If i may give my 2 cents :From what i have learned.Putting in a smaller airbleed,will richen up the mixture at high rpm.

With a streetport ; a little bit richer mixture is required,as there is less atomising(sp?) of the fuel.(Smoothed runners and ports)Allso ,with the overlap caused by the intake and exhaust ports being open at the same time,the intake charge is diluted more.(Which you knew allready!)

From the numbers you posted,it looks like the `77 carb is quite a bit richer than it would normally be.Is the carb one that was originally on a 13B 4 port engine?Seeing that their primary venturie size is a bit larger than the 80`s Nikki`s.

Karis

 

My testing showed that a dual plenum is preferable on a streetcar. I would also suggest that design for a SP engine. I think the 4 seperate runner design will definately choke a SP and not allow it to shine at hi rpm's which is where it operates best. As for the re-jetting, the carb doesnt know what engine its on. It simply meters fuel based on the air passing thru the venturi. Of course the stock carb is jetted a bit rich, especially on the top end, which like Karis stated can be controlled with airbleeds.

 

Yeah, all these carbs were on 4 port 13Bs as original equipment. The '74 carb does have larger secondary air bleeds with smaller main fuel jets than the '77. I guess it sort of works out in the end?

Maybe I should just stick with 100% stock secondary fuel jets then? What do you guys think?

Carl, is it ever a good idea to take a dremel or die grinder to a seperate runner manifold to add channels? I've got two, maybe three rebuildable '81-'85 Nikkis and two, maybe three seperate runner manifolds (I think one is a '76 but I haven't removed the carb from it yet to check if it's got a dual plenum).

 

I looked at my jets the other day in my hitachi it's 106 primary and it had 155's secondary I put 160's in the secondary and it worked much better. but then I hadn't used that carb in over 15 years since I got rid of my last RX3.

 

Thanks for the info. I thought RX-3s had 12As in them. Did yours get a 13B swap at some point?

Do you know what year your carb is? Do you also know the air bleed sizes? Thanks.

 

I have five '81-'85 Nikkis and I think only one has water damage.

It actually worked out rather well because the Nikki that was installed on the '73 engine that used to be in my MG Midget didn't have any throttle brackets (they installed some aluminum angle stock on one of the bellhousing bolts presumably to match the throttle cable length). The safety return spring was also popped off of its perch and a '79-'80 return spring was used, which appears to be maybe half as long as the '81-'85 type. No wonder the stock throttle cable was worn down to only one or two strands when I got the car. Also, the Nikki was a never-opened stocker with several vacuum tubes simply blocked by screws. The good news is that it's a low mileage carb.

I was able to steal the bracket off of the water damaged carb for the low mileage one. The linkage and brackets of the water damaged Nikki were a little bent, but after a little straightening, they should work great. Even most of the cadmium plating is still there. I think I can breath new life into this low mileage Nikki and then have three more to send to Sterling and/or Carl.

Hey, even though this carb sat since '90, it looks well preserved. I'm sure the AP and needle/seat rubber tips are no where near their former glory, but I think I'm going to try this carb on my 13B anyway as I don't have a running 12A to test it on. The 13B has a manifold that's been drilled and tapped for the Nikki stud pattern. Infact, when I got the manifold, it had a water damaged '79-'80 Nikki on it.

What are the clues I should look for the determine how much fuel pressure to use? There are two nipples at the top of the air horn which seem to comunicate with the altitude compensater. Can I remove the altitude compensation device and block off the holes? Should I use the dashpot I removed from the waterdamaged Nikki? It seems to function properly.

My future modding plans are mechanical secondaries, AP mod and choke simplification. This carb will eventually go back on the '73 12A untill I can get a Banana carb (the engine is ported, and unless Sterling carbs are now available for ported engines, the Banana carb is the one I've been told to get).

 

Hey everyone, how stupid of an idea is it to want to try installing one of my older, non Sterling/Carl carb candidates on a single rotor 13B project?

My reasons are because they're rotary carbs (so I'm at least starting with a carb calibration that ought to be close to the engine's needs vs a single or dual barrel dellorto/weber or whatever), I have three to choose from (well, two intact and one missing brackets etc), they're old and collecting dust, and I wouldn't want to try to modify them for power. No sense worrying about souping up these ancient Nikkis. In other words, if I want an excellent Nikki, I'll let Sterling or Carl mod it. As for these early Nikkis, I don't have much to lose and may even learn some valuable stuff while messing with them.

