I just took out my driveshaft, and the U-Joints are in excellent condition, But I do have a vibration when coasting in a low gear, and low rpm. I spoke with a driveline shop, and they said it may need rebalancing. My question is, Is it common for after 20 years a drive shaft to go out of balance? What else should i check to try and cure the vibration, I've already ordered a new tranny mount. Thanks, Matt

 

Coasting in a low gear at low RPM?

Driveshaft vibrations are unrelated to gear, and are all related to the driveshaft speed.

Do you mean a vibration, or the engine is bucking? Rotaries are known for bucking at low RPM/no throttle. That's why Mazda instroduced the shutter valve in '81 - to combat the bucking.

 

Its more of a buck really. My emissions are off. Is there any other way of combating the bucking? Would vacumme leaks contribute to low Rpm bucking?

 

The "problem" is that you have a rotary. That's just what they *do*. At lowish RPM (say 1500-2500) and high manifold vacuum (coasting in gear) they have a cyclic misfire, aka it goes buck buck buck.

Increased port timing alters the vacuum threshold for this bucking, until it's down to the point where it "bucks" at idle and under low load. Which is why bridge ports and peripheral ports sound so fargin' evil.

 

Ok, i guess I can live with it, there is also a clunk sound when going from accel to decel. I think its the differential, and when i took off the driveshaft, i noticed i could turn the flange about half an inch both ways with out any movement in the wheels or resistance. Is it a good time to upgrade to an LSD?

 

Half an inch is about right. That's all the gear lash being taken up, nothing you can do about that.