Valve that will open at a specific vaccum level - does such an item exist?

[Corellian Corvette]

So - I'm looking for something that I'm not sure exists. And if it does exist, I don't know what it's called. 

Here's what I'm looking for - I would like to find some sort of small in-line check valve, or regulator, that will only open at a specific vaccum signal or higher.

Here's my situation. I have a VERY nice running 470. It was dyno tuned, jetted, etc. with the carburator on the car. Engine has GREAT power and cruise, but I have been trying to chase down the fact that the idle quality is bad. I was able to diagnose the problem as two issues, and one of them was solved by reducing the timing (I had too much initial advance on the engine, got that solved with springs and bushing in the MSD)

The other problem is frustrating. The carb I am using has a faulty PORTED advance port, and they machined the slot too low in the venturi so that it acts like manifold vaccum - the ported slot is always getting a vacuum signal. This means it's impossible for me to run the vacuum advance canister beacuse the low and sligtly erratic vaccum signal at idle is made worse when the timing surges with the vaccum canister connected. It's fine when I disconnect the canister, but I drive the car enough at speed that I would like the vac advance to eek what meager mileage I can.

So, what would be awesome is if there was some sort of in-line valve that only opened up when the vaccum reached, say 15hg - 17hg - effectively re-creating a ported signal. My engine doesn't produce that much at idle, but it does at cruise so, it would allow me to run the vaccum advance canister.

Does anything like this exist?

[BrianShaughnessy]

Adjust the vacuum advance tip-in with an 3/32 allen wrench thru the vacuum nipple by turning it counterclockwise.

This moparts thread seems to contain enough info to get you going.
http://board.moparts.org/ubbthreads/showflat.php?Cat=0&Board=QuestionAnswer&Number=5436564&Searchpage=10&Main=5436026&Words=vacuum+%2Badvance&topic=&Search=true#Post5436564

[John_Kunkel]

Are you sure the ported slot is too low or is it just a case of the butterflies being open too far at idle and exposing the port?

Bumpity-bump cams often require that the butterflies be open more than normal at idle, this can be rectified by drilling small holes in each primary or opening the secondaries to let in more air at idle which allows closing the primaries.

[Corellian Corvette]

Great question and I've confirmed it's not the butterflies. The cam isn't *that* agressive (certainly not a "bumpy" cam), but enough that it doesn't like to idle below 700 RPM. I mean it WILL idle, just rough.

I've confirmed it's a machining problem it a couple ways - first you can back the idle screw almost all the way out, to the point where the car wants to die (like well below 600 rpm) and you still get a vacuum signal.

Second, I confirmed it visually. You can see the slit for the ported vaccum signal is really low in the bore.

Third - I put another carb on (lower CFM), jetted it, and ran it, and the car runs and ported vacuum works correctly (e.g. no signal at idle).

So there is def. something wrong with my carb. Problem is I don't want to buy another one. I was hoping there is just some sort of check valve that would only open up at 15hg. That would solve the problem...

[maxwellwedge]

What kind of carb are you running? I was going to post the same answer as John_Kunkel - I still think there is validity in his answer.

[Corellian Corvette]

It's an edelbrock thunder series AVS 800cfm.

I thought exactly the same problem re: butterflies but that's not it. If you looked at the carb venturi from the bottom, you can see that the transfer slot is REALLY low - like it's exposed even with the most minute amount of blade opening - it's much lower then other Edelbrocks I have.

All other carb settings are perfect - idle screws 1.5 turns out, float is correct with no drips... i'm fairly certain it's a manufacturing defect with the carb.

[firefighter3931]

If the transfer slot has been improperly machined into the main body your only real option is to drill some small holes in the primary butterflies. This will allow you to close down the idle speed adjustment and make the idle circuit functional again.  

How much vacuum are you making at idle ? What cam are you running ? Where is the timing currently set ?  

FWIW ; vac advance cannisters are designed for stockish engines that produce 15in of idle vacuum. They don't play well with hotter cams. I allways delete the vac advance on my engines....none of those ever run anything remotely close to a stock cam. 



Ron

[maxwellwedge]

It's an edelbrock thunder series AVS 800cfm.

I thought exactly the same problem re: butterflies but that's not it. If you looked at the carb venturi from the bottom, you can see that the transfer slot is REALLY low - like it's exposed even with the most minute amount of blade opening - it's much lower then other Edelbrocks I have.

All other carb settings are perfect - idle screws 1.5 turns out, float is correct with no drips... i'm fairly certain it's a manufacturing defect with the carb.

Is the idle transfer slot also the source for ported vacuum on those carbs? The carb having a defect is possible. I'm going to do a little research but if all else fails you may have to go the route that firefighter3931 suggests - drilling the throttle plates or a different carb.

[Corellian Corvette]

Timing is ~ 20deg at idle, 36 all-in mechanical + centrifigual at just under 3000 rpms. I'm using an MSD ready-to-run. Vacuum at idle (~ 800 rpm) is a shaky 10-12hg. After about 1200 rpm it's a rock-steady 20.

This was all done on the Dyno before the engine was installed in the car. Max HP was made at 38deg advance, but my engine builder suggested that I might want to be a tad more conservative on the street.

Cam specs I don't know. It was a custom grind hydraulic roller-lifter, with comp-cams roller lifter conversion.

My worry about messing with anything too much is that this was all dialed in on a Dyno. Rookie mistake that I never really looked at the idle situation (wish I would have). Buying a new carb is $$ I wish I didn't have to spend, which is why I was hoping there was some little check valve that would fix the ported problem. If I could restrict that valve to only work at about 15hg, I could keep all the settings the same, but still have my vac for cruise.

[Corellian Corvette]

Sorry for the bump. So I take it there is no sort of valve made like I'm thinking of...

[firefighter3931]

Timing is ~ 20deg at idle, 36 all-in mechanical + centrifigual at just under 3000 rpms. I'm using an MSD ready-to-run. Vacuum at idle (~ 800 rpm) is a shaky 10-12hg. After about 1200 rpm it's a rock-steady 20.

This was all done on the Dyno before the engine was installed in the car. Max HP was made at 38deg advance, but my engine builder suggested that I might want to be a tad more conservative on the street.

Cam specs I don't know. It was a custom grind hydraulic roller-lifter, with comp-cams roller lifter conversion.

My worry about messing with anything too much is that this was all dialed in on a Dyno. Rookie mistake that I never really looked at the idle situation (wish I would have). Buying a new carb is $$ I wish I didn't have to spend, which is why I was hoping there was some little check valve that would fix the ported problem. If I could restrict that valve to only work at about 15hg, I could keep all the settings the same, but still have my vac for cruise.

Try bumping up the timing to 25* at idle and see if that helps. If the idle cleans up then you would just need to shorten the mechanical advance to achieve the desired total timing. 



Ron