Jeff20B

iTrader: (13)

Decided to try a different approach with this huge carb. I think it would do well on the reverse runner manifold. Knowing what kind if carb setup and engine the reverse runner originally came with stock, it seems clear that the carb needs to have big primary ventris and stock secondary venturis. The air bleeds need to be small and the fuel jets need to be a little small on primaries and really small on secondaries.

Now let's compare what I have in mind with what originally came stock.

Stock 76 Hitachi:
22mm primary venturis
100 primary jets
60 primary air bleeds
130 secondary jets
90 secondary air bleeds

Some parts won't translate over perfectly due to the differences between a Nikki and a Hiatchi and channeled vs separate runner manifolds etc, but I think I got them pretty close, or close enough to work.

Hogged boost prepped Nikki:
25.6mm primary venturis
130 primary jets (Holley .051" air bleeds)
70 primary air bleeds
150 secondary jets
118 secondary air bleeds

Of course I know I could have gone with 60 and 90 air bleeds like stock, but like I said, some parts will have to be different due to the basic fundamental differences between Nikki and Hitachi carbs, and the fact Nikkis like a channeled manifold, which is what I did to the reverse runner along with drilling and tapping the Nikki stud pattern. Then of course the obvious differences in venturi sizes and slow air bleeds which I won't bore you with here.

The goal is to hopefully eliminate the off idle lean spot on a setup that doesn't have an anoying brake booster leak. Thus less air will be coming in all the time. So it will richen things up a little which can be tuned around with the AF mixture screw. Thus going with 70 primary air bleeds makes more sense than 60s. Just not sure whether the long primary runners kind of warrant a slightly richer primary tune. Or maybe they will mask it better due to the enhanced low end torque that long runners provide. Or will they wet out more? Don't know. But it's ideas like this that kept me going when I didn't know what I was doing.

Oh and the longer runners flow into the end plates which are arguably a much better more efficient size and shape for side ports compared with the narrow intermediate port runners. Just ask peejay. So maybe the huge venturis will be a home run for me on this manifold?

The only drawback are the secondaries which flow into the intermdiate plate. But like I said, kinda small air bleeds and jets are what you gotta use, and they're what I have on hand. Might as well test them, you know? Really short runners too, which might enhance high RPM? Maybe make up for the less efficient shape?

Note all of the above tuning info is for a boost tune on the Nikki and an NA tune on the original Hitachi. Don't want to confuse anyone worse than I already have.

Jeff20B

iTrader: (13)

So then which carb will go in the big one's place? The 25.4mm carb. The "medium" one.

I think this one is small enough to have a less pronounced lean spot right above idle but still big enough to give me the kick in the pants fun I've come to expect by now.

I swapped some jets and air bleeds around today that should be appropriate for a normal runner manifold. This is just my best guess.

Hogged boost prepped Nikki:
25.4mm primary venturis
130 primary jets (factory)
70 primary air bleeds
155 secondary jets
118 secondary air bleeds


My thought process here is if I use the same 130 primary jets as the other carb and the same 70 air bleeds, the minor flat spot will be less pronounced while possibly upping the AF mixture at part throttle cruise by a small amount.

But wait, there's more! The 130 fuel jets are actually the factory secondaries from the 76 Hitachi mentioned above! I've discovered factory jets tend to be a touch smaller than the actual size in metric, so they are probably closer to about a 127 or 128 or so. Compare this with the Holley 51 air bleeds I'm using as fuel jets mentioned above. These are a lot closer to actual metric 1.30 or 1.29539999mm according to an internet calculator. So they are just a touch bigger and should do better on the long runner manifold with bigger venturis.

Yes I know I'm talking about a difference in size of a couple hundreths of a millimeter here. Probably nothing to worry about, but since I have access to them, it's kind of fun to OCD about them a little. And I know if I need a tad richer I can swap to the Holley 51s and/or the 60 air bleeds.

As for the secondary fuel jets at 155, I discovered last year that a set of 150s in the 24.9mm carb did well enough, if just a tad rich after filling the slow air bleeds with solder and dropping the stock 140 main air bleeds down to my drilled 118s, so I know going to 155 is going to be too rich, however the new larger primary venturis are bigger and allow additional air in, that maybe a setr of 155s isn't a bad idea right now. I also only have one 150 left. The other was lost ages ago. Again I'm OCDing over the differences between .05 of a millimeter. Oh well, it's better to start off too rich and go leaner than the other way around.

Jeff20B

iTrader: (13)

But then what if I needed to do an NA tune on one of these hogged out Nikkis?

It just so happens that I do need a temporary NA tuned Nikki for an upcoming project, before it gets a turbo. Looks like I'm in luck.

This is the "small" carb at only 25.15mm. Because it's smaller than the others, I can get away with smaller fuel jets. But because it's getting an NA tune, the fuel jets will still need to be big. Around 10 numbers bigger for NA is a general rule. Air bleeds may also need some attention.

I also need to mention I only have limited NA tuning experience, and that was with 24.7mm venturis for the most part. This carb's slightly larger venturis will need some careful jet and probably air bleed selection.

If I was going for a turbo tune, I'd probably look for a set of 125 fuel jets and maybe have the 60 air bleeds handy to keep things rich enough if necessary. However the drilled 118s did quite well in the 24.9mm carb, even with 90 air bleeds, and seeing as these carbs are similar in size, as long as I have the 60 air bleeds handy, maybe I can use the 118s again.

With that out of the way, my first impression for an NA tune is to use a set a Holley 51 air bleeds as primary fuel jets because the factory 130s seemed to work ok in the 24.9mm carb for NA use, along with 90 air bleeds. I'm hoping the 60 air bleeds will keep the AF mixture from going too lean.

Man I sure am relying a lot on the 60 air bleeds. Let's hope I don't need them anywhere else.

To keep things along similar lines to the above tuning parts lists, here is my best NA tune guesses:

Hogged boost prepped Nikki, tuned for NA:
25.15mm primary venturis
130 primary jets (Holley .051" air bleeds)
60 primary air bleeds
160 secondary jets
140 secondary air bleeds

Notice the secondaries are stock factory tune. I've found the factory tune to be fine for NA use.