This seems to be the place to ask about things like stock model Ts. Engine reaches a certain RPM and stops running properly, like it has a rev' limiter. Not loose coils as i pushed them in the box and they seem tight. This is a link to a video someone posted for me because i lost my password http://youtu.be/GkN7a79ACjU You can hear it when it is sitting still, on the gear shift and when it reaches its new top speed of 25MPH.
What could be wrong?
Timer roller skipping?
Start with the timer. Classic symptoms for your problem. Idles fairly well, breaks up at speed. If not that go to fuel flow.
Is this a new problem? Could be a lot of things.
weak valve springs
not enough timing advance
not enough fuel
Keep it simple. (for now).
Flapper timer but i did have the front cover off. It did it in the past but only in low gear, Cheers i will look there first.
You might have a coil that has shelled one of its contacts. I had the exact same thing happen and simply looking at each coil contact set I noticed immediately that one coil had the lower contact no longer there. It had fallen off and the lower vibrator spring was still with enough spring tension to come up and hit the top contact and the car ran rather well at low RPM but immediately began to falter at higher RPM.
Sounds to me like ignition, i cant hear the engine well enough in the video.
timer is likely and easy to check as others have said, also if you had the cover off did you re center it?
when coil condensers go bad the car will still run and the coil will still buzz, but the car will run poorly.
if it was fuel delivery, your engine would begin to cut out, and die. another easy check.
seeing as a t valve spring has a little more tension than a rubber band, its highly unlikely but not impossible.
Tried a new day timer and it runs great again. Guess the cover needs to be centered. Thanks again. (you would think i would remember this check from reading about it on here in the past)