Dionysos

So I recently completed a manual trans swap in my '83 FB, I used a RB lightweight steel flywheel so everything went pretty smooth and the car feels a lot better in 1st, and I get much better mileage with a 5th gear now, so needless to say I'm pretty happy. I plugged the vacuum line running to the old A/T, and accidentally broke the hard line from the air pump to the cat, so now I have a gaping hole in the cat leaking exhaust and an air pump spraying air on the ground, but other than that everything else is the same as before. I did have to dial in the carb to bring the idle way down, but now I've got it idling around 650-700 or so when warm (which, maybe a touch low, but it feels pretty smooth when I'm stopped at a light).

I have a new issue though which I'm having some trouble diagnosing, but have an idea of what it may be. When I'm coming to a stop or something and am off the throttle, as soon as the car gets around 2k RPM it begins to buck and shudder fairly hard. The shuddering goes away immediately if I'm out of gear, have the clutch pedal pressed in, or back on the gas (even very very light throttle application). Even if I hold the throttle just a touch as I pass through 2k RPM down toward ~1k, it doesn't shudder at all. It also doesn't shudder at all if I have the choke out a little bit (lifting the idle to ~1000-1100, usually I begin driving with the choke slightly out like this and then pop it in as soon as the temperature needle starts to move which doesn't take long). I noticed that one when I forgot to pop the choke plunger back down as soon as I hit my first stop sign on the way to work and thought the shudder miraculously went away!

Sort of seems to me like there is something going on with the carb and a throttle/coasting solenoid (not very familiar with carbureted engines yet, this is my first) or something. The car only behaves strangely in that very specific circumstance (no high speed shudder, decel shudder above 2k, etc. which makes it hard to compare to similar threads on this forum). At first I thought it may be the clutch/flywheel, since I was thinking about what has changed, but the fact that very slight throttle or choke out cures it makes me think otherwise now. I do plan to remove the carb and rebuild it, and delete the air pump, put on my double sheave pulley, delete the OMP, and clean up the rats nest, etc. at some point, but I wanted to wait to do that stuff for when my RB exhaust came in - which seems like it may be a while yet. If this problem seems familiar to anyone and it's an issue I can solve without removing the carb that would be great, if not, I can keep just pushing in the clutch pedal under 2k.

Sidenote - other things I've done to the car in pursuit of what I thought was an ignition/misfire issue: replaced spark plugs, spark plug/coil wires, ignition coils, inspected all distributor components (they all look clean and fresh), and replaced the battery. The issue was a leaky/overheating battery. Replacing the plugs gave me an increased power feeling and the new Optima red top gives me a very powerful startup, but none of that had any effect on the shudder - positive or negative.

LongDuck

Two things; check your Throttle Position Sensor and it could also be your Pilot Bearing or Throw-out Bearing on the transmission. In order;

The TPS tells the computer (*yes, you have one, its under the passenger footwell carpet) where the driver wants the throttle to be. When set properly, it will tend to hang out in the lower end of the range, as thats the most common place for the carbon track and slider to sit. The problem becomes, over time, the carbon track and slider which sends a resistance signal to the ECU starts to get intermittent, which results in quickly varying signals being sent to the computer. Thinking the driver must be doing weird things with the throttle cable via the accelerator pedal, it tries to respond to helm orders from the captain behind the wheel. This can feel like shuddering, sudden drop-out of power, and is usually tied to a specific tension on the gas pedal, because it happens at a specific position of the throttle (*hence Throttle Position Sensor).

Secondly, the transmission you installed may or may not have been in good shape. Usually, anytime the transmission is out of the car, I replace the front cover gasket, the front cover, the throw-out bearing, the pilot bearing and pilot bearing seal, along with the clutch cover and clutch disk. I'm pretty horough that way, because it's a lot of work, and you're already in there. If you didn't replace this stuff with new, it could be a number of things, most likely that the input shaft of the trans isn't centering in the output shaft of the engine (the eccentric shaft). What causes this is usually a Pilot Bearing that has grenaded in there and its grinding itself to dust, but this is usually only felt at idle, with the clutch pushed in, and the transmission in gear. Trans in neutral, clutch out, both engine and trans are spinning together and usually get along fine - even with a bad Pilot Bearing.

