Help Me Diagnose 12A Stumbling Issues
 

A few months ago I picked up a 1983 GSL (my first rotary) knowing it hadn't run in a year or two. I drained the fuel tank then replaced the spark/fuel components typical for this situation: plugs, wires, coils, cap/rotor, fuel filter, air filter. After letting some MMO soak in the chambers for a day and fixing a starter ground cable issue it started right up and seemed to run OK (white MMO smoke notwithstanding.) The tach didn't work but swapping the trailing ignitor took care of that. The idle was a little high so I brought it down to around 750 RPM. Didn't touch the mixture. The leading timing needed a tweak but the trailing looked fine.

Generally it seems to run fine now until I try to 1) run it hard through first and second gears or 2) go up a decent hill. I can take it up to redline in first but as soon as I approach 5K in second it starts to stumble. I have to back off then keep blipping the throttle to keep it running for the next 30 seconds. There is long, steep hill near my that I just can't get up. Whether I drive normally, baby it or floor it, it just wants to stumble and stall. One last symptom is that if it's at idle and I hit the accelerator hard it drops to almost a stall for a half second before it revs normally. I've used carb cleaner and it looks decent in there but I haven't gone any further with the carb other than confirming I'm getting spray into the secondaries around 5K RPM. I replaced the fuel pump too but that didn't change anything. All the vacuum lines look OK and the propane trick didn't indicate any leaks.

Any ideas what I should do next? I really want to start enjoying this car before summer is over!

A little background about me... I've owned roughly 25 cars and collectively driven them literally a million miles. And I've always done my own maintenance and repairs and I consider myself a pretty competent backyard mechanic. I'm thinking I might have a carb issue, but one thing I've never done is meddled with a carburetor as complicated as this one. I have to admit I'm a bit intimidated here!

Have you considered checking for a clogged cat or muffler?
I mention this because I had hesitation above 6k RPM that felt a lot like fuel cut. I believe the PO though so too because the car came with a new (uninstalled) fuel filter and pump.
As my friends drove behind me on a long downhill the car was after firing and ejected cat material. Issues running up to redline suddenly resolved.

Hadn't considered exhaust as a possibility, but that's an interesting thought. Once thing I didn't mention was that car has 166K miles on it and I believe the exhaust to be original. Maybe this is my excuse to pop in a Bonex race pipe.

Before you go looking at tearing into the exhaust (unless you just want a new exhaust, in which case have at it) check a couple things. If you remove the air box lid and and look down the primaries while actuating the throttle, you should see a squirt of gas from the pump nozzles. There should be one jet for each primary barrel, and the amount of fuel should be equal. Also, have you checked the fuel level in the bowls? The sight glasses are on the front and rear of the carb. A mirror and flashlight will make viewing the rear one much easier. Fuel should be right about halfway at idle, and shouldn't change much while revving. If the levels are off a lot at idle, shut off the car and let the pump run for a little. If this fixes the problem, it's an issue with fuel delivery to the bowls and not the float adjustment. Floats don't change themselves and are not a fun thing to adjust if you don't have to, so first check other possibilities. Another thing that will cause carb issues is if the vent system is plugged or faulty. When you turn the key to the run position, the carb vent solenoid should make an audible click. This died on my car and caused many mystery problems. The vent solenoid is located on the driver side of the carb near the top. It has a single power connector and is grounded through the carb. I believe there are some that that have a two prong connector (ground and power) in the later years, but I could be wrong.

If any of those are problems let us know, otherwise there are a few other things that it could be. Start with the basics though (as I have learned the hard way more than once), and more information is always a bonus for forum trouble shooting.

Those cats do have a habit of clogging as detailed by GSLSEforme in some other posts but Benjamin4456 is correct as well. Usually if the cats clogged the exhaust header will get really hot on extended drives and maybe be glowing red or the pipe to the cat will be doing this. When I first read your post I thought it was a fuel delivery issue until you said you swapped in a new pump and see the secondaries getting fuel.

Thanks guys. Looks like this weekend I'll be checking cats, floats and vent solenoid!

Solved!
 

This morning I checked the cats but it didn't seem like I had any blockage. So I decided to dig into the carb. 4 bolts in, I pulled the screens and found they both looked like this one, completely clogged! After a few seconds with a toothbrush I put everything back together and the stumbling is completely gone! I raced through first and second, and drove up big bad Simsbury Mountain like it was nothing. Always start with the simple stuff!

https://cimg6.ibsrv.net/gimg/www.rx7...50e1db96d3.jpg

Next step is to figure out why when I stomp on the gas from Idle it drops to 100 RPM before revving. I'm thinking pump diaphram but I'm open to more educated guesses.

Nice! I don't have those screens anymore since I hogged out the nikki. If you had gunk like that on the screens, you need to pull the carb and do a through cleaning and refrub on it. Will probably do wonders it. Just don't touch the float adjustment, don't use the new needles or seats and take it apart on a big white sheet or other contained area so you don't lose critical small parts like weights and balls.

Yeah, I think you're right. After driving the car for a few days it feels underpowered. I've never driven a FB before, but this falls somewhere between my '90 Civic automatic and my '11 Mustang. And a lot closer to the Civic :) So I guess I'm going to learn how to rebuild a Nikki.