Symptoms/conditions:
-At a steady or slow acceleration/deceleration within the 23-30mph range the car will buck forward and back.
-Happens regardless of driving directly straight or turning.
-Happens regardless of 2nd or 3rd gear
-Only happens with the clutch engaged (this makes me want to say it isn't suspension or tires)

Otherwise the car drives perfectly fine before and after this speed. Even hard accelerating through this range the car does not experience this issue.

Previous work:
-Engine rebuild (From memory this bucking issue did not occur directly after this, but I will include it anyways)
-Mazda Edit


More Recent work:
-Suspension + brakes
-JDM 5 speed transmission

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I want to point my finger at the transmission, but I simply don't know enough about the internals of transmissions to say for certain. I have to find a leak in one of my tires this weekend so maybe I will just swap back to one of my 6 speeds and see if the issue goes away. I planned on rebuilding the 5 speed this summer anyways.

 

Are your plugs 'recent'?

Sounds like just a mis-fire.....

 

Plugs within 2000miles, but those were with the rebuild. I'll check plugs and coils tomorrow as well.

What would make the misfire happen only at that certain speed though? Last time I had misfire issues with fouled plugs on my previous low compression engine I was getting misfire codes, but so far no engine codes this time around.

 

Well I swapped the plugs to a new set I had laying around. Not much changed in terms of the bucking, started nice though. I might have to look into diff mount bushings or maybe the driveshaft.

I am still unsure of the internals of the JDM 5 speed transmission. I was not able to replicate the bucking in 1st gear at the 23-30mph range. I would think the 5 speed would be like the 6 speed 1st and 2nd would be on the same fork and 3rd and 4th on the same fork. Which this makes me think it might be an issue with final drive or the driveshaft.

 

Been paying attention to the issue more. It appears to be happening in that mph range when I first lightly touch the throttle, but doesn't seem to happen if I go hard on the throttle in the mph range.

Edit: I should add that it isn't just a single hard buck. It will last for 10+ seconds if the car stays within the 20-30mph range. Still not really sure why it is at a specific mph range, I suppose I'd just write it off as hitting some natural frequency type event.

I probably should have done the diff bushings, considering I did poly engine mounts now the diff is probably too soft and moving enough to upset the phasing of the driveshaft. Good thing the weather is warming up.

 

used to have that issue some a long time ago, but am not entirely sure what completely eliminated it along the way of making numerous mods; stiffer drivetrain mounts, engine tuning including making the DBW map more linear, higher idle speed, etc.

 

With the nice weather I went ahead and pulled the original 120k miles New England diff out of the car. The bushings are cracked around almost all of the rubber, the picture doesn't show it very well. I can tell getting the steel sleeve out of the aluminum is going to be fun.

I also checked the driveshaft U joints and noticed that both ends have a soft lock when straightened in-line with the driveshaft. Meaning the spline does not droop and remains in-line with the driveshaft.

 

So the bucking issue didn't go away, but it is less drastic for sure. The bucking now dies out within 3-4 bounces instead of infinitely. Might look into see what on/off throttle tuning can be done in mazdaedit.

Also a mill makes it much easier to take out those steel sleeves. They slide right out with one or two taps after it buckles.