I had an "M" chassis that was just a dog. It would take off ok but would not reach much top speed. I pulled the motor apart (you can do this with the "M" but not the "T" chassis) and discovered that the brushes were sticking in the barrels. It's kind of a cheapy set up but it didn't cost much so I can't complain. I replaced the Lifelike brushes and barrel springs with a set from a scrap Tyco 440 chassis. The Tyco springs and brushes are slightly narrower and a little longer, so there is no problem of them sticking inside the barrels when they get a little dust or oil on them. They will last longer too as the brushes are a little longer. It runs at least as good as new but now I'm looking for a better arm for it. It has nowhere near the speed of the "T" chassis but you can't swap in a "T" chassis arm into it. Maybe one from a Super G. I think the super strong downforce of the M chassis's motor magnets slow it down a bit too. It seems pretty durable if not terribly exciting.

Are you running the same gear setup as the T? Some of the older Ms have a different gear setup.

Rich

It looks to be the same as in the "T" chassis. The T chassis is wild fast..faster than any Super G that I have.

Something you'll want to look at is see if the armature is straight in chassis.
Sometimes the bulkheads on a M car are not true, that can make them run bad.

I already replaced the front bulkhead in this chassis and it was easy to see,
it was over to one side at the brush barrels.

http://i37.tinypic.com/2woyn28.jpg