One thing I do to reduce resonance is that I always add a thin strip of rubber (a 5" strip made from a single layer of bicycle tube) between the fuel tank and its diagonal retaining strap.

Check your engine steady bar(s), if it has poly bushes I'd bin them and go back to rubber.

Alright, I checked the valve cleareances, all is within spec.  Went for a long hot drive today and it seems better after adding some dynamat.  I also removed the bottom engine steadies.  I will continue to try differnt things through out the summer.  I can live with it like this now.  Thanks again to everyone who gave me ideas and help with troubleshooting.

Subguy

I had what sounds like a similar issue with my 1275 a few years ago.  There was no more vibration in it than a 998 when I first installed it.
I later installed a lightweight flywheel.  I then had the vibrations you describe.  It didn't feel right.  The only thing I changed was the flywheel, so I pulled it off.  I had installed the three straps ( backplate ) ever so slightly "off".  I corrected this and it ran smooth as silk. ( well, cotton at least )

Ok thanks.  I'll check the valves in a few days and let you know.

wait, dont give up yet... you havent tried adjusting the valve.  i experienced same thing , vibration is notorious, idles high, then after i adjusted the valve to the factory spec which is 0.012, vibrations gone and idle'd low and steady pulls better as well.

no i dont think the exhaust is touching the tunnel, you will hear a much distinct noise when it happens. 

its either the fan blades not aligned / little crooked or it needs valve adjustments.  try it first and report back the outcome.

The subframe is solid mount from factory.  It's a MK3, 1975.  Thats what I meant about solid mount.  It does handle great though.  I don't mind adding some weight to get rid of this vibrating shell.  It's working so for.  I did also add some rubber edging around the bonnot, that was the first and worse thing to vibrate.  Slowly getting rid of them.

This mini I just bought has a series of vibrations. The worst noise was coming from the bonnet. Actually thought it was something in my dash or gauge cluster initially. Made a terrible racket at various rpms. I narrowed it down to the bonnet by touching my way forward on everything I could think of. A piece of rubber on the lip of the windscreen panel fixed her right up. Like a different car. The next issue was my steering shroud was rattling also. A little dab of silicon on the pieces where they adjoin did the trick there. My car is also solid mount.

If it were me, I'd consider putting in softer subframe mounts - rubber or poly. That's probably what your car was built with.. for a reason! Solid mounts are supposed to improve handling, from what I've read. But to my way of thinking, if you're adding weight in the form of Dynamat, you've chosen to compromise handling by making the car heavier. As stated, I have a standard "lumpy" MG Metro with header and SS exhaust, but no Dynamat, and I don't have the vibrations you describe. My mid-80's car has poly subframe mounts and my only major rattle/vibration complaints are for the 3-clock insrument panel at idle.

Update.  Thanks to everyone for there ideas.  I tried almost everything that was posted.  Thats why it's taken a while.  I found the panels are resonating, worst in the firewall, floor area.  Coming from the front solid mount subframe.  Added dynamat to certain areas, floor, firewall, doors and it's made a big difference already.  I will be installing more.  I can live with it like this.  I feel better knowing that it's a mini 1275 thing and not just my car or something I did wrong during the build.  Thanks again. 

Put a jack under the gearbox and lift it slightly just taking the full weight off the mounts (use a piece of 1' x 1' thick plywood between the jack and the gearbox so as not to damage it). If it still vibrates loosen up the subframe mounts and see what happens, it is just a process of elimination but some Mini's especially 1275's vibrate more than others.

Thanks for all the great info.  I have a maniflow LCB header, i looked and the exhaust isn't touching anywhere.  I'm going to try the roof blanket idea this weekend and will let you know.  It's a like drone or a buzzing in the cabin, starting to think it's the body panels having a slight vibration from the motor.  Will investigate more this weekend.

a solid mount subframe -BL/Rover went to flexible subframe mounts for a reason... you're probably experiencing it.

vibration gets a little better when revved but the resonation stays the same if worse. - +1 for exhaust system touching somewhere. Make sure hangers have resilient (rubber or equal) components - no "hard-to-hard" contact.

You mentioned things like the steering wheel vibrating at idle. Are you now talking about resonance (hum, drone or roar) in the cabin? Sound absorption materials work in two ways: add mass or dampening (like the dampers on piano strings) to panels to stop them vibrating in sympathy, or absorption (like carpet, underpad, upholstery etc.) to collect sound waves travelling through panels or in the air in the cabin. Dampening materials are heavy and dense. Absorptive materials are light and porous. Adding material to the roof will stop it from drumming, but that doesn't sound like your problem. You can test-dampen the roof by tieing or taping something like a heavy blanket (e.g  movers pads) on the outside to see if it makes any difference.