Ok, we know our 12As and 13Bs can run on one rotor. It's a drag, quite literally, and yet the carb works because air is still flowing through both primaries. I've experienced a condition where a grass seed somehow made its way past the fuel filter and clogged one of the primary jets in my Hitachi carb. I was still able to drive it home alternating between floored and not floored. Both secondaries still worked and I was able to drive it up a steep grade without too much difficulty. It simply felt a little weaker than usual when on the secondaries. I pulled the top of the carb off, blew some compressed air into the veturi and out popped a seed. I replaced the fuel filter with a better one and there've been no problems like that since. Anyway, the point is that air was still flowing through both primary barrels and the vacuum secondaries will open fine. So just what am I getting at?

Ok, at first I was thinking I could install a totally internally stock Nikki on a street ported single rotor 13B and have it work ok. Then I was like wait a minute. How can a single port sucking a single port's share of air through both primaries be anything like a stock ported 12A's two ports? Not to mention the larger (street ported) port's lower velocity having to pull against both idle and primary circuits. It just didn't seem like a very good idea. Or at least not as good as soldering up the primary and secondary fuel jets in one of the float bowls. I'm sure the secondary circuit would be ok regardless but the primary side has a channel between both butterflies to share the vacuum signal (I think). I'm not sure how this will affect fuel metering etc because I don't think I grasp the whole carb concept as well as I'd like to.

Since we've crossed over into the absurd, how about if I leave the primary port either stock or streetported and then do a half bridge on the secondary port? I guess I could only solder one of the primary jets and allow both secondary barrels to flow into the bridged port. Mechanical secondaries will probably be a must and I think the AP shot is higher in volume on the older carbs too. Of course I might as well mod it anyway going to mech secs.

Oh, and here's something that's a little off topic: I'll try the FC aluminum waterpump housing with FB aluminum waterpump (and cookie cutter impeller) mod, as seen on another thread recently, for weight savings.

 

Thinking about the AP, modding the diaphram section may not be necessary because I'll block the squirter on the soldered jet side of the carb. This means the other barrel will get the full pump shot.

Our carbs are made out of pot metal, right? It seems easy to mold with little machining required; quick production times. Matchbox and HotWheels cars are die-cast metal. Is there any similarity?

Edit: Ok, the AP volume should be sufficient. I think increasing the lever length is still required for the mech sec mod though, right?

 

Here is the FB Nikki that was on my '73 engine. I've since added a throttle bracket, simplified the choke, and removed some of the hoses which had screws in them. I also popped the emergency return spring back onto its perch. It looks like this carb has a '79-'80 return spring for some reason. Maybe I should find an '81-'85 return spring? Is the dashpot good to keep? Should I use some mechanic's wire or SS wire to lash the choke valve to the choke linkage? How about adding a small spring so it can have some movement? Should I keep the little choke vacuum diaphram in workng order? Should the altitude compensator be reinstalled?


Here is the water damaged Nikki that I got the slightly bent throttle bracket from. I also swapped secondary vacuum boxes because it was in better shape than the one I took off my carb above (somebody forgot to replace the screws after removing that emissions device and corrosion compromised the diaphram gasket).


Here is the water damaged '79-'80 carb. It looks like somebody installed the air horn gasket upside down.


Here is what looks like a '73 Nikki.


Here is what looks like a '74 or '75 Nikki. The chopped throttle cable is on the other side.

 

Here is a rather clean looking '73 Nikki that is missing some brackets. I wonder if I could swap the '79-'80 carb's brackets onto this carb?


Two Sterling or Banana carb candidates.


This one looks to be an '83 with a bad accelerator pump, according to the tag. Another Sterling carb candidate, I think; it's a junkyard carb so I have no idea of its history or if it's got any water damage. Am I supposed to open it up and check before sending it?


And here's the Hitachi. It looks a little different from what most of us here have seen. Very uncomplicated and they seem to work really well. Check out the mounting hole spacing on the throttle body.

 

Hello? Anybody home?

 

hello haven'treally followed much are you still planning on hitachi? why not just go with nikki since more people are moer familiar with them

 

one thing to keep in mind throuigh out all of this musical carbs is no matter what your go with your gonna wanna most importantly check to make cure your OMP is getting the correct signal from the throttle shaft, if ya follow me.they all look pretty close to the same to me. The only bracket I have on my nikki is just the throttle cable brackets I am however having trouble with my return spring, easy fix though. Not sure what else your askign but I left my vacuum secondary dashpot on when converted to mechanical secondaries. if you take it off you have to plug it somehow. I just figured if I wanted to go back to vac secs then it would be easier that way.

