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MG MGB Technical - Just thinking of a rebuild when whump whump whump

That'll teach me to day-dream. I have a new and unwelcome noise from my GT. It's a low, bass "whump whump whump".

I only hear it under load and it goes when I lift off, but it's been getting louder all week. It was initially from about 3000rpm but now I hear it from 2500rpm, repeating at a rate of maybe five times a second.

Performance recently has been very lacklustre, which I put down to a visibly worn cam (valves on #1 cylinder don't move much in comparison to the others).

I am almost set to rebuild, but would appreciate a guess on whats wrong and how many miles I have before I've nothing left to work on!

Thanks,

Steve
Steve Postins

Steve, sounds like it's time to park it before you have a crank break or a rod come through the side of the block and you really won't have anything left to work on. If the cam lobes are that worn theres a lot of metal that's been circulating through that engine and I'm sure the bearings have suffered some damage as well, that would account for the low pitched noise, a loose main bearing.
That or the kid next door just got a new stero system in his car! Lots of bass Whump Whump Whump from those as well!
Bill Young

Many thanks Bill. Sometimes you just need someone to tell you!
Steve Postins

Steve, as you know its difficult to diagnose a noise without actually hearing it but you say five times a second. Does the frequency vary with engine revs or is it road speed. Five times a second seems rather slow for an engine problem and I have a feeling that it could be a UJ on the propshaft that is causing the problem. This does not excuse the fact that the camshaft is in dire need of replacement though.
Iain MacKintosh

I can't really separate road speed and revs as it's only audible in top and quietens as soon as you take the load off the engine. It sounds a bit like when you ahve a whel out of true but without the vibration. Cam was ruined 20,000 miles ago so you're right about it being time up for that.
Steve Postins

Sorry, on refection it's not like a wheel out of true, more like a tyre completely flat.
Steve Postins

Have you looked at the Harmonic Balancer?

Just a wild guess.
Daniel Wong

If it's only on load it's unlikely to be anything that is load independant like the harmonic balancer. What does it do in different gears? If it is at much the same rate in 3rd as 4th at that rpm, then it is going to be in the engine or input side of the gearbox. If it is at a slower rate then the output side of the gearbox, propshaft etc.

It does sound like when my son's Metro had a worn big-end though, sort of a thump felt through the floor as much as anything. And see this from John Steinbeck in 'The Grapes of Wrath' - "But that thudding as the car moves along - can't hear that - just kind of feel it. Maybe oil isn't gettin' someplace. Maybe a bearing's startin' to go..."


Paul Hunt

Thanks for the suggestions. I've listened really carefully and can just hear it in 4th, and it is at the same rate versus rpm, so engine/input side.

Paul, you must have an encylopaedic knowledge of literature as well as MGs! It's not a bad description too. If we're going for prose I'll revise mine to it being the akin to whump you get from a sturdy pair of leather bellows used on a faltering log fire.

Anyway, I'm going to take the hint and park the car. I'll let you know what lurks inside when I open the engine.
Steve Postins

I do read quite a lot, Steve, but most of it goes 'in one ear and out the other' so to speak. Two passages in Steinbeck's book really struck home though so I copied them out, the full text of them can be found at http://www.mgb-stuff.org.uk/wn_hammerframe.htm, below the Golden Rules.
Paul Hunt

This thread was discussed between 01/05/2009 and 04/05/2009

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