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MG Midget and Sprite Technical - Engine or gearbox noise

OK. I know that it must be just this side of impossible to identify a noise without hearing it but give it a go please.

All this year I have experienced an engine or gearbox noise but I can't tell the source. The noise is a mechanical tapping that is engine speed related, occuring from around 3000-3500 rpm. It happens in all gears. It appears under load but stops if I lift off the throttle. If I release the clutch and rev the engine I cannot reproduce the noise.

The noise sounds just like a very noisey tappit (it does not sound like little-end knock or a big end rumble) but I recently fitted a new rocker assembly and triple checked valve clearances and neither made a difference.

I thought it might be 'pre-ignition?' but I have tried three different distributors with increasingly 'performance' advance curves (the last being an Aldon Yellow) and retarding the ignition by various degrees and nothing seems to make any difference. The gearbox was professionally reconditioned around 3K miles ago so should be OK but who knows.

Is there a way of positively distinguishing the source of this irritating and potentially worrying noise. If it is the gearbox I will simply drive it till it gives up but if it is the engine then that is more of a concern. Any suggestions???
Chris H (1970 Midget 1275)

timing chain?
David Smith

UJ's?
G Lazarus

I too have thought of UJs but it's definitely not UJs as I checked those earlier this year and they were solid (much to my regret as replacing tem would be easy). I had not thought of the timing chain - so is that a possibility? How would I check and isn't that an engine out job?
Chris H (1970 Midget 1275)

Chris - Funnily enough, my 1275 has a similar noise to yours. Sounds like an old OHC Ford Pinto engine with a knackered camshaft but not quite as loud.
I'd rule out the gearbox as the source but wouldn't rule out a little end knock, especially as it's only there under load. All my A series engines have sounded a bit 'tappety' in the past but yours sounds as though it's slightly worse than that.

Matt
Matt

Manifold leak?
Jeremy Cogman

Matt - your description fits the bill very well. It does sound like a knackered OHC Ford. I hope it's not little ends. Perhaps I'd better get that spare engine rebuilt!

Jeremy - I'd like to believe that it was a manifold leak but how does that lead to a tapping noise?
Chris H (1970 Midget 1275)

I've had some hard to stop rattles, ranging from the passengers headrest, rubbing throttle cable, worn pedal pivots to oval spring hanger mounts.
What tappet clearance are you using & are you on a standard cam / rocker ratio?
If your still on a standard gearbox i'm surprised you can hear yourself think at 3500rpm let alone distingush any other type of noise.
Brad
B Richards

Everything is sweet up to around 3-3.5K rpm and then a distinct tapping. Beyond that it is true that it is hard to hear anything very clearly. It is very clearly related to engine speed in any gear so I think we can rule our door rattles and the like.

I'm using a standard cam with high lift rockers (new) with valve clearance set at 15 thou.
Chris H (1970 Midget 1275)

My first thought was little end except you said "it does not sound like little-end knock"
David Smith

I had thought that little end knock was a bit more high pitched but I may be wrong. From the suggestions so far I'm beginning to think I might be wrong and it could be little end knock. Howver, I don't understand why there is no knocking when the engine is just revved and not on the road.
Chris H (1970 Midget 1275)

I have what I reckon is exactly the same issue but haven't managed to diagnose the cause.

The engine runs great but the rattle starts at around 2.5 - 3k rpm. It seems to disappear under heavy load but returns on a light throttle.

At first I thought it was the clutch release bearing or fork but it doesn't seem to stop if I depress the clutch peddle (a bit difficult to be sure because of all the other noises).

I am concerned that it is little ends but not sure how to properly diagnose this without stripping the engine down (I suspect this is the only way!).

Does anyone know if little end noise should disappear under load?

Rob
Rob Storer

Rob

I would have thought that heavier loads would give rise to more mechanically-induced knock.

Don't forget one difference between heavy load and light load is vacuum advance, i.e.

- no vac adv on heavy load

- vac adv on light load - and if too far advanced for CR (etc) this could give a knock.

This might be an explanation. When I installed a high CR cylinder head, I had a knock that was so bad I had a look at the big-ends, which were fine; so I retarded the ignition (a long way) and it went away - and so did the performance. The fix in my case was a different dissy curve.

That's why your knock could be different from Chris', which sounds like a b/e or little end.

BTW Chris - dropping the sump allows inspection of b/e's and (to an extend) the l/e's. Might be worth a go. Having done this many times, I can say that a drip of engine oil directly into the eye is a joy.

A
Anthony

Anthony - thanks for the comments. If it is a little end bearing it is probably the result of trying to catch you on the way to Spridget50!!! I'm interested in what you say about dropping the sump. I thought you could not do that on a 1275 or have I got that wrong? If it is possible, how do I do it?

I'm also wondering if the world is full of midget owners whose cars make a tapping noise! If so, should we worry?
Chris H (1970 Midget 1275)

Chris - The reason I thought it was more likely to be little end is that you think the noise is there when the engine is only under load. That's when you're more likely to hear it knock - it's less likely when you just rev the engine when it's out of gear.

The noise mine makes is there regardless of whether it's under load or not.

Cheers, Matt
Matt

Well this gets stranger and stranger. I'm working at home today and it's lunchtime - so time for some fiddling with the car. I check the oil in the gearbox and changed it to on the safe side. A trip around the block confirmed no change in the nasty noise. On a whim, I removed and blanked off the vacuum advance. Bingo! No noise!! What gives? That suggests knocking from too much advance. But why when under load (perhaps I unconciously take my foot off the throttle when I hear that noise)?

I don't understand this, as performance seemed as usual. I always set up the ignition as suggested by someone on this BB (4500 rpm and set mechanical advance to 30 degrees). I currently have an Aldon distributor supposedly for a fast road set up and my previous distributor was supposed to have an advance curve tailored to my set up (it was elecronic ignition) but both produced the same rattling/tapping noise at 3000-3500 rpm.

Obviously the short term fix is no vac advance but what are the implications of that (poor fuel consumption I suppose). Perhaps I need a trip to a rolling road sooner rather than later.
Chris H (1970 Midget 1275)

Ha! brilliant. You usually only hear pinking/detonation when the engine is under load anyway.
Do you use an octane booster in the fuel? You could offset how far you have to retard it by using the high-octane shell unleaded and/or an additive. Unless you already use these of course.

Funnily enough I'm at home today too ('gardening' leave unfortunately). However, the sun is out so may go for a run in the Midget!

Matt
Matt

Vac advance works in the opposite way to many people's idea... the less throttle, the more vacuum, and the more advance needed (since the flame frotn travels slowly with less compression, you need to advance the spark so the bulk burn happens at the right time).

So on low and mid throttles, the vac advance is (well) advancing the ignition. It adds to the mechanical advance through the bob-weights. If you have too much mech advance, then the additional vac adv may cause pinking or detonation.

It seems removing the vac is a temporary fix.

I'd check the static timing which should be around 7 deg BTDC.

I'd also check you have the correct spec of adv unit, since there will be at least 2 different types: early one for direct connection to the inlet manifold; and later ones for the 'ported vac' - connected to the carb bodies.

Using a ported vac unit straight to the inlet might give rise to too much adv.

BTW - the detonation due to over-advance you were hearing is like hitting the piston on its way up with a sledge hammer; best avoided.

A
Anthony

My vac advance is connected to a take off in the carb (HIF44) body. If this is detonation then it has been happening for aged (gulp!) - probably since I fitted my Metro cylinder head when I was first concious of it.
Chris H (1970 Midget 1275)

This thread was discussed between 24/09/2008 and 25/09/2008

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