Welcome to our resource for MG Car Information.
MG Midget and Sprite Technical - Clanking noise and misfiring
Well i set off for MGLive this morning with car running like a dream, after about an hour the throttle stuck and I had to switch off the ignition come to a halt an fix this issue (end of cable snagged on the heat shield it appeared). I then set off again throttle working fine and almost immediately started to get a clanking noise from the engine and misfires. The clanking noise sounds like a nut in a tin can no rhythm to the sound but it sort of sounds like something free in the engine that is hitting against something that is rotating in a random fashion. I know this sounds like a child's description but how do you describe noise? I had the car recovered back home and am now wondering how to approach this problem. Any suggestions very gratefully received (Off to MGLive in the modern day estate car tomorrow which is just not the same).
First thing I would do is remove the spark plugs and rotate the engine by hand, inserting a probe of some sort - a stick, pencil or similar into each cylinder in turn. This would be to check that the pistons are all going up and down as they should.
Just a guess, You may have a broken con rod, with one piston stuck at the top of the bore. The random clanking being the end of the conrod rattling against the side of the bore.
|Have you checked the oil for metal fragments etc? This could help in identifying whether a con rod has gone.|
|Busted rod usually takes out the block or stops dead. I'd pull the rocker cover and look for broken pushrod, rocker, valve spring. Also look in the distributor - sometimes the advance mechanism comes apart with similar result. (I once had a Spitfire come in with the entire distributor cut in half by the advance - drove in with nothing but the wires holding the top half in place. But that was a Delco distributor.)|
|easy things first!|
Take off the rocker cover to check the rocker gear/push rods
put the car in gear and push it to see if all moves as it should.
If all is fine there pull of the sump and check the rods and crank.
As said alway check the easy things first!
Fletcher, I have had a con rod break up near the gudgeon pin ('though it wasn't on an A series) and the piston stay at the top of the bore. The broken end of the con rod simply thrashed around inside the bore and actually did very little damage. And I have also had a con rod punch its way through the side of the block, and that wasn't an A series either!
Onno, You are right, easy things first. Removing the spark plugs and checking with a pencil or screwdriver that the pistons are moving seems pretty easy first step to me!
|Yes Guy but that can give a very false reading. |
With a broken piston or rod the piston will still move up!
And a broken bolt or cap can create very strange noises without catastrofic failure
| Nobody mentioned a spun bearing|
...vary doable esp if the throttle gets stuck wide open
But I agree, pick the low hanging fruit 1st
Its hard to belive a broken rod in an A-series...those rods are just so meaty, but I never thought id see a crankshaft bust completely in half at the #3 main location ither
I hope its not a big issue
Indeed, but it doesn't match the " nut in a tin can no rhythm" very well. Busted rocker or even just a dislodged pushrod does, especially with the stuck throttle - near certain over rev. I think all the bust A Series engines I've seen came to a sudden stop, at least until they created some clearance.
Onno -"With a broken piston or rod the piston will still move up!"
But it won't go down!
Once knew a guy blew up a Ford flathead V8, drove about 50 miles home, rattled a bit and had a miss, not bad if you kept the revs down. When we took it out, you could look in the exhaust port and watch the rod go merrily up and down.
|Hmm, l did day it was only a guess. Just a possibility, but very easy to check or eliminate with just a plug spanner and a pencil ! And no oily gasket to reseat.|
But a pushrod problem would certainly be a better outcome!
|Well I have removed the plugs and no 2 plug has clearly been hit by the piston..... The gap has been reduced to nothing and actually the top of the electrode is now bent over the central core of the plug (I am trying to upload a photo which seems impossible on a macbook) . I have carried out the pencil test and all 4 pistons move up and down the bores without any problems. Any further ideas and suggestions for my next move. I was going to drop the sump next is this a good idea at this point in the diagnosis ?|
|On the cylinder with the bent plug electrode, does the piston go back down the bore while the crank is rotated, or is it pushed up by the con-rod and then left there as the rod goes back down?|
If the piston does stay at the top then obviously the con-rod is broken or the piston has become detached from it in some way.
