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I have a '62 MK1 and recently dropped in a rebuilt 1275 (out to 1310). It's an everyday driver and loving it, except the harmonic vibrations in the range of 1800- 2200rpm (edit) is significant. I've only had small bore mini's in the past, but girlfriend did have a GT Clubman many years ago, and I don't recall it suffering as much as this. Everything was balanced, have standard 1275 harmonic balancer, standard flywheel, road cam, HIF44 carb, exhaust etc.

Most people ask me if I have solid mounts! I'm driving around in 3rd gear all the time to keep it above the threshold at 60 k/h. At over 3000 rpm it's pretty sweet, at 4200 it's as smooth as an A series could ever be and just hums quietly along.

I've done search here and found lots of interesting things, but can ask everyone's thoughts about changing the harmonic balancer to a ROMAC or other? Would swapping out the flywheel to a lighter one help - or make it worse. Any other thoughts?
Many thanks.

Does it vibrate at that RPM Range regardless of throttle opening ?

ie, in the cruise as well as acceleration ?

Are you sure it’s harmonic? My money would be on it not being in tune.

Spider wrote: ↑Sat Apr 18, 2020 9:20 am Does it vibrate at that RPM Range regardless of throttle opening ?

ie, in the cruise as well as acceleration ?

Yes, I even feel it when I slow down from higher rpm i.e. if I take my foot off the accelerator when slowing down from a higher rpm in any gear, it's there as soon as it gets into that rev range. Once it falls below, its smooth again. It's also there just revving the motor through the rev range.

I don't think it's tuning, because it pulls like a train even in that range.

OK cheers.

It doesn't sound like a Harmonic Resonance that a Damper would sort out and these you can't usually feel in the way you are.

To me, and I hate to say it, it sounds like the Balance job you've had done isn't up to scratch. They need to check the bottom end for balance at several speeds, and balance for that. I'd have a reasonable guess here that it's only been done at a single speed.

Spider wrote: ↑Sun Apr 19, 2020 3:51 am OK cheers.

It doesn't sound like a Harmonic Resonance that a Damper would sort out and these you can't usually feel in the way you are.

To me, and I hate to say it, it sounds like the Balance job you've had done isn't up to scratch. They need to check the bottom end for balance at several speeds, and balance for that. I'd have a reasonable guess here that it's only been done at a single speed.

Hmmm, I didn't really want to hear that. Thanks though, much appreciated.

Try and check for simple things first

Are you sure the exhaust is not touching somewhere for example?

Was the clutch and back plate marked and did you assemble it the same way as it was balanced?

That's part of the problem, I didn't do it myself. The engine builder (very reputable) assured me all was balanced and it's just that these big bore engines do suffer from this, but mine just seems extreme. It's a shame because it takes a lot of the enjoyment out of everyday driving. Maybe I'll have to pull it down and go through the whole exercise again.

Definitely no exhaust touching, no broken or worn engine mounts, in fact everything is new on it having gone through a full re-build. Even replaced engine steadies with original rubber bushes.

Jt850 wrote: ↑Mon Apr 20, 2020 9:12 am That's part of the problem, I didn't do it myself. The engine builder (very reputable) assured me all was balanced and it's just that these big bore engines do suffer from this, but mine just seems extreme. It's a shame because it takes a lot of the enjoyment out of everyday driving. Maybe I'll have to pull it down and go through the whole exercise again.

Definitely no exhaust touching, no broken or worn engine mounts, in fact everything is new on it having gone through a full re-build. Even replaced engine steadies with original rubber bushes.

I don't know if this is relevant or common but I recently had a flywheel, crank etc full balance. When I sent the flywheel/back plate to the machine shop I'd lined up the 'A' marks on the diaphram and the back plate but when the unit was returned the diaphram had been rotated and new paint marks put to show correct placement. After a bit of thought I came to the conclusion that the machine shop had rotated the components to achieve the best balance location before actually doing the 'proper' balancing sequence. To my mind that meant that they had to remove less metal than if they'd stuck with the 'A' mark locating position. So now I have white paint on all parts, including the release bearing pad with wire clip.
Maybe if your machinist had done similar but the new marks were not noticed on assembling with centre plate then the whole thing could be out of balance.
Discuss.

