Need someone to do a quick carb test for me....
 

I have managed to get my hands on an old school Rx3 intake manifold. It's really awesome, and I can't wait to try it out with my reworked Nikki carburetor. However, I'm having difficulties finding a ported vacuum signal for the distributor. I sent my carburetor to Sterling to be modified, so I don't have my carb available right now for testing.

I won't be able to use the typical black carb spacer with my Rx-3 manifold. My air filter won't fit under the hood if I do. It's pretty tall. So as a result the black carb spacer is a goner. Plus, the black spacer's tiny holes would be a bottleneck to my air flow if I use it.

So I need to find a nipple somewhere on the carb that offers me a ported vacuum signal, just like the farthest forward nipple on the black spacer does. I know some of you just use manifold vacuum for your ignition advance, and that option is available to me, but I really believe firmly in using a ported vacuum signal for the dizzy. So I'm trying to find a nipple on the Nikki carb that has ported vaccuum. I think I might have figured it out already, but I need someone with an actual carb who can verify it for me. I can't test my theory since Sterling has my throttle body right now. See picture below:

https://img15.imageshack.us/img15/81...foldvacpor.jpg

1) Does the nipple in the picture REALLY offer ported vaccuum?

2) If I seal off all the holes and ports on the bottom of the throttle body, does it STILL have ported vacuum? Basically, can that nipple stand alone and offer me a ported vacuum signal even when the black carb spacer isn't underneath it?

I'm not sure how all the ports and passageways on the bottom of the carb, the black spacer, and the top of the intake manifold are intertwined. So if you could do the following test for me I would be GREATLY indebted to you! The following to check should answer the question:

1) Pull the carb off
2) Cover up all the small holes on the bottom of the throttle body. Maybe with masking tape? Doing this will simulate the top of my Rx-3 manifold, since it has no ports on it.
3) Remove the vacuum cap from the "ported" nipple in the picture.
4) Blow some compressed air into that nipple, or carb cleaner or something, and see if it comes out of the slits that are inside of the throttle bores, next to the butterflies. If it does, then that is indeed ported vacuum. If not, then something went wrong.

If I have confused the crap out of you guys my apologies. Please post with any questions.

Thanks in advance,
Jamie
Glazedham42

I'm the wrong person to respond to this thread because I have never used vacuum advance on a 12A. I won't be using it on the old school RX-2 manifold (separate runner) that's going on the half BP either.

Now that that's out of the way, one thing you can do is hook up a vacuum/boost gauge to the nipples of a spare carb and go for a drive. One nipple at a time. Find the one that pulls vacuum only during primary circuit use. Do not use one that pulls at idle. Unfortunately you may find that the nipple you're looking for does not exist on the outside of the carb itself and your only choice would be to find which small hole in the carb's TB/thick black spacer plate works (ask kentetsu I think) and mod the manifold to work. I'm suggesting to drill a small vertical hole downward a little ways into the manifold, also drill a horizontal hole to join like the factory does and tap in a nipple pulled from a parts carb say like an OMP nipple as those are long and presumably easy to work with. This sounds to me like something wackyricer would do, well maybe not on a Nikki since he only believes in 2 barrels (puke). Heck I'd do what I'm suggesting if I was interested in using vacuum advance on the RX-2 manifold.

Hmm... they must have used vacuum advance on the points engines, otherwise why have the vacuum diaphragm things on the carb? Which nipple did they use? It's gotta be on the front of the carb. I'm talking about an old school carb. These had like maybe one hole in the cast iron TB, not the four or five of a modern Nikki...

i would do as jeff suggests, boost/vacuum gauge on teh nipple and go drive....

Unfortunatley, my car's not going to be running any time soon. It's in pieces all over my garage, and will be for many more months. I'm in the middle of a complete build up.

IIRC correctly, my modified nikki ran better with vaccum than without it. but that was years ago and my memory is really vague and probably I didnt exhale back in the days yada yada yada lollllllllllllllllllll

I'm telling you guys, if you're not taking advantage of VA then you are losing out on low end torque and better mpg.

I guess the stock setup is manifold vacuum, but it involves a thingamajig that makes sure it doesn't work at idle and was used for emission control rather than power creation. So with a rat's nest, it gets complicated. But I think rat's nest or no, you'll bet better performance with ported vacuum.

Just think of it like this; more advance means a torqie motor, but too much and you start knocking at the high rpms. You get good low end performance though. So this is like hiring a tiny little midget to ride under your hood and crank that dizzy back and forth for you.

"Just starting out from a red light? well please allow me to crank out a bit of power for you here".

But if no rat's nest, and you hook it up to manifold vacuum, that little dwarf starts acting like you mother in law. Just when you need the power, the little shit cranks the opposite direction and robs you of it.

Anyway, enough ranting, I need to get back to work. :)

If you will be using mechanical secondaries, and will never be switching back to vacuum secondaries, why don't you use the little nipple that supplies the vacuum secondary box? The one that is revealed when you pull off the box. That one is a ported vacuum source for sure.

I will definitely be using mechanical secondaries. However, when I removed the vacuum box there wasn't a nipple there. Just a bunch of holes. How do you know that hole is ported vacuum? Not doubting you, I'm just trying to understand how it works. :bigthumb:

Jamie

cause the secondaries open with air flow...

so why dont you just make 10 louder?

if you want more advance @low rpm and less at high rpm, it sounds like you want to change the MECHANICAL advance curve....