 What do you have for an exhaust header/manifold and exhaust system. A cast iron exhaust manifold absorbs a lot of the noise coming out of the ehaust ports which are pointed at the cabin; steel tube headers just transmit the sound, stainless is worst (apparently). A stainless exhaust pipe below the floor is harder than carbons steel and transmits sound more.
What do you have for muffler(s) - twin box is much quieter than sngle box and standard-types are quieter than peformance.

I have a plain steel Freeflow header connected to a stainless single box RC40 system and it is loud in the car when at speed.

Drive with a passenger who can explore to see where the sound is actually being generated from. They can change seat positions and search for where the vibation is loudest or for things not properly secured. Sitting in the driver's seat, it is not easy to verify where sound actually originates.

Is there any chance that your exhaust is touching the tunnel? 

Thanks for the replies.  I have a new harmonic balancer on there now and tried old ones to compare, no difference.  Did the same with the motor mounts, new and old.  I believe it's a big journal motor.  Does have a metro cam in it, but don't think the motor was balanced.  The first motor i tried was balanced.  The car has a solid mount subframe (75)  The vibration gets a little better when revved but the resonation stays the same if worse.  Would not having sound proofing or dampening on the roof be a factor?  I did do door panels and floor.  I just want to make sure my car is acting and sounding like other 1275 minis out there.

For what it's worth my Mini with a MG Metro (high compression) un-rebuilt transplant vibrates some things like you list at idle. It has the stock-for-it cam which is decribed as being lumpier. To smooth out the engine a little, one of the improvements was to set valve clearance a bit bigger than spec. ( I don't have the info handy). At idle in my car, forget about the rear-view mirror unless you want vertigo, the instrument panel squeaks, the sterring wheel hums.

That suggests to me that part of the problem may also be the cam you installed.  Another issue is that everything resonates at some frequency or other. You can adjust the generating frequency (idle speed) or the resonant frequency of the individual items. Tighten fasteners or mount on absorptive material (rubber grommets etc.), add or remove mass, add dampening material to metal or plastic panels, modify shapes (stiffening creases in sheet metal), etc.

What subframe nounts to you have - solid or flexible, rubber or poly?

My daily driver (base model Suzuki SX4, 2WD CVT transmision) is a nice, quiet car, until I'm stopped at a traffic light, when the economy seats (why am I always in the "cheap seats"?) become the luxury vibrating ones. At well over 200 lbs, even my weight can't dampen the vibration. It would be not too bad, except I suffer from muscle fibrillations... I don't need vibrating seats too!

So the simple answer is ... don't sit there idling! DRIVE 'ER!

 

My first mini was a 1960 850 with a balanced and blue printed engine based on the books of Clive Trickey. As happy and smooth an engine as I have ever had. I traded the car complete plus $300. for a 67 Cooper S less power unit. I picked up bits till I had everything to build a proper Cooper S power unit. By then money was tight and I rebuilt the engine but failed to have it balanced. Once assembled and in the car I strated it and adjusted the timing and carbs. Before taking it for a drive I noticed that the steering wheel, rear view mirror and the dash were buzzing. I have always been a if it's not what I want I don't want it type. I pulled it back in mom's garage pulled the power unit stripped it down ordered new rings and had the assembly balanced. I always have a English 4 cyl. balanced when I'm building it. Steve (CTR)   

do you have a broken motor mount?

Or  check the harmonic balancer on the fan end of the crank. It's a disc in a disc with rubber molded to isolate the vibration. If it's shot, it could be the cause.

Start with the simple stuff.

does the vibration go away when you rev it up?

1275s all vibrate more than the smallbore motors do. But, some are worse than others. It depends on what's in the build- conrods? Small journal or big journal crank? Flywheel weight?

I find the big journal crank A series motors with their heavy lump conrods and poor crank counterweighting have more vibes than the small journal ones do.
Lightening the rods and flywheel, wedging the crank and doing a good balance job minimises the above.

Well where to begin?  1975 mini restored with a rebuilt A series1275.  It has a resonation/ vibration in the cab, when idling, its like sitting in a drum. (not that i have done that)  Steering wheel, heater, dash all vibrate.  Didn't happen with the 998. I have tried everything i can think of.  Different old and new motor mounts, bulkhead mount, engines steadies, harmonic balancers, even a different 1275 motor was dropped in.  I have not driven in another mini.  Is it the motor?  Solid subframe?  Are they all like this?  Any suggestions would be appreciated.