Rx-7 Nikki jetting help?

G-Man

G-Man

New member
Need some help with jetting the stock Nikki carb on my 1985 12A IT-A Rx-7, please. And I hope this research will be helpful to others.

Please note - I run in Colorado at about 5,000 feet above sea level. About 18% less oxygen at this altitude and summer trackside temps exceed 95F (very low humidity).

Here's a comparison of stock carb, current race carb jetting, and one recommendation. First number is jet size in millimeters, and second number is jet size in inches.

Main Circuit Primary Main Fuel Jet
Stock 85 Manual 0.92 / .036
Race Carb 1.25 / .049
Recommended 1.02 / .040

Main Circuit Secondary Main Fuel Jet
Stock 85 Manual 1.60 / .063
Race Carb 1.35 / .053
Recommended 1.85 / .033

As is, the car runs extremely rich during partial throttle. Like 7.7 mpg - too rich!

Also, it was recommended I remove the spring from the secondary venturi diaphragm. This should make the secondaries open super fast. Thoughts?
Thanks in advance for the help!
 
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My carb setup is way weird. I'll have to dig out the files. But, I also have an O2 sensor, and record the entire session. And I run at sealevel mostly. So my numbers might not make much sense, or be much help. I will say this, though, nearly 8MPG isn't that bad. I think I get worse....really.
And part throttle? What's that!???
 
Are you still needing help with jetting?
To be completely honest, the recommended jetting looks about right!
and what you have in your current race carb is whack..
1.25mm Primary jets? That's huge!

If you're running a stock ported 12A with a full header, a slightly bigger primary get like a 90 or 100 will suffice, as for secondary jets run anywhere from the stock 160 to 190! Remember, if you lean you secondaries, that will also lean your primaries at WOT, and vica versa.

Hope this helps.
 
Secondaries must remain as original, according to the rules. I've seen carb shops build them mechanically linked, and it was common back in the day. But illegal.
In theory, it shouldn't make a difference, if you adjust your driving.
 
Late response here, but wanted to give an update in case other folks need this information.
Reminder - I'm at 5,000' elevation.
I ended up going with 1.02 primary (up from stock 0.92) and stock 1.60 secondaries. This carb has been race-prepped - it's definitely different from a stock carb but I don't know all the details. Also, I've cut the spring to the secondary diaphragm so they will open faster.
It's been cool - air temp in the 60s - and the car has been running very well. Much much smoother. Still some stumbling on partial throttle but not badly. Much more driveable, much nicer sound, overall a big improvement.
Thanks to IT.com forums for the help!
 
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