The channel who connects the two primairies:




Why does it have such a horrible aerodynamic shape?
Why are the primairies connected?
What happens if i block the channel?
What happens if i make the channel more aerodynamic? Or make a big open spacer?


Just try to understand things .......

 

It is to share a vacuum signal so the primaries work more efficeintly. Notice the imprint from the other channels in the manifold? Same idea. Air doesn't actually flow through there though the reversion pulses will "pulse" there. Does that make sense? I wonder if anyone else will add their take or understanding of this subject? Anyone?

Bart, I must ask why are you interested in this subject? Are you trying to gain more power? If so I'd recommend doing some Nikki mods and leaving the manifold and spacer alone. There are lots of posts about how to increase air flow through the Nikki. Less posts for modding the manifold and like none for modding the spacer, other than to trim off the heat shield (not needed if you're doing the cool manifold mod) and which nipples do what (like vacuum advance etc).

 

I am curious how thing works.

It seems to me that both primairies are feeding one rotor.


I have stock port, free flow exhaust, and free flow air filter, mecanical secondaries.
Looking for a bit more power on the primairies.

 

That's what I figured. The stock Nikki has pathetic primaries. The only way to make more power is to increase air flow through them. The only way to do that is to mod the boosters (they come out if you pull straight up carefully with a little wiggle). Then enlarge the venturis from their stock 20mm to Sterling's size of 22mm, or Yaw's size of 24mm or the old school racing mod of 26mm. This last size is extremely large and can cause some glitches (flat spots) in the part throttle drievability but it's easy to drive around them. However the tradeoff in power gains are worth the risk. I have one of these rare modded carbs, taken off a GT1 race car that was internally water damaged. This carb has excellent low end power thanks to the oversized primary venturis. I then did the Sterling accel pump mod and a variation of his mechanical secondaries using wire ties. This carb can compete with a Racing Beat Holley 600 tested on the same engine (74 spec ported R5 13B with full RB long primary "SP" exhaust).

Some will recommend to remove the venturis from the carb for machining. That's risky. Others will say to just hog them out while installed. They will also suggest to machine the throttle shaft down. This carries risks too as bending/twisting is far more likely (I haven't machined mine and don't plan to). One mod that isn't risky is to chop off the venturi booster support. Then reshape the other side into an aerofoil. There are pictures somewhere. I'm sure someone knows where they are.

 

I am aware of the mods.
I do have a set of enlarged venturies, waiting to be build in.

But i am curious about the channel.
Is it tried to open up, or close the channel? Does changing this making things worse?

 

Oh, ok. Glad you're modding the carb. If done correctly you will really like the gains.

Not sure how opening up/closing the channel will affect things. Could end up hurting performance or helping. Maybe someone else with experience will chime in?

 

How does a Holley carb/intake look like? Are the intake of the two rotors joined?

I think that the nikki was not intended to have the primairies joined. If it was intended, they would have made a proper channel.

One benefit from joined primairies is, a more stable vacuum source for the vacuum advance of the ignition.

Once I was playing with a different carb / inlet. I had the vacuum advance hooked up on the intake of 1 rotor. (separate of the intake of the other rotor) I found that the vacuum advance actuator was rattling. Caused by the vacuum pulses of one rotor.

 

The holley manifold is separated. The vac advance ideally goes to the 2nd nipple from the front.
That said, I'm sure the engineer in charge put the channel there for a well researched reason.

 

There is a cut out on the bottom of the throttles to. This is the outlet of the idle circuit.
There is one idle circuit feeding both rotors.

So that's way there is a channel connecting both primairies.
I guess the small cut out on the bottom of the throttle wasn't effective, so nikki made a second channel in the spacer plate????

It seems to me that (beside of idle) joining both rotors isn't the best way to get most out of the carb?


By joining both rotors, when one rotor is sucking mixture in, it is also sucking on the second rotor intake. Is that second rotor blocking the intake port? Does it matter?

Some brain waves:
Make a propper aerodynamic channel to join both primairies. Close the channel which joins the primairy and secondary in the manifold. That way the secondary is exclusively feeding 1 rotor on the outher housing port. And the inside housing port is fed by both primairies.
Just some thoughts......


A holley, does that have two idle circuits?

 

I like this idea...one more thing I'll be doing to mine..

 

Joining the 2 primairies will increase flow.
So i don't think enlarging the venturies and modifying the boosters will bring more. In fact, i think it's better to leave the venturies and boosters alone.

Apart from cutting of the booster arm, and shaping the remaining booster arm to reduce turbulance. Reducing turbulance is a good thing. Rounding of sharp edges, as shown on the youtube link above is good.

But when you leave the intake Original, then i open up the venturies makes sense. In fact, that the second plan if my little adventure faills....


I have ordered some lasercut aluminium. 2 sections to close the channel between the primairy and secondary, And a spacer with a propper channel between the primairies.
This spacer have to be milled at an angle, to compensate the angle of the inlet manifold. I have my engine mounted level, and the angled spacer will make the carb sit level.

 

probably there just to equalize/?? pressure between the two venturis. it probably doesn't flow much, as it doesn't need too.

some of the old british sports cars use two carbs with a balance tube, same principal, different packaging
Triumph Net Nuggets - cross over balance pipe

try it, it'll do something!

 

Having run an engine recently without the spacer and I can tell you that it basically made the engine have higher and steadier vacuum at idle and part throttle. It was really weird noticing my brakes didnt grab as hard as easily with an SA mani+no spacer. otherwise, it ran great.

 

Mildly confused... So it had higher + steadier idle with channeled spacer OR without it? Is the SA mentioned the same car originally mentioned? Which had better brake feel + vacuum?

IIRC, I once read somewhere that PP'd engines prefer Weber's due to the completely divorced (separate) runners...

 

this is true, but its because the exhaust ports are open when the intake ports are open, so the separate intake runners keep exhaust gasses from sharing between rotors, and other craziness.

not really an issue on a stock port

 

It was the same peak vac readings with or without, but it remained steadier at idle and didnt drop as much on part throttle.