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MG Midget and Sprite Technical - Vaibration on over run

Hello all. I hope you all had a great weekend and got out and about in the midgets.
I didn't manage to get the car off my daughter so no fun for me.
Anyway, the next issue I need to sort on the car is a little rumble/vibration on over run. Take your foot off the throttle and there is often a metalic rumble/vibration, not always but in certain conditions that I have not nailed yet. Usually around 50-60 mph an din top gear I think.
It feels and sounds like it is coming from the prop shaft or rear of the gearbox.
The engine is new (1275) and the gearbox was reconditioned by the PO and is not that old.
The wheels are wires and slightly buckled but have been properly balanced and there is no vibration when driving.
My question is how to identify where the problem is. If, as I suspect, the UJs on the front of the prop shaft are worn how do I confirm this without taking the prop shaft off. Once I have the propshaft off I might as well replace the UJs as they are not expensive. Do I need to drain the gearbox first?
What else could be causing a rumble? It stops as soon as the clutch is pressed or the throttle is opened.
Thanks for your help.
Dave
D Brown

Dave I'll leave the more technical to answer



just thoughts for now

intermintent could be because of transfer of weight and effort moving along the car forward and back

checking underneath would be a good idea in case it's just loose bolts somewhere (prop, UJs, rear springs)

assume your rear axle is fully topped up with oil

I know you'll have to mark up the prop so it's fitted back in the excact possition it came out to keep the balance

I think you can feel if there's movement in the UJs

I know you don't have to empty the gearbox but as you're bound to loose a bit of oil (Sod's Law) and want to top up you might as well replace all the oil in the gearbox if it's not been done in the last three years
Nigel Atkins

Dave

I am sure you have thought of this but could it simply be a piece of metal panel,trim, heat sheild, exhaust etc. This could be resonating at the particular frequency of vibration you get on over run. Any change in this frequency ie when you depress the clutch or increase the revs would cause it to stop.

From my limited experience these can make a suprisingly loud noise.

Tim
tm wainwright

does indeed sound like a UJ problem
You can try getting underneath and lifting the uj to see if you can decern any play, or rock it slightly/gently to see if that highlights issues.

As you say UJ are very cheap and easy to change so it is sometimes worth just eliminating the possibility if nothing else shows up.

The oil will run out of the gearbox if the prop is removed

And sorry to Nige but marking the position of the propshaft is a waste of time. It only fits in 2 positions and the balance is IMO an urban myth from whence it came I have no idea?
Robert (Bob) Midget Turbo

You can also sometimes eliminate other possibilities by repeating the same test - 60 down to 50 on the overrun, but do so slipping the car into neutral to take engine and clutch out of the equation. Also try "weaving" the steering a bit to see if the noise is accentuated which would indicate a wheel bearing rather than the prop shaft.

I would suspect the UJs on the propshaft.
But I would contradict Nigel on some of his comments - worn UJs may well not actually feel slack when you grasp and try to move the propshaft. They can also run dry and get tight, which also causes vibration.
There is no need to mark the propshaft before removal. It is a balanced unit in its own right so won't go out of balance. It only bolts up in two positions at the rear flange anyway - so check that all 4 bolts slot in before putting the nuts on or it will inevitably be wrong.
What you do need to mark is the yolks before tapping out the old bearings. These do need to re-assemble in the same relative positions to maintain the balance. Maybe that is what Nigel meant You can get away without draining the gearbox if you jack the rear of the car up quite high to gain access, but this may be higher than you would normally jack it so be sure that it is well supported.
Guy

Bob as you know it wouldn't be the first time my memory has been faulty, I should prefixed as I normally do with - I think

At least I got the oil leaking right

Still worth checking all nuts and bolts are tight first, costs nothing but a bit of time and effort

The 5 minute edited is far too short for me to get my posts correct or even the way I mean them
Nigel Atkins

Sorry Guy missed your post please contradict, you know I get things wrong (or a very few times type wrong)

it's many years ago I helped someone with prop or UJs
and I thought we had to mark something

I did put I think about the UJs but didn't put possibly I'll try and remember next time

I mainly posted to get the thread noticed more

I dislike working on cars dirt, messy business that I'm no good at and have next to no knowledge about, I've got involved in it when I've no other choice because of the time, location or lack of more money (as now)

nothing wrong with the basics though like checking all nuts and bolts are tight and changing gear oil if it's not been done for a good while
Nigel Atkins

And carefull with jacking up the rear end very high. Gearbox oil can run out on to the clutch.
Alex G Matla

what Alex says. I'm lucky that my drive has a slope.. when the back jacked up the car ends up horizontal so no oil dribbles into the bell housing!
David Cox

Thanks for the warning Alex.
I always used to rely on tipping the car at a steep angle when removing the propshaft and never had any problem with oil seeping into the bell housing. Don't they have an oil seal at the input shaft? - or maybe its a question of what condition the oil seal is in?

I now have a type 9, so the problems are different for me these days, but worth knowing for my half-Sprite.
Guy

Guy, no seal at the front end. That's why there's a hole at the bottom of the bell housing with a "keep it clear by rattling" spilt pin in it..
David Cox

Pretty sure mine didn't leak out of the front when I tipped it up, seal or no seal. Well, not in any quantity. Maybe the oil was to old and thick and gloopy!
I know about the split pin in the bell housing, although I know it mystifies some people. But it was a common feature in many bell housings, including those with gearbox input shaft seals!
Guy

Thanks for all of your replies.
I am taking it out for a spin tonight to a local pub that is having a classic car meet. I will try to work out exactly when the vibration occurs.
I don't think that it is a panel vibrating, it sounds too deep for that.
I suspect it is the ujs, I don't really know why. I think I am going to have to replace the ujs just to eliminate those from the equation.

Thanks again.
D Brown

This thread was discussed between 11/04/2011 and 12/04/2011

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