Fawkes2277

Hey everyone, Im looking for advice on my rough idle issue. 1985 RX-7 GS 12a, 78k miles. Car has been parked since 1994 and I got it running and driving over the past month after the previous owners could not. The car idles ROUGH and idles around 550 RPM. My first instinct is a vacuum leak, but I have used a can of carb cleaner and an unlit propane torch and can not find one. The car runs rough at idle but at higher RPM and driving speed is fine. It'll rev to 7k all day long. Car does not smoke and starts right up. If I give it a little more gas from a bottle while its idling it smooths out. Looking down the carb I can see the OMP working and the venturis are pulling fuel evenly, its not flooding.

Fuel System: Cleaned/sealed fuel tank from Riggs Rotary, new fuel pump, filter, lines, sending unit, pickup tube and the carb has been rebuilt using a hygrade kit. (original needles/seats, no added base gasket) Idle speed and air/mixture screws do not change anything.
Ignition System: plugs and wires are old, but serviceable. I have NGK plugs, wires, and a new cap and rotor in the mail. Adjusting the timing has not affected the idle issue
Vacuum lines: Rats nest and factory emissions parts are all present. 99% of the vacuum lines are new and were replaced when I rebuilt the carb. No vacuum leaks detected using carb cleaner and a unlit torch
Compression: I have not done an official compression test, but the compression feels strong and the cadence is good. Videos attached.
Intake manifold: There is an open tube on the back of the intake close to the firewall. I figure its an emissions thing, plugging it has no effect.

I plan to do a compression test this week but i don't think its weak compression because it runs strong and can actually idle unusually low. I am going to replace the cap/rotor/wires/plugs this week but I dont suspect thats my issue. If I have no luck i am going to delete the rats nest/emissions components. Any guidance or suggestions are greatly appreciated!

Starting and Idling Rotor #1 Cadence/Compression Rotor #2 Cadence/Compression

rxtasy3

iTrader: (2)

damn, and i thought mine fired off quick. a vacuum leak will cause a high idle. there's an adjustment on the carb. i think it's just idling too low. have u checked the foxed.ca site for the manuals?

Foxed.ca - Mazda RX-7 Manuals

FührerTüner

iTrader: (5)

What about the throttle set screw? The one that sets the openings of the butterfly valves. Try turning that up?

Also, have you checked timing? Could be retarded.

FührerTüner

iTrader: (5)

Fawkes2277

Update on the idle issues. I replaced the original 1984 dated plug wires with a new set of NGK wires. Also replaced the cap, rotor, and put in some new NGK plugs and it helped a lot. Before, the idle mixture screw and idle speed screw did basically nothing. But after fixing the bulk of the misfire issues I adjusted the air mixture and idle speed screws and got it to run and idle A LOT better. I also noticed I forgot to reattach the arm for the secondary's so that was corrected before driving it 35 miles today without any issues which I am super happy about. It is noticeably slower than my NA Miata but its always exciting being the first person to drive something in almost 30 years. I did a compression test using a piston compression tester with the schrader valve removed, and used the rotary compression calculator from foxed.ca and got 98 PSI for the front rotor pulses and 93 PSI for the rear rotor pulses, consistent on all the faces. BUT, there is still a little bit of a misfire or stumble every so often. I still think that there might be a vacuum leak somewhere that I just haven't found. But now that I know the engine is healthy, I think ill follow through with the rats nest delete and get rid of most of the emissions stuff since I would eventually like to put a header on it and change the exhaust.

If anyone has any further suggestions regarding the occasional stumble at idle I would love to hear them.
Updated Idle video

FührerTüner

iTrader: (5)

Glad you got it fixed, but your rx7 should blow your Miata out of the water.

Fawkes2277

I dont know, a bone stock NA8 miata has more power and weighs less than an FB so its about what I would expect.

diabolical1

iTrader: (3)

it just goes to show how much difference a simple tuneup can make.

glad you got it sorted out.
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