Jeff20B

iTrader: (13)

I need to know whether a couple of 105 jets originally recommended by Racing Beat for '74 spec ported 13Bs will work ok in a Nikki on a 12A.

The Nikki is stock internally with only the accell pump and mechanical secondary mods. Also the choke has been simplified. The main concern with going to jets this large is the potenial for spurting out of the primary venturis at idle. I've had that happen before and it makes for a couple of glowing header pipes.

So far I've tried 94 and 95 size primary jets across two different Nikkis with good results. I only noticed a little bit of spurting out of the venturis on a stock ported 12A with J-spec 12A turbo rotor housings (later exhaust port closing than US spec, hehe). Then I played around with the throttle stop and AF mixture screw and it settled down.

The engine I'm dealing with right now has '74 spec intake ports and slightly enlarged exhaust ports. The primary jets are 94 which was an upgrade from the stock 92. The stock eshaust was replaced with a Racing Beat dual pipe "street port" exhaust system.

While it was off I could see the exhaust ports sleeves, and they were rather light in color indicating a lean condition. I need to increase jet size.

I was hoping to go up in stages first to 100 then to 105 but I don't have easy access to 100 so I'll just have to start with 105.

Has anyone else experimented with larger than stock jets in a Nikki? How did it turn out?

By the way, the primaries feel a little sluggish but when the secondaries open, it's a huge difference. I want a little better primary operation and the evidence suggests the primary jets are simply too small. Thoughts?

flight_of_pain

i run 140 primaries and 160 secondaries at on a very large sp 13b.

no bog anywhere in the rpm range, and smooth acceleration all around.


isaac

p.s. if you have a drill press buy the bits and drill them yourself

DriveFast7

iTrader: (1)

The AEM UEGO wideband air/fuel meter I installed in the REPU made it so much simpler to tune a carb. Worth the money Jeff. Otherwise it's just guess work really. You could always disconnect the secondaries and just try different primary sizes.....check the plugs and tailpipe color. I'd imagine the motor would want a few sizes up from stock 74 12a jets since the exhaust is ported and RB, but exactly what it needs only the wideband can tell you accurately.

The fuel jet is also made by Weber and can be ordered online. It looks like the same main fuel jet in the 36/36 DCD.

IanS

iTrader: (4)

Not to threadjack, but what kind of AFRs would someone be looking for as far as tuning for all around street performance. I am sitting at 13.8 pretty much across the rpm range. Should I be running richer or leaner, and where should I be running richer or leaner (in the rpm range I mean)?

DriveFast7

iTrader: (1)

13.2-13.4 at WOT for n/a. Any richer and it's just wasting fuel. Keep in mind it'll richen up when the motor hits peak volumetric effeciency and is drawing a lotta vacuum and therefore fuel. Say around 5000rpms???? And overall the air/fuel ratio should look like a curve that's a tad leaner on left and right side, and dips a tad richer in the middle.

13.8 works great for the progression circuit (very light throttle).

What air fuel ratio does your motor idle best at?

Jeff20B

iTrader: (13)

flight_of_pain, has your Nikki been modded a lot? Hmm, it sounds like a Nikki would be the answer to some of my 13B carb problems as well. The Hitachis probably only work well on stock or near stock ports.

I'll definitely have to check the plugs a lot because I'm just borrowing the exhaust.