I have an 84 GS manual trans. When I take my foot off accelerator in any gear, I notice the car start to buck when coming to a stop. I also have a 2009 RX8 and this does not happen. I should be able to cost to about 10 mph before I have to put in the clutch but I have to put the clutch in much earlier.

 

Stock motor, stock carburetor?

 

Yes

 

10 mph is way too slow for a rotary in 4th gear. These cars have no grunt at low rpm.

Your trans mount may be toast, have you checked that?

 

Mount may be separated,most are with any kind of age/miles on them.
OP,you're lugging the engine. 10mph in 4th gear would be below idle speed by a bit.Bucking likely misfire from engine turning too slow under load in gear. Your RX8,like other modern cars,actually turns fuel off on deceleration and back on at some rpm point,usually a couple hundred rpm above base idle speed for seamless operation. I would imagine like other FI cars,that point at which fuel is turned back on under closed throttle there would be a mild surge or bump at that time.

May i ask why you leave clutch engaged while decelerating at such a low rpm? A habit you've grown into? The bucking is the engine's way of communicating to you it doesn't like it. Like a piston engine subjected to that condition regularly,there would have to be some accelerated wear on rotating components.

 

It’s not only in 4th gear, it’s all the gears. I leave it in gear to slow down and down shift to the right gear according to the speed.

 

If it’s in all gears,what rpm are you shifting to the next lower gear?
Check ignition timing to be sure base timing is correct and vacuum advance hoses are hooked up properly and working right. Carburetor idle mixture/idle speed are properly adjusted.
Consider downshifting before the engine gets to rpm where bucking occurs,better yet,save wear on transmission and use brakes. Brakes are much cheaper to replace.

Again,FI is more flexible and has better driveability than a smaller carbureted 12A engine.
Really can’t compare driveability traits on cars 25+ years apart.

 

The downshifting shouldn't be based on speed but RPM. I like the neutral method of slowing down.

 

Is it possibly driveline slack? Mine has a bit of play in the driveshaft/differential and bucks when decelerating with low or no throttle.

 

Mixture is too lean , richen it up a bit and play with the timing ... it'll help .