1985 GSL shifting issues
 

First post here so i’ll explain my situation a bit more than i will for other posts. 17 with no experience driving a manual besides in racing sims (very different but i got my techniques down) and a few months back i’ve bought myself a 1985 rx7 in otherwise great condition. My problem arises when driving where during gears 1-3 when shifting, as i let out the clutch the car violently jerks and rattles, no matter how slow or how fast i let the clutch out a smooth shift is simply not possible, you may figure im not rev matching but indeed i am shifting within ideal revs around 3-4k but the car is absolutely not having it. i took a prybar underneath the car to see if my transmission or driveline was loose, no dice they seem solid. so next i had my dad (who has been driving a manual since he was 14 and agrees the car doesn’t feel right) get in start her up and put her in first, let out the clutch slowly with his foot on the brake so i could observe if the engine was jerking, it seemed to bind up the tiniest bit but i figure it’s the bushings in the engine mounts doing their job of allowing a little bit of motion. The only time the engine really shakes is when it starts up she kicks back and forth pretty hard but during the little test we did it hardly moved. Clutch pedal feels great there’s no issues there, definetly feels like a leg day driving it which i honestly love but the crazy jerking and shuddering of the car between first third and second is killing me, 4th and 5th is like a dream and downshifts aren’t too bad(struggling with heel toe in the cramped space with the gas being so much farther behind the brake) but otherwise nice. Anyways here i am at a loss making an account for these forums. So hi everybody love to be here love to own a mazda and would love to hear your thoughts. If there’s simple solutions or tests for me to do id love to hear that rather than drop your tranny and take it apart, i’ve heard from a buddy that my shifting linkage could be shot and the car isn’t getting into gear all the way but it sounds great when i’m driving and the shifting isn’t horribly hard, feels just about right for a 38 year old car

I don't mean to sound condescending, but are you sure your clutch pedal feels okay? The reason I ask is that most of the sim clutches that I've used are very light, and don't really emulate the feel of a real clutch. On the face of it, it sounds to me like your clutch is engaging too "low" in the pedal, or possibly your slave cylinder is sticking. About how far would you estimate that you can release the clutch before the engine starts to load (AKA the "bite point")?
Is this problem only noticeable between gears 1-3, or does it also happen when you start from a dead stop?
To some extent, a bit of shuddering can be normal on these cars, much more so than any piston engine car I've driven. It is pretty difficult for me to make buttery smooth low gear shifts in my car. The lightweight flywheel and street/strip clutch certainly don't help.

I certainly wouldn't go straight to dropping your transmission. Performing a full restoration of the clutch master and slave cylinder is relatively inexpensive, and a worthwhile investment even if it ends up not fixing your issue. You could start by having your dad hop in the car and cycle the clutch while you watch the slave cylinder. You should see it extend and contract an inch or so (IIRC). Another thing to check would be to put the car on level ground, then put it in first gear but keep the clutch pedal down. The car should remain stationary, if it lurches even the slightest bit when you engage 1st gear, that would indicate that your clutch isn't fully disengaging from the flywheel. Also check basic things like your clutch fluid level, and hose condition.

I'm sure some other guys on here will have some good things to check, lots of different issues could cause what you're describing. I've never driven a stock 12A car, so I'm not sure if there's any specific common issues that could cause your problem. Just my 2 cents.

I have a stock clutch in my car that I changed in 2022, along with new master and slave cylinders in 2021. The nikki carb is stripped but otherwise stock. My car will buck and shudder under little to no load from idle to about 1800rpm. It is particularly bad when coasting into a stop, I've just learned to clutch in at 2k rpm. I think some bucking and shuttering is expected in carb'd rotaries, at least this has been my experience with my car. I can shift around 3-4k rpms without issue though, although again, the engine prefers to be under load at lower rpms so at slower speeds I sometimes drive at 4k rather than up shift when the engine won't be loaded.

I think @Eydes2Rotor is onto something wrt to checking the hydraulics. I have a stock 12a with OEM style clutch...pure vanilla. It should be a really smooth and progressive takeup when bringing the pedal up, not grabby at all. A failing slave or master cylinder will be less progressive and will make it hard to shift, especially in the lower gears. And first has no synchro.

Black fluid in the clutch master reservoir is a sign. If you do find that either the master or slave cylinder is at fault, replace both of those, not just the failed side. You should also replace the rubber hose. All of these parts are cheap, maybe $75 all in. And fully clean the hard line, flush and use all new fluid. Over years the rubber hose interior degrades and fouls the seals in the master/slave cylinder. They might leak externally or internally blowing fluid by when applying pressure. And if you put in one new part rather than all new, my experience is it will fail quickly (a couple of years) due to the contamination.

Really appreciate this! i’ll be taking a look as soon as possible and report back.

Yeah my fluid in there is black and smells like ass and chemicals(maybe rubber?) but when i press the clutch it sucks the fluid down into a big bowl shape and goes back up when i let off the clutch Any measures i can take to test if this is the issue though? Fluid change is obviously in order but is there anything else i should try to see if it’s shot and i should just replace the unit all together

You need to check to see if the clutch slave cylinder is moving when you cycle the pedal. It’s on the bell housing of the transmission, near the oil filter