The Throw-Out Bearing will make a lot of noise when it's going out, because with the clutch pedal down, it's the only thing keeping the engine from spinning the transmission gears. When it fails, you'll have all kinds of problems with not being able to get the car out of gear and inability to start it, but before then it can cause a lot of vibration and could be speed specific, because it spins at engine speed with the clutch out.

There's a lot of information on these if you search. Good luck, and please report back with what you find so others can learn from your experience,

Dionysos

Thank you for the reply LongDuck. I don't believe it to be the pilot bearing or throw out bearing, as those are brand new. I too replaced the front and rear seals, and made sure all mating surfaces were nice and clean and bearings appropriately lubed, along with using a brand new clutch and pressure plate. I wanted to make damn sure that nothing would have to come back out! I initially believed it to possibly be shaft misalignment but since it goes away with faint throttle application in gear (a condition sure to make misalignment produce worse vibration), I want to believe that's ruled out. Throttle position sensor sounds like a possibility, I didn't even know there was one until yesterday when I started reading some threads and the manual on carb rebuild to prepare myself. Although now I'm more inclined to now think it may be something the throttle sensor is controlling, like the shutter valve maybe? Honestly I probably won't try singling out components from the carb as I've always intended to rebuild it, and now I have a lead on some Racing Beat exhaust parts locally, so I will probably be doing that soon.

I will certainly keep the thread updated after the carb rebuild to say if that improved anything, or if anything was obviously wrong with it. Should be in the next few weeks or a month, I'm thinking.

LongDuck

Could be the front U-joints on your driveshaft. I've replaced them when deceleration caused a significant rumble that could be felt through the trans tunnel. When it's the rear U-joints, you'll know - there's no mistaking it. When you get into your carb rebuild consider keeping the stock needle valves and seats. Its common knowledge that the originals should still be good, and replacing them with aftermarket poor quality needle valves and seats results in flooding.

Keep your originals in place and un-messed with if at all possible. Good luck,

diabolical1

iTrader: (3)

why not try adjusting the mixture a little and see if that helps? maybe just a touch richer?

Dionysos

Well, my busy life has meant that this has taken a lot longer to come to fruition than I wanted, but I do have some updates. I got my hands on a RB header and presilencer and installed them about a month ago. After putting them on and starting the car up, it smoked like unholy hell out of the tailpipe, which the cat was probably masking beforehand. I played with the mixture and idle until it was dialed back in and burning clean and it actually ran a lot smoother, the deceleration vibration is now substantially less, almost unnoticeable at times. Something I've noticed after doing a lot more driving however is that the idle doesn't hold rock steady, it sort of "bounces" around ~50-100 rpm, I feel like this is likely linked to the same underlying cause as any decel shudder.

I finally got back into the engine bay this weekend and completed the air pump delete (it was running with the air pump still, not hooked up to anything lol), blocked off the ACV port and switched a double sheave pulley set-up. Made sure the air cleaner vacuum ports were blocked off, and then also changed the oil (unrelated, obviously, but mentioning it for posterity). The car runs about the same as when I'd switched to the RB exhaust but left the air pump on.

I've also discovered a probable fuel leak from the carb, on the drivers side of the intake manifold. So now carb rebuild has shot to the top of the priority list, strongly suspecting perhaps the accelerator pump to be the cause of both the shudder and the leak. I may not get around to it for a while but I will follow up when I do!

Triangle Man

I had to register just to reply as our situations are very similar and I see you still haven't had any luck.

I recently swapped an 82 from auto to manual and began experiencing the same symptoms. Racing Beat references it in number 4 on the page below https://www.racingbeat.com/mazda/per...misc-tips.html
However in my opinion from further research this is a characteristic of the rotary that many people reference experiencing after switching to aftermarket carbs or doing full emissions delete, and i suspect the coasting valve/richer solenoid that we don't get on the A/T models is specifically implemented on the m/t to assist with this.

I ended up satisfied enough with the above since its only really noticeable once the revs are low enough that for the 12As sake its time to downshift anyway so I've just come to live with popping it into neutral a little earlier on the way to a stoplight.

I do think its worth trying the fixes racing beat suggest, getting my carb adjusted properly did reduce the severity of it.

My ultimate solution was a Bridgeport since now it bucks much higher in the rpm range so I don't even get the chance to experience it anymore 😁