The piston moves up and down the bore as I turn the crank. I have taken the rocker cover off and all seems ok here
|On an A series I thought the piston could not hit the plug (unless a lot has been planed off - not sure even then ?) therefore it could be that a 'foreign object has gone down into the cylinder - i.e nut etc.|
I have seen this before with a nut bedded into the piston and again something else spat out the exhaust valve !
This ties up with your 'nut in a tin can'.
I would take the head off for a look - but first can you see anything missing from the air cleaner cans etc ?
|If you crank the engine with the plugs out do you still hear the sound?|
Do the valves go up and down?
|You ought to be able to see the top of the piston through the plug hole, and if something is embedded in it. One possibility is a valve head, broken in the over rev, or a dropped seat if it has had inserts fitted. Another is piston rings. I had a Spitfire that had been run for a year or two with broken rings after having been "stuck" and freed up. The rings finally ate their way out the piston top, and pieces of ring were hammered into the piston top and cylinder head, and also traveled through the manifolds to other cylinders and did the same there.|
You do have air filters on?
Doesn't really matter, it's head off at minimum.
|If there is some metallic "foreign body" you might be able to gather bits of it up with one of those little magnet-on-a-stick things, via the plughole. But in truth the only option - and next logical step, is to remove the cylinder head for an inspection. Its pretty quick and easy anyway and you will learn a lot about what caused the problem, and the extent of any damage.|
|Taken the air filter off and cranked the engine. No sound of the not in the tin can. Cannot see the top of no 2 piston but all of the valves seem to be moving as expected. Probably need to get the head off to be sure of the condition of no 2 piston. Am tempted to stick the plugs back in and start her up (not sure what this will achieve though). should I just bite the bullet and get the head off?|
|I would certainly not start it up until you have removed the head and checked inside. You may have just got away with it as it is. But if there are loose fragments in there it could do a LOT more damage!|
Be patient. Remove the head, you know you should. Its only half an hours work.
|Head off am pull the oil pan....is my vote..take photos|
|As Guy said - pull the head off to find out what killed the spark plug before doing anything else.|
|Sounds like a piece of piston. If you free rev an engine in neutral with no load (like with the throttle stuck open) the piston will go up a tiny bit further than normal. If the engine has got a few miles on it and the bore has a good size ridge at the top of it the ring catches on this and it can break the ring land or perhaps in this instance taken a piece out of the top of the piston.|
|You Guys are Legends. I have taken the head off and have found two foreign objects in number 2 bore. Photo attached. Anyone got any ideas where this might have come from. I have left the carbs attached to the head at the moment.|
|A closer picture of the cylinder invaders. Any ideas?|
|It dosnt look like it came from the lower end, and no appeart damage to the cly....im not sure this is your issue, they look like they have been there awhile|
Was there any damage to the seats of the intake valves
As to what the pieces are....thats a puzzle,
On another note....might be time for some new rings
|Can you upload a pic of the no2 head chamber or better still take out the valves to no2 and photograph the seat as well.|
Check nothing missing from carb/air cleaner - though the bits don't look obvious.
Prop - they must be part of the problem - bits can't live in a cylinder for long without causing major grief !
|Okay I got a guess, but its not a good one|
Its those golf ball tees in the smog port of the cyl head,i cant imagine how they swam agianst the exhuste current to end up there, but thats my guess
|I got to agree with you richard...|
but those 2 parts are about 1/4 inch max and it dosnt look like the oil residue on top of the piston has been been disturbed, if they where fflying around, id think there would be cly wall damage and the top of the piston scared up
but thats just my guess and its a pure speculation ... But im glad to know we are thinking the same on the valve seats
|Is it by any chance the center electrode from the dead plug?|
Are they magnetic?
Clean up the top of the piston and the head, looking for dents. Also mouse nibbles at the piston edge,though it does not look like ring fragments. Even the Spitfire mentioned above did not have bore damage (except the rust rings where it had been stuck) though all pistons had broken top tings and a couple had two broken.
