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MG Midget and Sprite Technical - ignition or gearbox

I've run out of ideas - any suggestios regarding a problem I have.
For the past 18 month-two years I have been trying to cure what I have thought was pre-ignition when the engine is pulling around 3K-3.5k. There is a definite metallic 'chattering' when pulling at those revs, although lifting off the throttle makes the noise go away. It does not always do this but it does it much of the time. Sometimes 'thrashing it' makes the problem go away (the opposite of what I would expect).
Over the past year or so I have tried everything I can think of to cure this. I have:
* retarded the ignition (sometimes excessively)
* fitted richer needles in case it was running lean
* fitted an Aldon yellow dizzy for a better curve
* fitted a 123 dizzy set on its least advanced curve
* use super unleaded petrol laced with octane booster
* fitted colder spark plugs (NGK BP7ES)
* driven with and without vacuum advance.

No matter what I do - the problem seems to remain. I have reached the point where I am wondering if I have diagnosed the problem incorrectly. There seem two possibilities: a worn little end bearing or a gearbox problem. Neither seems likely to me (little end noise would be all the time surely and why would a gearbox chatter at just 3-3.5K revs?

For information: 1275, MG Metro head (p+ported), HIF44, LCB, maniflow exhaust.

Any suggestions gratefully received (and apologies for the long thread)

Chris
Chris H (1970 Midget 1275)

Let someone have a run in the car that knows what pinking sounds like, just to be sure.
Robert (Bob) Midget Turbo

Spridgets can have all sorts of weird noises.

A couple of the more usual ones are...gearlever zizzle and rattling centre caps on Rostyle wheels.
Dave O'Neill 2

How about the BENDIX rattling (tinkling) The little buggers have been known to do that sort of thing.
Sandy
Sandy Sanders

Have you made certian that the exhaust system is up to par, no leaks. worn muffler, loose bolts, Cracks in the header....warped header flange, (that will do it every time) missing/worn donuts, loose/unsecured exhast pipe, rock or debrie in exhaust pipe system

Prop
Prop

... exhaust touching chassis/body?

A
Anthony Cutler

Timing chain?
Brad (Sprite IV 1380)

Thanks fellas - I can discount most suggestions. There are no worn donuts in my car, they get eaten too fast (thanks Prop). Exhaust is in good order and not touching and I don't have Rostyle wheels. I might check out the Maniflow LCB for cracks - that might do it. My main concern is that it occurs under load at the rev range that is usually associated with pinking. I can't get the chattering when the car is stationary (so probably not the timing chain then) but when stationary it is not under load. As obvious as it seems, I agree with Robert that the best bet is to get a second or third opinion from someone knowledgable about midgets. Chris
Chris H (1970 Midget 1275)

Chris

Your attempts to eliminate detonation in your first post all seem logical, and should have done the trick if that was the cause.

The only other explanation seems to be mechanical - esp. as 3-3.5K would seem to be max torque for your engine. Assuming that some torque-induced movement of the engine is responsible, then the problem should be worse in the lower gears (since the torque created is multiplied by the gear ratio).

So... is the noise the same or worse in lower gears?

Seems to have been going on some while - would expect big or little end noise to get worse over a period.

I'm down in Malvern if that's any help... and plenty of hills here to accelerate up to create the right conditions....

A
Anthony Cutler

Chris,

I reckon I have the same problem that I’ve been struggling with for a similar length of time so I’m watch this thread hoping to get an answer!

I get this noise at a constant throttle opening, if I accelerate hard it disappears and like you I can’t get it to make the noise when stationary. The following list of things I’ve done over the last 2 years and that haven’t made the slightest difference may help with the process of elimination:

Replaced gearbox
Fitted anti-rattle kit to gear leaver
Changed from twin SUs to single hif44 and back again
Compression test (results good)
Fitted electronic ignition

I’m now wondering if it’s a little end or maybe the clutch / release bearing / arm. The frustrating bit is the cars drives really well.

Please post if you get a fix, I’ll post if I make any progress.

Cheers

Rob
Rob Storer

Hi I would be tempted to having the car checked/set up on rolling road , a good rolling road would tell if , where and when its pinking in the rev range also your end up with possibly a quicker car and probably a smoother driving car , just a thought :-)
Darren 2 litre frogeye

What about the diff and/or UJs....?

Only mentioning it because from experience where you think a sound is coming from isn't necessarily where it is coming from.

Think I read somewhere (maybe even in the archvies here!) that very knackered UJs on their last legs can produce a metallic screeching/tinkling etc, think the diff can produce similar noises.
James30

This thread was discussed between 20/05/2009 and 22/05/2009

MG Midget and Sprite Technical index

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