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MG MGA - Chasing a ghost

I feel like I'm chasing a ghost.

Is this plinking??

I have a rattling noise that sounds like ball bearings in a cylinder.

It occured during acceration and sustained running in the 2500 to 3000 rpm bandwidth. It would also appear when I let my foot off the gas and the engine fell through that bandwidth.

The sound had pretty much gone away earlier in the season after my first tuning.

I then realized that I had tuned the carburators too rich. So I leaned them out slightly and the sound reappeared. I then retarded the spark and the sound has diminished somewhat however I've reached the limit of retarding the spark without dramatically effecting acceleration.

One item to note (a clue)is that I've noticed that the sound gets worse if I have a passenger over 130 pounds in the car.

As an added test to see if this is not plinking, I disconnected the vacuum advance and the sound continued to occur!!

What in the name of Old Speckled Hen is this???? I'm starting to drink (more) and mumble to myself.



T McCarthy

Ball bearing combustion noise is knocking, or pinking.
Too far advanced, too high compression ratio, low octane, or carbon deposits.
Art

Running lean causes detonation too. My car also hates 95 Octane fuel, but that's because it's a high compression engine with a Supercharger, and running on 98 solves the problem immediately.

Tysen, you ned to tune your car from first principles, setting timing, mixture, plug gap, points gap and idle in the sequence explained in the manual. Fiddling ad-hoc gets you nowhere slowly. It may seem a pain in the ar*e, but it's really easy to get it all running correctly if you start at the beginning and do everything is sequence, balancing the carbs on the way.

If you can't get it right, take it to someone who knows how t do it (are John Twist or Carl Heidemann far from you?). I showed someone how to do thsi a few weeks ago, and he says his car runs much better since then.

dominic clancy

In support of Art's comment, try backing off the timing, some premium fuel, or both and see what happens.
GTF
G Foster

Those are the exact symptoms and times in which they occur for pinking.

This could be from carbon deposits if the engine has had many miles of use and still has a build up in the combusion chamber but is most likely because you are not running higher octane. Here in the States, look for 92 octane fuel minimum. It is also best to try and find non-oxygenated fuel which burns different then corn-gas. Here in MN, the pumps are labelled as such and also read for collector cars and off road use only.

As mentioned above, there are other possibilities but using cheap fuel to save $2.00 when you fill up is the most common reason.

-BMC.
BMC Brian McCullough

Thanks Dominic,

I don't think I've ever seen the order of things described in the owners manual but intuitively it seems that it should be:

Mixture
plug gap
timing
idle (air balance here)

Is that the correct?

GTF and BMC, for fuel I always run 93 minimum and usually 94 from Sunoco.

I just went out to get a color tune to see if I can get the mixture right and drive everything from there.

BTW, as an added bit of info the noise is particularly noticable in 4th gear when I let off the gas and its still in gear and there is a little bit of engine braking....particularly on down hills.

Tysen
T McCarthy

An additional thought...Tired centrifugal advance springs could allow full advance too soon.
FWIW,
Doug
D Sjostrom

plug gap
valve clearance
points gap
timing
throttle balance
mixture
idle speed

FRM
FR Millmore

The order is not really important, apart from setting up the carbs, which is a subroutine that mus be done in the correct order, and which is explained very well in the manual. What is important is to set up each individual item to the correct spec as described in the manual. Note that timing will NOT be the same as the manual if you are using unleaded fuel.
dominic clancy

I wouldn't expect pinking to happen when you let your foot off the gas as described in the original post. Possibly a rattle caused by engine vibration which may be more pronounced at those revs?

Does it happen at those revs with the car parked and the bonnet up? If so it would be worth having a close listen around the engine bay. I had an annoying rattle which turned out to be the spring on the bonnet catch being the wrong way round and therefore close enough to the body to hit it when vibrating at certain engine revs.
Keith Morris

Big progress today.

The engine was running significantly rich. (the carbs probably need a rebuild) In any event I put the gunsons Color Tune on and it was running a sloppy orange color, I leaned it out considerably. The car doesn't like a full bunsen blue setting, which is likely a result of worn jets and needles. I synchronised the air flow and got the timing right.

Its no longer makeing the noise under acceleration, though it does make it slightly when the car engine brakes down a hill.

Thanks to all....I'll keep you posted.
T McCarthy

A little more.

This morning the car made "the noise" while idling (which it has done in the past under various settings of timing and mixture)

The noise sounds very much like the clatter of a starting diesel engine.

It still doesn't do it under acceleration, only when there is slight engine braking and the occassional at idle.

Is this plinking or is this valve flutter do you suppose?
T McCarthy

Check that the bolts holding the fan blades and water pump pulley are tight
dominic clancy

If it's not during acceleration, then it's not pinking, in my estimation. At idle and when you're decelerating in top gear sounds like something else entirely.

How's your oil pressure? If oil isn't reaching the valve train, you'll get more noise and probably damage there. But if the noise isn't constant, I wouldn't think that's the problem, although that can sound like a diesel clattering. Maybe just normal noise from the valve clearances? Loose timing chain?

As I'm typing here, I tend to think maybe a transmission thing or loose clutch shaft or something like that. But it sure doesn't sound like preignition/pinging/pinking, whatever you want to call it.
Tom

I had a similar noise from the engine which only occured when slowing down. Eventually I tracked it down to the fan which only had a tiny clearance from the edge of the radiator. If the engine moved forward under braking, one fan blade just touched the surround with a pinging noise. - Check it out.

I have known the genuine sound of a ball bearing in the cylinder head. Many years ago as a kid, my dad took the family to Scotland in our Humber Hawk. On a remote mountan road the engine suddenly sounded as if it had a ball bearing in it! After much head scratching at a village workshop they removed the cylinder head and sure enough there was a ball bearing, and the top of one piston was pitted all over. The culprit? a previous owner had installed a "patent" fuel saving device in the inlet manifold. It was a small turbine which rotated in the mixture and it was mounted in a ball race. One ball missing!

David
David Marklew

I too thought pinking only happened under load - I recently replaced my dynamo and had to fit the cooling fan and pulley to it - it made a slight tinkling noise when first tried as the blades touched the body of the dynamo - could be worth checking - good luck in your search - cheers Cam
Cam Cunningham

Thanks all.

I'm starting to focus on something at the front of the engine in the fan/generator area.

I just caught the noise happening while the bonnet was open and it sounded like it was coming from that area.

So far the fan bolts are tight and the blades are not hitting the radiator.

Whatever the noise is it varies in intensity depending on the mixture and timing of the motor. Before it was happening at 2500 to 3000 rpm, now it happens at 2300 rpm.
T McCarthy

Check that the crankshaft pully bolt is tight.
John H

I had a noise that came and went, I added a drop or two (2) of oil to the back of the dynamo it went away??
Jones

This thread was discussed between 06/07/2007 and 10/07/2007

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