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Old 09-11-2004, 06:35 PM
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vacuum secondary tuning

I feel like an idiot. I've read about 4 books on Holleys and I can't seem to tune my vacuum secondaries correctly. I have a very good understanding how it all works and what the different tension springs do but something isn't right.

During my test drives I can't tell any difference in performance between my lightest spring and the stiffest spring. I think that my driving technique isn't working. I read somewhere that you should test the opening point of the secondaries in high gear. Well when I do that, I can't tell the difference between the secondaries kicking in and the transmission kicking down. So I started my test runs in first gear. There doesn't seem to be a bog
or any type of performance difference between springs when I do this.

What can I do to tell if the secondaries are opening at all and if they are then how should I find out which spring is best without a dragstrip? Thanks in advance.

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Old 09-11-2004, 06:44 PM
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were you able to set the air bleeds with the vaccum gage and are certain there are no vaccum leaks and you have the vaccum line hooked up to ported vaccum above the throttle blades?
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Old 09-11-2004, 09:25 PM
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What size carb and what size motor? If you have a 750 carb on a stock 305 you might not ever open the secondaries. Holley doesn't recommend it but I find that if you remove the check ball from the passage leading to the vacuum diaphragm for the secondaries it will respond much faster.
Keep in mind that open gradually as rpm builds rather than snap open at once so you might not hear them kick in real hard when you get it tuned in correctly.
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Old 09-11-2004, 09:35 PM
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Quote:
Originally posted by GoneNova/406
were you able to set the air bleeds with the vaccum gage and are certain there are no vaccum leaks and you have the vaccum line hooked up to ported vaccum above the throttle blades?
Ya lost me there dude

The airbleeds are the orifices at the top of the venturis, to either side of the accelerator pump nozzels.

Vacuum line hooked up to ported vacuum?
Those are for vacuum advance, not secondary actuation.

A 750 Holley will still open the secondaries on a 305.

Do not remove the check ball. If you wan to speed up flow, (you don't need this mod with your carb and engine size) you can take a phillips screwdriver and tap it lightly with a hammer to make 4 slots underneath the checkball.

Okay now for the answer to the question.
Check to make sure you have the o-ring gasket between the secondary housing and the main body.

Make sure the vacuum diaphragm is intact, and installed properly. You can do this somewhat, by removing the housing and putting your finger over the vacuum port that I mentioned above. Push the linkage arm on the secondary housing up into the housing itself. It should have tension going up, and if you keep your finger over the hole it shouldn't release until you take your finger off.

Try that and then get back to us.
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Old 09-11-2004, 10:53 PM
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Ok. The carb is Holley's 3310-1 780 cfm. It's on my 383 chevy engine with 10:1 compression and a cam with 280 288 advertised duration.

I'm a little confused too on the air bleed comment since they are not easily adjustable on these carbs. I'm also lost on the vacuum advance relation too. But with that aside:

The gasket between the vacuum unit and the carb body is in place. The diaphragm is intact and I assembled it right (according to everything I've read and heard). I did the check you mentioned over and over again and it seemed to work but with a little variation. As I pushed the rod in, there was some resistance (more than the spring) and then I'd plug the vacuum hole with my finger and the rod would back out again most of the way but would return fully when I removed my finger from the vacuum port. It doesn't "stay put" as it's supposed to. It's a new diaphragm and I've tried others as well as a different housing. It all does the same thing everytime.


Also for schitz and giggles, I put a stack of washers where the spring goes to keep the secondaries from opening at all then drove it. There wasn't a big difference at all in the performance of my truck. Shouldn't there be a significant difference between them opening and not opening at all? Am I overlooking something?

Oh and for the record I tried it with the check ball removed and it does the same thing.

thanks for all of your input so far!!!
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Old 09-12-2004, 10:21 AM
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Looks like you have a leak in the vacuum diaphragm. Even if it is new if it slips a little while you're assembling it it can leak.or the hole for the vacuum signal could get blocked. If you have a hand vacuum pump make sure that you can operate the daiphragm with the vacuum pump before you assemble it, and leak check it at the same time.
As for removing the ball, anytime you do anything that speeds up the flow through the vacuum passage it's possible that it can cause the secondary throttle plates to flutter. The reason I remove the ball rather than making slots in the area that the ball seats is that it is much easier to put the ball back in than it is to repair the seat if you end up with a flutter problem. Only rarely have I had to put the ball back in.
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Old 09-12-2004, 10:35 AM
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lluciano
you just don't know when to quit do you?you'll be happy to know i just got a new elgin pro stock cataolgue and guess how they have the difference in cams listed when it comes to 1.5 and 1.6 rockers dude?yup you guessed it.you were wrong wrong wrong.no wait a mimute,maybe elgin is wrong.do ya think.i can shoot my mouth off to but who does it benefit.why don't you just go about your buisness and i'll go about mine and we'll leave it at that.something tells me you aren't the type though.
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Old 09-12-2004, 10:35 AM
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We used to tune Holley vac secondaries until they produced a bog when they kicked in, then back off the tuning until the bog is just barely eliminated. But then again, most of the time I used to drive around with a screw in the secondary linkage which made them mechanical.

