Jeff20B

iTrader: (13)

That title looks really weird.

I have some carb questions. Nikki and Hitachi. They can share jets and probably a few other things. I tried swapping jets in my Hitachi and the results sucked. I'm sure you all can figure out why; with carbs, if some is good, then more is better, right?.

I pulled the carb off the REPU and did a jet swap on it. I upped the sizes in too great a step and now all it will do is idle after a bit of frantic fuel and air mixture screw (yes, there are two screws) adjustments . When I open the throttle or even blip it, the engine stumbles badly and then slowly goes back up to idle. I'm going to have to tone down the jets and try again.

I can only assume the jet sizes in the Hitachi are stock. The Hitachi carb itself looks like number 77 at the lower left.



Has anyone ever come across a carb that looked like it? The only other one that comes close in 78 in the upper left. I assume the numbers refer to the year of manufacture.

Anyway the engine sounded very sweet when it was briefly running. It sounded like a bridgeported FC with a rotary engineering glasspack, giving it an awesome sounding brap brap brap brap. The only problem is that it did not behave like a bridgey because the brapping would continue when I'd get on it. It would then start to bog the further I depressed the go pedal. The only thing that seemed to work, if only for a second, was to mash the pedal quickly to get a good pump shot from the accelerator pump. Otherwise, it was brap city.

The tailpipe became blackened with carbon from a pig-rich condition. Now see why.

Stock Hitachi jets: Primary 94; Secondary 145
Stock FB Nikki jets: Primary 92; Secondary 160
New jet arrangement: Primary 160; Secondary 190

Yeah, a pretty big step. I have access to some 155 and 83 jets. Should I get a set of 100 or 105 or 110 jets for the primaries and use the 155 or 160 jets for the secondaries?

I don't have access to air bleeds, so that option is not currently unavailable.

hornbm

Did any of the 4 port 13b's come with nikki's or were they all hitachi's? Ive got a buddy with a rx4 manifold. He also has a nikki that doesnt look like any one that came off of any year rx-7. Did the cosmo come with a nikki perhaps? I know the rx4 came with a hitachi and im not sure about the repu

Jeff20B

iTrader: (13)

I have an old '74 semi rusty Hitachi sitting around and a working '76 Hitachi on my '76 Cosmo. The tops look similar in that they both have cast fuel passageways (like in the pic above). The later Hitachis went to steel tubes like the 1st gen Nikkis have. It guess it flows better or something.

All four port 13Bs came with Hitachi carbs. I've seen a couple Nikkis on 13Bs, but they were always custom jobs.

So, with your knowledge about jetting your Weber, what jet sizes do you think I should try in my Hitachi?

By the way, I installed the rejetted Hitachi on my 13B powered MG Midget. It's the one that sounded like a bridgeported FC. The REPU I mentioned above is the one with a rebuilt engine in it. I didn't want to risk screwing up the break-in process, so I threw the carb on the old 13B in the MG that will eventually need a rebuild (it blew some blue smoke today, hehe).

hornbm

With the webers being two barrel carbs my jetting experience isnt gonna proove much use, BUT,

I am running 180 main jets, 160 air correctors, f11 emullsion tubes, 60F9 idle jets. And that is all on a stock port S4 6 port 13b. Im gonna try playing with the air correctors a little bit and see what that does.

I used to use 65F9 idle jets and my god I gained so much power by going to the 60F9s. The thing runs so much smoother and now im getting 17 to the gallon!!!

I wish I knew more about conventional carbs.

Jeff20B

iTrader: (13)

Thanks for the info. It may help some Weber users here. I too wish I knew more about carbs, conventional or otherwise.

It's occuring to me that I should just try upping the jet sizes in small steps. Maybe 100 or 105 primary and 150 or 155 secondary.

numan2

I'm currently running one of my old hitachi 13b carbs on my 12a and here's a little info about them....they are way more touchy about changes in jet sizes than the nikki III think going from 94 to 160 is waaay too big a jump for it. I would start with something closer to a 100 if your trying to fatten the primary up some or even the 145 from the secondary side would be better than the 160. And they did put the hitachi on a 12A for 1 or 2 years right around 1977 - 1978 I have seen one of those manifold/carb setups from the factory many years ago in a bone yard.......I shoulda got it...sigh.

Rx7carl

Quite a collection you got there. 160 is wayyyyyyy too big on the primaries, just ask Carlos Have you pulled the emulsion tubes to check the design? If they are are undrilled, open ended tubes, see if the Nikki tubes will fit.


EDIT: Haha, numan2 beat me to it. I need to type faster.

Siraniko

iTrader: (13)

I say JUNK the HITACHI and use a NIKKI carb on a 13-B manifold. Yet, junk them all and get a 48 IDA.

Jeff20B

iTrader: (13)

I wish all those carbs in the pic were mine.

Thanks for the responses, guys. Yeah, this Hitachi paints my spark plugs white. I need something to fatten it up, but I don't know what jets to use. I'm sure after yesterday's little outing that my plugs are now nice and black.

Ok, I'm going to try the 160 jets in the secondaries and probably the 145s in the primaries. I might possibly put the 94s back into the primaries just to get a baseline performance seat-of-the-pants rating first though since it's been a year since I last drove this car. If only I had some 100s kicking around or even 105s. I've looked inside three Nikkis so far and they're all basically the same jetwise.

