Well, this isnt really me seeking information on the topic, more of putting an actual thread out there with some of the findings ive had with the stock Nikki carb on a stock port 12a (pretty common) when changing exhaust systems.

My dd FB (the one I just painted) for a long time has run the collected RB header and short primary exhaust. I had a totally stock nikki carb running on it very well, and noticed no real issues. Was returning around 19 average mpg, and made decent power.

I just switched to the RB road race header+RB dual exhaust and noticed I feel way more power and torque in midrange, but it almost feels like its nosing over at almost exactly 6k rpm, and surges a bit on part throttle. A quick spark plug reading shows a lean mixture, but im not sure exactly how lean...

Which is why I put this here. I have an idea what to do as far as jetting at this point, as im sure my weber redline fp is enough pump. I can shut the car down right as I feel the surging and check the float levels and they are normal. Ive searched and searched for anyone having similar results when using the RB dual with the stock nikki, and it has been very vague at this point.

Im going to try 95 primary/ 170 secondary as per a gentleman at mazdatrix' recommendation, but was sorta curious of anyone elses ideas. My Sterling'd Nikki has a radically different jetting/air bleed combination than stock, but I expect it to be different in that way. I wanted as close to a stock carb as possible on my DD for MPG, and reliability.


So any thoughts on this? Maybe its just my engine? I get mixed opinions on it the more I ask, so I figured it be a good idea to post here.

Engine is a 85 12a, no flapper valve (intake not ported though, so rotor 1's intake runner is very different from rotor 2's), Accel coils, Stock ignition otherwise. Weber Redline 60 GPH inline fp, RB dual exhaust and is a manual. Engine is at about 84k original miles, good compression. No vac leaks that I can find anywhere, and im leaning on a vac leak not being the issue bc of how well it ran with the short primary system installed.

Ill be trying the slightly larger jetting this weekend, ill let you guys know how it goes.

 

In my slow motion don't do too much at a time journey I recently bored the primary jets in my stripped Nikki (85) to 1 mm. The result was noticeable, but not dramatic. When the opportunity presents itself I may very slightly bore the secondaries, just to see what it does. My exhaust is RB rr header into collector/presilencer with a Magnapack, all 2", with a borla 2.5 dual outlet muffler at the end.

Small mods one at a time allow one to find out when too much is too much. I'd do just the primaries first.

 

I like the thought of going 95/170 because I have those sizes too. All I need to do is mod a collected 12A header into a road race style to use with an RB long primary. The guy I'm building the engine for wants 74 spec intake ports but stock ports are an option. I'd love to test this setup parallel to yours and report my findings but if he wants 74 spec, it'll need more air at the same RPM and therefore more fuel and the carb becomes a bottleneck much sooner, and so on. Not really apples to apples.

 

here are some thoughts based on my experience, however i haven't had a stock 12A in a really long time, so its on different engines.

1. with the P port/weber IDA, moving the collector made a HUGE difference in the carb's tune. i would expect a stock engine/nikki to have a less dramatic change.

2. i would try the bigger jets, and see what they do. IMO since the carb works by sucking fuel out of a hole its a little hard to predict how picky it will be about jet size (going from 85 to 95 might be a huge change or it might not be noticeable)

3. have fun!

 

That RB RR exhaust will definitely make your car lean out on top with the stock nikki jetting. I
would mess with the primaries first until your cruising well and then look into the secondaries for
when you mash the fun pedal.

When I went RB RR on my Dell I had to retuned it a bit richer on the air bleeds to make it run better.

 

I think based on the small size of the primaries, and extreme air speed through them that it should respond to jet sizes very well. The secondaries being larger maybe not, but those primaries should be subject to large changes from small differences; if that makes sense.

 

its does, and that is what i'd expect to happen, but you never know

 

95/170 (primary, secondary respectively) confirmed to be exactly what the engine wanted. Its a slight bit richer at cruise, helped smooth the decel out a little, and made a bit more grunt mid range at part throttle. At WOT the engine pulls smooth to 7k, where I shift. I think this would provide enough fuel for higher rpm's, but I dont think ill be going any higher.

I returned 218 miles from 12 gallons of gas this weekend, mixed driving. Runs great, im very pleased.

This HAS increased power at both part throttle AND WOT. Ill have to dyno tune to confirm results for an official number...

 

Been driving this for about a month now, still very pleased. Doesnt seem to have affected gas mileage at all, and the power and smoothness is great.

To be clear, this carb ran great running the RB short primary exhaust and stock jetting. Only when switching to the long primary did it run lean...

 

Splendid! The Nikki is the perfect 12a fuel delivery device, and as you have shown, is quite tuneable to accommodate varying engine requirements.

 

Yup, only thing id really like now is a reinstall of the dashpot and id call it perfect lol

 

Are you still using the emergency return spring?

 

So where does one get jets for these carbs, or are you simply modifying stock jets?

 

Mazdatrix has them. Throttle, Carb and Choke Parts

 

Yes, pedal was sloppy without it IMO.

 

I prefer not, I like the lighter feel.

 

This jetting is still running strong for me, although it seems my secondaries are opening too quickly and affecting my gas mileage when im cruising at 70-80 mph. It must be only slightly, but just enough to suck down the fuel from that side of the carb. Just changing secondary jetting has affected my highway mpg...

 

Have you tried mechanical secondaries?

 

I'm feeling like I'm having the same experience, but I also need to rebuild the carb and get an upgraded fuel pump, I'll probably go Carter again. It feels lean in the top end I have no idea when the carb was last rebuilt but I figure if I'm going to open it up I should probably jet it up a touch. I didn't bother with the SA and short primary setup it never short on power up top I think because it was jetted richer for the thermal reactor.

I'll need 2-95's and 2-170's is that right?

 

iirc the jetting was only like .1mm larger for the primaries only on thermal reactor carbs. So it IS bigger, but only just.

I do have a 78 nikki that runs richer than my 84-85 nikkis with the same jetting. I always heard the early carbs werent as good, but mine actually runs much smoother than any of my other newer nikkis lol.

I can get you in touch with people that can get you a perfect nikki carb pump. Its a werber "REDLINE" product ive installed on several cars now. I use it on my 45 dell car and it keeps up with that too lol.

 

dbl post?

 

You're right about the jets. The 79-80 had 93 primary jets and the 81-85 had 92 usually and sometimes 91.

The reason the 79-80 runs generally richer (and better on your long primary) is a set of slow air bleeds at 150. The 81-85 carbs had 170 and 180. Possibly 190. These can hurt idle quality if conditions aren't perfect. And with carbs, they seldom are.

Oh and 79-80 carbs had separate air and fuel screws which makes life easier if you can handle adjusting two screws simultaneously. I don't really like the single "mixture" screw on the 81-85 carbs.

The accel pump is also bigger on the 79-80 carbs.

I've gotten a newer Nikki to run just as well or better than the earlier carbs by using early parts or modding later stuff like the accell pump banjo bolt to be more like an early one. Drill out the tiny jet.

 

Played with my jetting last night reverting to all 79 jets/bleeds for test, very lean on primaries but the secondaries seem to be perfect.

 

The 79 main air bleeds are 90 which in your case would be too large, letting in too much air too quickly and thus leaning out your primarys. I'd say go back to the 70s or 80s the carb had originally, but keep the 93 fuel jets. Or stick with the 95s recommended by Mazdatrix. This should bring the primaries back to where you want them without changing the secondaries.

By the way, where is your wideband O2 sensor mounted in the RB long primary?

 

I have 2, on each header pipe at the flange to the mid section.