IMO you could change the mechanical advance curve, no knocking, no mother in law

and then you get the vacuum advance for part throttle and mog like its supposed too.

not trying to make a big deal, but the dizzy is 3d, you might as well use it

Right, as demand and air flow increases, the vacuum on the secondary diaphragm increases too. The secondaries wont open when the throttle blades are shut, when manifold vacuum is at it's highest.

I believe the second from the top is the hole you'll want. It looks like some carbs have a nipple, others have a hole.

It's the unthreaded hole. The nipple you speak of is probably a plastic tube thing. I say tap a metal nipple into the hole and Bob's your uncle.

^^^Yes! Good point

That sounds like venturi vacuum to me, not ported vacuum.... Am I wrong? I thought that venturi vacuum was a funtion of airflow.

Venturi Vacuum?

Easiest way to find your ported vacuum line is turn the carb upside down and have a look.

There should be a hole just above the throttle plates which provides ported vacuum (not all nikkis have it). So there is no vacuum signal at closed throttle, but as soon as the throttle plates open a little, the signal has high vacuum, which then decreases as the throttle plates open further. (There is a thread around somewhere about the pros and cons of ported vs. manifold vacuum for the dizzy vac advance)

Anyway the hole for ported vacuum will be just above the throttle plates, you'll have to push the throttle open by hand and shine a torch up there probably to see it. IMO just squirt some carby cleaner or whatever through the nipples outside the carb, and see which one comes out of the ported vacuum hole above the throttle plates.

Those are basically the same thing Jamie.

We are only dealing with two types of vacuum; manifold & ported (venturi).

I don't believe that they are. I think all 3 are different.

Manifold vacuum
Ported vacuum
Venturi vacuum

Check out this article. It changed my mind, and clearly explains the difference between the 3. Yeah, it's written by a Corvette guy, but the principles are the same.

http://www.lbfun.com/warehouse/tech_..._explained.pdf

I traced back the port that operates the vacuum secondaries. It is definitely running off of venturi vacuum, not ported vacuum. It is connected to two small holes in the bores of the actual venturis, one in the primary venturi and one in the secondary venturi. Ported vacuum is measured just above the butterflies in throttle body. The pressure relationships are very different there than they are in the venturis.

Very interesting. I stand corrected. :)

I'm learning as I go here, but I have to say that article above really straightened out a lot for me. Makes sense now why you would have 3 different types of vacuum and what they are all good/bad for.

Can someone please test their carb and see where the "PORTED" nipple below terminates at??? I don't have my carb available to test.

Thanks!
Jamie

https://img15.imageshack.us/img15/81...foldvacpor.jpg

I would but I don't have access to a modern Nikki at the moment. Maybe later today when I can get out there to finish up the side plate prepping.

Okay, if you get a chance that would be great. I'm not positive that all the modern Nikki carbs have those two nipples. Or if it is just an 84-85 thing? Either way, mine has them.

If you can blow some compressed air, or carb cleaner into that "ported" nipple I'm thinking it will exit inside the throttle body. If you look very carefully inside the throttle body there are some small slits about 1/4" long, and probably 1 centimeter wide. They should be right next to the butterflies, inside the throttle body bores. That is where I am hoping that nipple terminates.

I'm bringing this thread back from the dead so we can make some conclusions. I managed to get an old parts carburetor so I could trace down all of the passages inside of it. I also took my black phenolic spacer and I ripped the gaskets off of both sides of it so that I could trace down all those annoying little passages inside. I traced down the front nipple on the black phenolic spacer (the one that Kentetsu likes to use) and I don't believe that it is really ported vacuum like we thought. It DEFINITELY terminates inside the secondaries throttle bores, just below the throttle plates. I believe that would mean that nipple is under manifold vacuum? Can someone confirm for me? I have attached a picture of the two holes that terminate inside the secondaries so you guys can see them. One interesting thing that I did figure out is that the famous Kentetsu nipple is also available on the base of the carb, not just the phenolic spacer. The front nipple on the passenger side of the throttle body should be exactly the same as the front one on the black phenolic spacer. I'm not going to be using the phenolic spacer because of my Rx-3 manifold, but I still would like to try the vacuum source that Kentetsu uses just to see how it works. Now that I know which carb nipple offers that I can still try it and see what I think.

I also checked those two nipples that are on the driver's side of the throttle body. See my very first post in this thread. That picture shows the two nipples. They both terminate right inside the primary throttle bores. The nipple marked "ported" terminates just a tiny little bit higher in the throttle bore than the one marked "manifold". Are those truly "ported" and "manifold" vacuum sources like I have them labled?

Oneiros, you said that the ported vacuum source will be just above the throttle plates. Is one of these the hole you were talking about?

https://www.rx7club.com/attachment.p...1&d=1310436343

https://www.rx7club.com/attachment.p...1&d=1310436343

if the hole is UNDER the throttle plates its manifold vacuum.

if the hole is above the throttle plates, i guess that would be ported, it wont see any signal until the edge of the throttle plate gets near it.

In theory, I understand the idea of having the "ported" hole above the primary throttle plates, and that way it doesn't see vacuum until they open up. But in reality, I just don't see how putting the hole above the throttle plates really is any going to be any different. The thing that has me confused is that, even at idle, aren't the primary throttle plates open just a little bit? Essentially, that is what the idle speed adjustment screw does. It just adjusts how far open the primaries are at rest. When are the primaries ever fully shut? Won't they always be open, at least a little bit? Or am I understanding wrong?