MrMinerMan50

Just recently finished the rebuild on my 12a, but I'm fighting an issue from before I rebuilt it.

Symptoms:

Requires rich mixture for smooth idle
accelerates fine under gentle throttle
falls on its face at about 5/8 to 7/8 throttle (all gears, any RPM)
Full throttle, pedal to the floor clears it's throat and scoots like a stabbed hyena. (Except from 0%-100% throttle, then it gasps and coughs for a second)
SEEMS to do better when holding the choke out. It will still do it, but it's much less aggressive

Engine info:

'82 12a, Rebuilt with resized GSL-SE housings (this has resulted in a very small overlap, but Vacuum pressure still reads decent, idles great, and this issue persisted before the rebuild)
'82 factory carb, rebuilt and cleaned, main body parts dunked in can of Chem-dip, and sprayed and wiped clean afterwards - including air and fuel passageways
Carb has been opened up a few times, and blown air through the air bleed holes and emulsion tubes
Rats nest temporarily deleted, running line direct from intake to dizzy vac advance, and aftermarket vac gauge inside car. Other Vac plugged off, no audible leaks. Very small rise from starter fluid around carb base
Mechanical Secondaries temporary modded - vac solenoid verified working

General Description:

Car idles fine, engine has been rebuilt with refurbished housings, stock porting, good vac signal, slightly richer than expected on carb (I expect about 2-3 turns out on mix screw,but screw nearly falls out to run the best).
Takeoff is normal as expected, unless you get too heavy in the throttle, then it falls off, and chokes hard. Back off to about 25%, or give it the onions, and it drives perfectly again. Vacuum jumps to 20, and slowly falls with engine RPM.
All in all, the engine drives as I expect for the most part. But, essentially in the 3/4 throttle range, it all just falls off for a moment and has to think about life. It makes driving a bit difficult. My father has experience working with carburetors, and is as bewildered as I am.
A carburetor mechanic expert gave some advice that a passage in the carb may be plugged up with some old gasoline varnish -- Possible, but everything was super duper spray cleaned! I've had the carb off and on the car so many times I could win an Olympic medal!

Calling the Nikki carb experts out here to help me out. Any ideas are super duper appreciated!

Benjamin4456

So a couple things caught my eye on first glace. Firstly, it sounds like your vacuum advance may be connected incorrectly. The way that vac advance works on these cars is that the pots should see no vacuum at idle, vacuum at part throttle, and then of course no vacuum at wide open (assuming no emissions solenoids are connected). With the pots connected directly to the intake they would be seeing vacuum at idle, which is incorrect. I know it's commonly set up that way on some engines, but not for us. Instead you need to use a timed vacuum source like the second to front port on the phenolic spacer. On most Nikki's this is the ideal port, however I have found some that have this port blocked off internally so I'd double check that you can blow air through it before just sending it.

Second thing is the mechanical secondaries. Getting these carbs to work well with mechanical secondaries can be a treat. Usually you will have some degree of a bog or a hiccup (depending on how you timed them), especially if you haven't modified the accelerator pump or anything to help compensate. This could explain your issues around 3/4 throttle since that's when the secondaries begin to be active. If I might ask, what's the reasoning behind running the mech secondaires? Also what solenoid are you referring to? The secondaries have no solenoid related to their function that I am aware of. Perhaps I've misunderstood your "mechanical secondaries temporary modded" statement.

Lastly, if it were to be clogged passages, the only sneaky ones are a couple small tubes that connect either the main and long slow air bleeds, or the no.2 and long slows, can't recall which. It goes sideways between the two within the main body and is seldom an issue, but it's something that you could investigate. Otherwise if you can see through all the jets and bleeds after cleaning then they shouldn't be an issue. I'd begin with the vacuum advance tweak and perhaps try putting the vacuum secondaries back in action if possible.

Figure I would also mention that the baseline idle mix is 3.5 full turns from seated. Not sure where you landed beyond expecting it to be 2-3 turns out, but it should be around the 3-4 turns range. Hope this helps.

MrMinerMan50

Fantastic things to look at, and thanks for the reply!

So, the Vacuum advance, I've looked over a few times, checked with a timing light and a vacuum gauge. It appears to be timed correctly at idle, and seems to advance properly according to the manuals I've found here: https://charm.li/Mazda/1982/RX7%202R...g/Adjustments/
I've also done some checking on foxed.ca manuals as well.
I also have no change when I remove the vacuum at idle. It's been doing this, on and off of two different rats nests. I also believe I have them hooked into the same vac port, the one you mentioned, that the rats nest (Identical carbs and plates) carries them to. It may be possible that having the L and T advance signals running through the same vac hose may be causing issues, but I'm not 100% sure.

Mechanical secondaries, I zip tied the linkages together, so there's not really much "timing" to it, as much as backyard redneckery. My primary reason for doing this was because I figured maybe the secondaries were sticking closed, and then WOT forced them to open up, so the zip tie was just meant to help them out. I did find the secondary blades were a bit stiff, but I cleaned em up a little better and they opened much smoother, but that definitely was not a big change. Apologies for the "solenoid" confusion, I was referring to the vacuum diaphragm that opens the secondaries.

I have done a bit of playing and tinkering with it to experiment, and see what works. Right now, I've actually got it running (somehow!) with the secondary air bleeds swapped with the primaries. In this condition, it's running with the mixture screw all the way in. This was more of a little "let's see what this does" sort of experiment than anything. I'll be returning it to the stock configuration very soon.

Do you have any images or diagrams showing the long slow air bleeds your mentioning? I'm sure I bubbled through them, but I may have missed one or two. I'd like to double check myself and make sure I've got em.

As far as the idle turns go, just going with what I remember from my old RX-7 that was running correctly centuries ago, I believe it was about 2.5 turns that got it running best. I'm in the Rocky Mountains with a little bit of altitude, so we usually run a little leaner.

Thanks for the suggestions and help, I'm definitely going to take a closer look at the vacuum advance stuff and revert most of my mods. I hope you're right! I've been fighting this issue since I got the car last August, so it'll be good to have it running right!