Vince
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Old 09-12-2004, 11:22 AM
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Quote:
Originally posted by GoneNova/406
lluciano
you just don't know when to quit do you?you'll be happy to know i just got a new elgin pro stock cataolgue and guess how they have the difference in cams listed when it comes to 1.5 and 1.6 rockers dude?yup you guessed it.you were wrong wrong wrong.no wait a mimute,maybe elgin is wrong.do ya think.i can shoot my mouth off to but who does it benefit.why don't you just go about your buisness and i'll go about mine and we'll leave it at that.something tells me you aren't the type though.
woah tiger. all he said was he doesn't know where you're coming from on the comment. I mean, do either of you two build these carbs professionally? Either of you work for holley? Seems like there's really no professional edge with anybody here, we're (mostly) all just hobbyists.

Lighten up bro, nobody's saying you don't know what you're talking about, you've given some good advice in the past to me at least-

K
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Old 09-12-2004, 11:56 AM
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Quote:
Originally posted by GoneNova/406
lluciano
you just don't know when to quit do you?you'll be happy to know i just got a new elgin pro stock cataolgue and guess how they have the difference in cams listed when it comes to 1.5 and 1.6 rockers dude?yup you guessed it.you were wrong wrong wrong.no wait a mimute,maybe elgin is wrong.do ya think.i can shoot my mouth off to but who does it benefit.why don't you just go about your buisness and i'll go about mine and we'll leave it at that.something tells me you aren't the type though.
Alright foolio. You just won't get it will you. They measure the increase in duration at .050". The total duration does not increase. The rocker cannot predict that the cam lobe is coming, or that it is about to return to zero.

You are and idiot, idiot , idiot.

You need to learn how to read, read, read,

And you can eat a big d*ck, d*ck, d*ck!

I went about my own business. Notice how you just hijacked this post. You are rehashing an old argument. You couldn't leave it at that. Talk about the pot calling the kettle black.

Quote:
Originally posted by GoneNova/406
were you able to set the air bleeds with the vaccum gage and are certain there are no vaccum leaks and you have the vaccum line hooked up to ported vaccum above the throttle blades?
Everyone knows the airbleeds can't be "set" on standard Holley carbs. You can only drill them. That would most likely ruin a perfectly good carb.

It sounds to me like you are purposely giving false information. People have been suspended for that recently. Keep it up *sshole.

Sorry dirtydave. I just had to set the board's newest Troll straight. I don't think it is cool to knowingly misinform people.

I think the diaphragm is probably in correctly.

Keep the check ball in.

I have tried putting a screw in the secondary linkage to make it mechanical before. You will get a bigger bog when the secondaries open. It may feel like the car gets faster, but what you actually fell is a hesitation before the acceleration. That gives the biggest seat of your pants feel.



Here is the original thread that GoneLoser is referring to if anyone is wondering what we are talking about.
http://hotrodders.com/showthread.php....5+1.6+rockers

Last edited by lluciano77; 09-12-2004 at 12:29 PM.
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Old 09-12-2004, 02:33 PM
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Old 09-12-2004, 09:20 PM
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Well thanks for all the comments and the settled argument (if that's what you want to call it). I still have some questions though.

How is this bog that I'm looking for supposed to feel? Is it sort of a hesitation in acceleration? Is it like a dip or a stumble? I guess I'm still a novice at recognizing these symptoms.

Shouldn't there be a noticable difference between the secondaries opening and them staying shut? My truck seems to pull hard at WOT even when I rig the secondaries to stay shut. I would think this would mean they aren't opening at all when I don't have them rigged shut, unless the difference between them open and closed is small. But that just doesn't seem right to me.

With the research I've done, people have made 450 hp + with this same carb on a similar set up 383 as mine. So I don't think the carb is too big. How else would too big a carb act (besides the poor off idle performance and unresponsive bottom end)?

thanks a lot for bearing with me here


dave
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Old 09-12-2004, 09:29 PM
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Maybe they aren't opening. Try resinstalling the secondary diaphragm.

Then try the lightest spring. You should feel a bog with it. A bog feels like you let your foot off the gas for a second.
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Old 09-13-2004, 05:48 PM
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I reinstalled the diaphragm and put the lightest spring in. No bog!!!

Why aren't these things opening?

I checked the vacuum passage and it isn't blocked (in the housing or the carb body).

the cork gasket is in place.

The diaphragm passes the leak test.

I checked that the secondary throttle plates were able to move freely.

The length of the operating rod on the diaphragm is the same as the one that came with the carb.

Am I test driving this thing wrong? I push the petal to the floor when I have enough speed to prevent wheel spin and then let her rip up to 6000.

Any more suggestions? What else could keep those puppies from opening? The only thing I can think of is that my 383 isn't moving enough air to get em going. How can that be though? My setup is common with this carb.


dave
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Old 09-13-2004, 06:53 PM
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Your engine is flowing enough air to open the secondaries.

Maybe they are opening. It is a little a little hard to tell you if they are opening or not without riding in the car.

One option is to take the hood off and watch to see if they open.

You could disconnect the secondary linkage arm so they definetely don't open and see if that makes a difference.
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