Could it be that the Crank Damper has failed ?

They are two pieces and can fail, that rubber is bloody old now......for sake of £90, might be wort replacing it, cant do any harm.

http://www.minispares.com/product/Class ... o%20search

Spider wrote: ↑Sun Apr 19, 2020 3:51 am

To me, and I hate to say it, it sounds like the Balance job you've had done isn't up to scratch. They need to check the bottom end for balance at several speeds, and balance for that. I'd have a reasonable guess here that it's only been done at a single speed.

I have never seen anybody with a machine or technique for achieving this? How many speeds and how fast slow?

And, you can't balance a 4 cylinder anyway.......

You can only balance the parts, and, even a lot of that is questionable.........

Main item is the crank/flywheel/damper assembly should be balanced, con rods end for end, and pistons one to the other if you want to make yourself feel good.......

If you are running a vacuum Dizzy, disconnect the vacuum advance, plug up the vacuum port.......My guess is, the problem will go away, or, be dramatically reduced.......................or not, but, it's quick and easy to check....

Just one other thing here to check before getting too serious, take the fan belt off and try it. By the sounds of how it is running, you should be able to do this test without the need to drive it.

bwaminispeed wrote: ↑Mon Apr 20, 2020 11:21 pm And, you can't balance a 4 cylinder anyway.......

You can only balance the parts, and, even a lot of that is questionable.........

Main item is the crank/flywheel/damper assembly should be balanced, con rods end for end, and pistons one to the other if you want to make yourself feel good.......

Spot on !

Hipwell wrote: ↑Mon Apr 20, 2020 8:58 pm

Spider wrote: ↑Sun Apr 19, 2020 3:51 am

To me, and I hate to say it, it sounds like the Balance job you've had done isn't up to scratch. They need to check the bottom end for balance at several speeds, and balance for that. I'd have a reasonable guess here that it's only been done at a single speed.

I have never seen anybody with a machine or technique for achieving this? How many speeds and how fast slow?

The machines I've used in the past and the guys who do mine now have an infinitely variable speed machine, just dial it up or down.

How many speeds ? At at least 3, but as many as you like.

There was a thread a little while back where this was covered and why.

Just thinking what cam/ rockers are you running,
Are you in the uk?
D

Had a problem many years ago - basically the flywheel was not sitting on the crank very well. Once it was lapped in the problem went

Saw a video a couple of years ago where Vizard talked about reducing vibration and really liked a viscous damper. Bought one, installed it and liked the result. Now I’m able to use the rear view mirror to read the license plate number of the car following me.....
Edit-
For what its worth, I found the video; David Vizard; Tuning the A Series Engine. He talks about engine vibration/viscous dampers from 1:28 to 1:34 in the video. The video is an interesting watch but most of the subject matter is way over my head.

Thank you all so much for your suggestions and thoughts, really appreciated. I’m in Australia. The cam is a “mild street” cam - will find out exactly what it is though, but it will idle quite happily, smoothly and without any vibration at 800rpm. Standard rockers. New electronic distributor - no vacuum advance. Big journal crank. Crank damper not new though.The shop that did it is very well respected, and they assure me it’s all ok, just big bore harmonics, but I’m trying to get my head around why it has to be so severe given the build is nothing extreme, just bored 0.040” with the few additions. Although it does go very well and it is an absolute joy to drive at 4000rpm Feel free to tell me to just suck it up, as that’s what it is. But thinking that maybe I should have gone for a nice warm 998 then?

croc7 wrote: ↑Thu Apr 23, 2020 7:26 am For what its worth, I found the video; David Vizard; Tuning the A Series Engine. He talks about engine vibration/viscous dampers from 1:28 to 1:34 in the video. The video is an interesting watch but most of the subject matter is way over my head.

Any link to that video croc7?

The whole lot can be seen here:
https://www.youtube.com/watch?v=ow5cGV7bXCw