Carl, one of the Nikkis I looked into is a '73 that has the hole at the very bottom of the air bleed/emulsion tube, while the '83 type Nikki has the cross drilled holes near the bottom. I'll go ahead and look at the tubes in the Hitachi the next time the top is off and see what they look like.

Ok, so the options I have right now are to change the secondaries to 155 or 160 (I only have one 150 jet, arg!). Change the primaries back to 94 or 145. What numan2 said about Hitachis is news to me, so maybe I'll just go back to 94 for now and only adjust the secondaries.

I'm going to go look for some 100s at least. If I can find some 105s or even 110s, I'll try them.

lovintha7

I'd try to go with around 100's on the primaries and 165's on the secondaries.

Just my 2 cents.

Jeff20B

iTrader: (13)

I didn't find any 100s today at the old local rotary shop, or any other jets for that matter. I think I'll either buy some new 100s from Mazdatrix or go back to the 94s and increase the fuel pressure (it's not quite up to the middle of the site glass). I'll also try either the 155s or 160s in the secondaries and see how they do. I've got a spare set of 160s in another old Nikki (I think; it's never been apart) which can go into my other Hitachi, should I need them.

So should I be conservative this time, guys? The Hitachi is more sensitive to jet changes than a Nikki. 155 vs 160 in the secondaries.

BMS2004

if you know anyone with "holley airbleeds" they will fit in there i think the entire kit is like $200 or so. Maybe someone you know has a set around, actually I might have some spares of the sizes, i'll look tommorow. Carl Mazda does jet sizes in mm and holley in Inches right?

EDIT: FYI the holley airbleed you would use as a fuel jet would be a 40 and don't feel bad when i was trying to find jets for my carb I put the equivelent of 170 in primaries and 182 in the secondaries.... it didn't run for ****, matter of fact i don't think it even revved passed 5k b/c it was so rich.

karism

Jeff:
Are you running conventional venturi sizes?
When my Nikki was stock,the sizes were :0.78" (20mm) Primary and 1.102"(28mm) for secondary.

Please refer to the attached picture:

If i look at the Hitachi used on the older 13B 4 port motors,it looks like the primary venturi is larger than the 12A Nikki.

I`am guessing a 110 mm primary fuel would be in the correct region?

cdrad51

Man you got unlucky, so sad it didn't work out in the end with the 160/190 combo. However, while it's definitely a good idea to downsize the fuel jets, be aware that small changes can create big differences. When I did the 160/190s combos in my nikki, with a 0.986 primari venturi, and it didn't work, I went down to 147s and 160s, and then started to up the sizes little by little. I ended up with 150s and 180s, which is only 2 sizes down from 160s/190s. At 160s/190s, the engine would behave the way you describe it. at 150s/180s, the engine feels very very powerful, more so than with any other smaller combo.

BMS2004

https://www.rx7club.com/1st-generation-specific-1979-1985-18/bigger-jets-nikki-231456/

sterling enlightens us on stock nikki jetting. some help for the hitachi I hope.

cdrad51

Oh yeah, that thread. Brings back memories. Good times.

Sterling

Is that the one where I made you my bitch?
Yer a good bitch, Carlos.
When are you coming out to NY?

cdrad51

No, that's the one where you tried

Don't have any plans on that, though I'd love to visit NY and check out your ride.

Jeff20B

iTrader: (13)

karism, 110 sounds like a good size to use. Before I buy any, cdrad51 has a point; since I have these 145s sitting here, it wouldn't hurt to try them. If the 145s brap like there's no tomorrow (like the 160s did), I'll have to get some 110s. Does that sound like a plan?

Sterling or anyone else, if a '79 to '80 Nikki has 83 jets in the primaries, what is the usual size of the secondaries? Thanks.

karism

Jeff20B :That spounds like a good idea to me!Try the 145`s and see how it goes.

Well, i had good results with 110`s.Still very driveable,but with alott of midrange punch.Please not that my primary venturies were bigger than stock

Sterling

Taking jetting advice from Carlos is like getting brain surgery instructions over the phone.
He might as well have drilled a hole through his carburetor to let the fuel dribble into the manifold.
He gets six mpg, ya know.

Carlos drive out here cause you wanna drive out here, not to see my ride. It's nothing special, I promise.

cdrad51

Hey! I get 12, not 6!

Well I'm sure you're ride is nothing special. I just wanted to see something I can use to compare to my own, and what better than probably the most tuned nikki in the US? Around here, everybody has gone dellorto, mikuni, weber, or is totally stock, which is not a good comparison.

cdrad51

Hmm, drill a whole thru the carb... not a bad idea!

I just realized though that your car is stockport, so that wouldn't work for comparison either.

Rx7carl

Is that all you two do is bicker? If I didnt know better I'd think you were an old married couple.

Jeff20B

iTrader: (13)

And if I were a newbie, I'd ask Carl to delete all the useless banter in my thread. I am trying to learn something here.

Sterling, do you even know the secondary jet sizes? Or have I been giving you too much credit lately? C'mon, I know you've got the info in you. Spill it!