So it's RPM dependent not MPH dependent? Have you tried to replicate the vibration in 3rd at 50-60mph? By shakes, do you mean misfire or and actual vibration? The last real Mini I drove had an upper engine stabilizer that bolted to the t-stat housing and attached to the bulkhead. In fact, every real Mini I've driven or worked on had that. Does yours?
Does it do the same thing in 3rd accelerating from 2100 rpm?
I just drove it to the parts store for some oil. Tomorrow I will finish buttoning it up and go for a nice drive.
Did you check motor mounts?
Whens the last time it had a tune up? What are you running for plugs? You are putting it under more load climbing a grade in 4th than 3rd.
Try pulling the choke when it starts shaking.
Also, I have an MGB GT that doesn't shake if you want to trade off your shaky Mini. It sounds totaled...
The torque rod on the engine has bushings that are replaceable. These are common wear items (well, everything on a Mini is) and easy to replace. They make urethane ones that are better. Usually, it will bang around when you shift and you can usually see movement in them just by rocking the engine by hand. The lower mounts are harder to check and a nightmare to replace with the engine/subframe in the car. If they are old, you may assume they are ready for replacement.
I've had some weird highway speed 'miss' from points getting out of spec but not a vibration as you describe. Still, check your point gap and make sure the wires, etc are all up to snuff.
1st thought was spark plugs. So +1 on tune up
i think u should trade me for my neon. that way you wont have headaches anymore:-P
Inner driveshaft coupling? Do you have the pot joints or the early rubber U-Joint type? I had the rubber type, and when they got old (read oil-soaked), they got mushy. Led to a wild vibration, under certain speeds, loads, etc.
So, to recap:
It does it at 50 in 4th
It does not do it at 50 in 3rd
It does it at 2000RPM in 4th
It does not do it at 2000RPM in 3rd
Pulling the choke has no effect = probably not a vacuum leak
Replaced plugs had no effect
Pertronix rules out points/condenser
Per the stated facts:
Speed does not seem to be a factor
Engine RPM does not seem to be a factor
Gear selection does seem to be a factor
Thinking out loud: Why only in 4th? 4th gear is generally "direct drive" , bypassing the layshaft. Of speed, RPM, and gear, the only variable that had an effect was gear. That seems to rule out any other drivetrain issues. They likely would have been apparent at either the same speed or RPM in 3rd. Can you lug the engine in 3rd at any other MPH/RPM and do it? Assuming it's something related to 4th gear, I would think a gearbox issue would be more pronounced than a vibration. When the layshaft went bad in my Land Rover, it vibrated in every gear, BUT 4th. Scratching my head... For the record, I'd be surprised if it's an internal gearbox problem.
Does it do it going downhill in 4th and accelerating from 2000RPM (no load)?
Finally, we're less than 400 miles from each other. Trading my 'BGT for your Mini solves your vibration problem and also solves my "I've always wanted to own a real Mini" problem. It does create the minor problem of "Why are you dragging home another car", but that's really more of a wife-related problem and I can fix that.
A lesser solution is to always drive faster than 60 and/or keep the tach redlined. There's a popular car movie franchise that can provide instruction on that.
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