proper" but if they have had fuel in them, then they should be wet, or at least a little slick.

Like DJCarbine said... fuel evaporates quickly.

Especially in a carb where it's hot with alot of airflow.

Cobain03 · Aug 2, 2006

so, what is the best way to tell if they are opening

Protowrxs · Aug 2, 2006

What kind of carb is it?

Vacuum secondaries open on engine vacuum demand, not necessarily at a certain RPM. Flipping the throttle will generally not force them open.. shouldn't anyhow.

It’s easy to adjust when they open on a standard Holley by changing out the spring that holds them shut. I actually went as far as having a micro switch and an indicator light inside my car eons ago when I was tuning the carb. Was nice to see when/how they came online when springs were swapped out.

I know it’s too much hassle for what you want. I bet if you disconnect the bellow and wire the plates down you’ll notice the difference if they ARE working.

-Stephen

D.Hearne · Aug 2, 2006

If you've still got the 600's original secondary spring, the secondaries will not open untill about 4500-5000 rpms. Buy a secondary spring assortment and pull the black spring out of the vacuum pod ( bet you $5 that's what's in there now ) and put the lightest spring in the pack in it's place. They WILL open then just after you plant your foot into it. Want them a tad later? use the purple spring.

D.Hearne · Aug 2, 2006

BottleFed70 said:
My question exactly.

Unless there is a mechanical obstruction keeping the secondary throttle plate from opening your engine would run very lean when you put your foor into it. It would bog badly and probably stall untill you back off on the pedal.

This scenario would only happen if the secondary butterflys open but no fuel flows. If they don't open, no fuel flows and the engine's rpm will simply be limited to whatever the primary side will support. It will not lean out on the top end and cause any damage nor will it stall.

Cobain03 · Aug 3, 2006

i do have the spring kit and have changed the springs a couple of times. i just have no idea whats in there now

D.Hearne · Aug 3, 2006

Try this: With the engine not running, open the primaries all the way, then while holding them open, see if you can open the secondary side by hand. If they do, then the only other reason they wouldn't open by themselves is the vacuum passage is blocked, or could be the o ring gasket between the pod and carb body is missing. Also check the diaphram and see that it's in place properly under the cover.

Max Power · Aug 3, 2006

You can't test vacuum secondaries by revving the motor in idle. It needs load and a vacuum drop, in conjuction with the primaries being open.

An old trick is to put a paper clip on the secondary linkage. Put the clip on the rod coming down from the secondary diaphram at the very top most location. Note the location where you started, go out and flog it, then check the location of the clip when you're done. If the secondaries are opening, the clip will have been forced down that rod and will be sitting near the bottom.

I have heard many times over the years where people are convinced that their vacuum secondaries are not opening. They usually think this because their motor isn't running as strong as they think it should. 99.9% of the time they are wrong. A 302 with stock heads and minor bolt ons is not going to need those secondaries nearly as much as you think.

ForceFed70 · Aug 3, 2006

D.Hearne said:
This scenario would only happen if the secondary butterflys open but no fuel flows. If they don't open, no fuel flows and the engine's rpm will simply be limited to whatever the primary side will support. It will not lean out on the top end and cause any damage nor will it stall.

Which is exactly what I was getting at.

If the car doesn't run very poorly, there must be a mechanical obstruction keeping to butterfly from opening OR the secondaries are working properly.

To check if the butterfly is opening, just look down the carb with the engine shut off. It should be wide open because there is no vacum with the engine off.

This is all you need to do to determine if the secondaries are working or not.

D.Hearne · Aug 3, 2006

BottleFed70 said:
To check if the butterfly is opening, just look down the carb with the engine shut off. It should be wide open because there is no vacum with the engine off.

This is all you need to do to determine if the secondaries are working or not.

They don't work that way. With the engine shut off and the throttle closed, the secondary butterflies are as well. They do not open after you shut the engine off. They as well will not open with the engine shut off and the primary butterflies pulled open. The secondary spring pushing on the diaphram keeps that from happening, they close by means of the linkage (drivers side of the carb) connecting the primary to the secondary throttle shafts. This link can also serve to convert the secondary opening to manual(or mechanical) by inserting a machine screw and nut into the slot that the secondary linkage rod operates thru, creating a sort of "poor-mans" mechanical secondary. But to make this work better, a larger 50 cc accellerator pump needs to be installed in the primary side to compensate for the temporary lack of sufficient fuel when the secondaries are opened ( which to those newbies out there is the reason for the secondary side pump on a "double pumper" Holley-type carb)

ForceFed70 · Aug 3, 2006

D.Hearne said:
They don't work that way. With the engine shut off and the throttle closed, the secondary butterflies are as well. They do not open after you shut the engine off. They as well will not open with the engine shut off and the primary butterflies pulled open. The secondary spring pushing on the diaphram keeps that from happening, they close by means of the linkage (drivers side of the carb) connecting the primary to the secondary throttle shafts. This link can also serve to convert the secondary opening to manual(or mechanical) by inserting a machine screw and nut into the slot that the secondary linkage rod operates thru, creating a sort of "poor-mans" mechanical secondary. But to make this work better, a larger 50 cc accellerator pump needs to be installed in the primary side to compensate for the temporary lack of sufficient fuel when the secondaries are opened ( which to those newbies out there is the reason for the secondary side pump on a "double pumper" Holley-type carb)

OK... so he may need to pull on the throttle linkage to get the secondaries to open, but I see no reason why you couldn't get them to open with the engine off.

Cobain03 · Aug 3, 2006

engine off, the butterflies are closed. i have pushed them open before while the car running and it sucked air like crazy and no fuel came out.

ForceFed70 · Aug 3, 2006

Cobain03 said:
engine off, the butterflies are closed. i have pushed them open before while the car running and it sucked air like crazy and no fuel came out.

With the engine off, did you pull on the throttle linkage to simulate WOT like D. Herne mentioned to do?

Also, generally you won't be able to see the fuel flow... when you do see fuel it's usually from the accelerator pump. The secondaries in a vacum secondary carb do not have an accelerator pump.

If the butterfly is opening but no fuel is flowing your engine would bog and barely run. The AFR would approach 25:1..most engines won't run any leaner than 17:1 You'll have to trust me on this... take it from someone who's had a carb that had this problem (plugged jet). When the secondaries opened the car bogged very badly and started to actually slow down... I would have to let up on the pedal to accelerate again.

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