 

I'm going to try the Nikki on my 13B manifold with dual sets of stud holes. I have four 13B 4 port engines and only one has smallish 12A sized ports ('76 Cosmo). Two are ported and one has '74-'75 ports which were big from the factory. According to all the reading I've done recently as to the current limits of modded Nikkis, they can comfortably be run on SP 12As and stock ported 13Bs. I assume that means my '76 Cosmo engine. I'm still waiting for the Banana carb to be released.

 

I know a guy w/rx3's 13b 4 port, want me to ask him what he does? are you willing to go to webber or like to stay with nikki/hitachi?

 

Yeah, the OMP is one issue I haven't dealt with yet. I'm not sure if there is a difference in height or location between Hitachio and Nikki. I'll find out when I swap carbs. I might have to go backl to the Dan Atkins method of wiring the OMP open to about the 9 o'clock position.

I'm currently reading a write-up that Sterling did about mech secs and the AP mod. I might try them both on the Nikki I'm working with at the moment.

I'm still waiting for the jets to arrive for my Hitachis.

 

I think I'd like to stay with Nikki/Hitachi. I've messed with a dellorto before and while it worked marginally for a stock ported 13B, it was too weak for a street port. The Hitachi worked a lot better over a broader RP range. I can't wait to try some different jets which according to RB, are actually the correct ones for the carb (three of my carbs have the same primary air bleeds which means all three can benefit from RB's recommended jet size ).

 

go to rotaryrefs.net they have the manual on the carbs which has recommended jets for each year. 7's of course but you'll get the idea.

 

very interesting read goes over fundamentals of carbs and stuff like that.

 

There seems to be a problem with those pdf files. The pictures don't seem to work. Do I need to upgrade my Acrobat reader?

Carl, do you have any spare throttle return springs? I need to borrow one from one of my carbs to go on the Nikki I'm currently working on, and wonder if you could throw one in if I sent you a carb to mod. I also have lots of other questions for you in my other posts.

 

You seem to be trying to find out the optimum jetting for each of these carbs as they are in their stock configuration...that is to say, stock venturi diameters, etc.
You are then working with a 313 CFM carburetor, at best.

 

Hey you!! I've been reading up on your mech sec and AP mod info lately. I have an FB Nikki to play with, and want to try my luck. If you go back one page and check out the pics, the first one is of the Nikki I'm messing with right now. I have a well-lit white formica work bench and the carbs are sitting on 2x4s.

I have some questions which should be a cinch to answer.

1. Would you be able to throw in a throttle return spring if I sent you a carb without one? I need something better than the '79-'80 return spring as seen in the pic. It looks awfully stretched out.

2. Can I remove the emergency return spring? Is it really worth the effort to keep it?

3. Should I remove the dash pot? Isn't it only for keeping the cat(s) alive when quickly lifting your foot from the pedal?

4. I've attempted to simplify the choke. Should I add a little piece of thin wire to lash the two tabs together? How about a weak spring to allow the vacuum diaphram to still pull on the choke valve? Should I remove the vacuum diaphram?

5. Should I go with mech secs right off the bat before having even tested this carb on a running engine?

6. Even if I don't do mech secs, should I perform the AP mod right now?

7. I have access to the AP banjo bolt from a '79-'80 carb. It lacks the pressed-in jet at the bottom. Should I throw it into my carb?

8. Should I not use the altitude compensator? I've already removed it. Should I just block the holes on the air horn?

9. This carb's air horn has two nipples on top near the alt comp. What year is this Nikki? What fuel pressure should I use?

Thanks in advance for your help.

 

1- Go to auto parts store. Look for the aisle with all the "Help" brand stuff. Look for a replacement return spring. I bought a nice one. Its actually a dual spring (coil within a coil). Pn 59208. Direct fit, but tight in the holes cause its a dual spring. And cause now you have a dual spring, you can do away with the emergency spring.
2- see 1 above
3 yes
4 - only way out that has worked for us is to remove the everything and only keep the fast idle linkage for cold starts. If you wire the 2 together, itll die when you try to drive when the choke is engaged.
5 I would hold off til I knew there were no other issues.
6 Shouldnt hurt
7 Yes, also check your nozzles, you may have one with bigger squirters. Then maybe the mech secs would be ok.
8 yes, bye bye
9 84-85 have additional nipples on top of carb for the MAB. Block them. 2.5psi should be adequate.

No problemo

 

Thanks Carl! I'll go ahead and start modding the carb now.

The 105 jets showed up today. They look a little different than stock jets. I'm going to see how well they fit.

 

the RX3 had a 13b in it when I got it and over the many years I've had a shitload of 13B's all of them carbed but this carb I'm using now always worked the best out of all of them.