Just tested the bits from no2 cyl and yes they are magnetic. There is nothing missing from the carb butterfly's. There is a very small dent in the piston but not right at the edge, the piston edges look perfect. I will take the valves out over the next couple of days when time permits. I am really puzzled how these bits got in......
|Id still be tempted to pull the oil pan for a look inside|
the fact that they are megnetic sorta explains why they were not washing around in the cly...
Not the plug electrode?
Came to me I don't know if they are magnetic or not, and it is likely different for different plugs. I think they are typically a nickel alloy (possibly magnetic), sometimes welded to a copper core, sometimes with tips of platinum or other. So far, I am unable to think of anything else that general size & shape.
|PJ does the plug you took out actually STILL have its centre electrode in it?|
Fletcher offered this question but you didn't actually say.
You describe the outer electrode as closed over the centre, maybe the centre dropped out and then bashed the outer one over to hide the hole.
I would take that plug apart to find out, I think some makes of plug have two part electrodes, maybe with a carbon radio suppressor inside which would pulverise and disappear if it fell into the chamber.
I really cant think of any other likely source of two handy small steel rods than the plug.
What make of plugs are they?
They are Champion plugs and the centre core is in tact the electrode is bent over but the centre core looks fine. I will take out the valves tomorrow and post some pictures. I really appreciate your thinking with this mystery, here is the photo of the plug.
|Time to definatly drop the oil pan for a better look, I think you got deeper issues|
Those 2 little pieces of swarf wont bend the spark plug electrode....its hitting the piston top, or something bigger was in the cly.
Im leaning more toward a rod/piston/crank/or bearing issue....
The only other way I can see that you got piston slap, was from really high rpm and the block was flexing....but even then thats more theory in my view
Im starting to think its a bottom end issue....drop the pan.
|Any chance the piston could have been droped or damaged when it was installed the last time....as in the air gap got bent out of spec...easy to do if your not careful|
I agree with you - well worth a check at this stage as clearly something is not as it should be.
Although damage is minimal "up top" Its still a mystery as to what those pieces are and exactly how they got there.
Of course, finding one fault doesn't mean it was the ONLY fault.
Its still possible that those small bits found in cyl #2 did damage the spark plug, but were NOT the cause of the engine clattering noise
|Wow, what a mystery. Those bits look almost like needle bearings.|
Could they by any chance have come from one of the carburetors?
Needle bearings are a great guess,,,,but I cant imagine where they would have come from....its like a splinter in the back of your mind
|>Any chance the (((piston))) could have been droped or damaged when it was installed the last time.<|
Opps,,,, sorry, please replace th word (((piston))) with the word spark plug
When I had my MoT at the beginning of June I had the garage crypton tune the car so that is possible if they removed the plugs. I am trying to find the time to get the valves out tomorrow (wife has list of jobs for me damn it)
|So whats the ... "Breaking News"|
Now thats what I call a pun...:)
|I have at last found time to remove the valves. photo attached is the valve seats no2. The engine is an Ivor Searle recon with about 10K on the clock.
|Now the valves. No2 exhaust and inlet.|
I plan to grind them in a bit with fine paste before re-assembly.
|Could it be a split pin (remnants of) or the end of one of the throttle return springs (shaped like a shepherds' crook?)|
I once bought a 'breaker' car that had an awful engine death rattle and it was a filter-to-carb bolt that had been ingested. It happened during the time it was in the hands of a well known tuning company. The bolt was beautifully polished, as were the port and intake valve!
PJ. Is the little piece soft enough to bend with pliers (splitpin) or too stiff (spring steel)? Is it flatter on one side?
one piece is 7.1mm long the other is 8.1mm long and they are both 2.5mm in diameter. They are hard and look like they are hollow where they have worn through the outer skin of steel. They are both magnetic.
This thread was discussed between 25/06/2011 and 10/07/2011
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.