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MG Midget and Sprite Technical - Pinking

Chaps

Went for a spin in the glorious spring sunshine yesterday o'er hill and dale amongst the hosts, and lo!... a pinking noise from the engine under load. Died off around 2700 rpm, but quite noticeable on acceleration.

Qs therefore

Will pinking cause overheating? I did note the rise in T faster than normal in temperature when going uphill, and the electric fan came on once or twice - it didn't before....

Any ideas as to root cause of this pinking.

Coolant is fine...topped to the brim
Distributor is electronic and positioned correctly off the cam drive
Plugs OK, leads are performance silicon things
Rotor arm and contact points are clean
Oil fine
Fuel only 3 weeks old


I fiddled about with the valve clearances recently opening the exhaust vvs a wee bit to about 14 thou - inlet (nominally) still 12 thou. It is a modded head with a fast road cam and 16 thou is what is actually advised for both - I think. Strangely it seemed to run a wee bit better with exhaust at 12 thou.....would duff clearances bring on pinking?

Marvellous in the sunshine though!

Obliged as always


Mark
Mark O

If your clearances are too tight there's a chance the exhaust valve may use up all the clearance when it expands with heat- and an exhaust valve that doesn't close doesn't have a chance to release its heat into the valve seat, which can cause the valve to get so hot it creates pinking. Valves that get that hot can also burn (partially melt) and crack.

If you change the cam you really should run the clearances the cam manufacturer advises. Tighter clearances will make the valves open earlier and close later, so it may well make more power, but the consequences of too tight a valve clearance are fairly unpleasant.



Growler

Growler - understood. It is a bit odd though that I have opened the exhaust out a little to about 14 thou - after a previous chat with Guy. Beforehand at around 12 thou I did get a very small amount of pinking, but now it is definitely more noticeable.

I will check the ex valves again and open them further if required ....
Mark O

The symptom is usually one of the distributor haviong more intermediate advance than it should, usually due to wear in the advance springs and/or their posts. Or if full electronic, it may not have the correct curve selected. You say electronic distributor - completely, or just electronic points? If the latter then it still relies on the original mechanical advance mechanism.
There are other causes of pinking, depending on whether the engine is modified or not the requiremens for plugs and intermediate advance can change. Or it could be as simple as wrong fuel octane as has been found in the past. See my website at http://www.mgparts.co.nz/advice/technical-notes/combustion-problems-and-detonation/ and http://www.mgparts.co.nz/advice/technical-notes/distributor-advance-rate-adjustments-compensating-for-high-mileage-wear/
Paul Walbran

Try retarding the timing slightly. Turn the distributor anti-clockwise by a couple of degrees, lock the clamp and retest, say uphill in 4th with the engine struggling a bit. You want it to be almost at the point where pinking starts, but not quite there!
Dominic Excell

Dominic and Paul

Yes I appreciate the point. It is the 123 ignition kit. I have set the distributor body by simply rotating it until the best rpm was achived on a high idle. Will try a couple of degrees CCW and see what happens.....
Mark O

Mark,
as your engine isn't standard, as Paul has put, have you got the 123 on the right setting for your car

Paul also mentioned octane, IIRC? your engine is higher compression so may favour 99 octane

as I missed it on Dave's and being a doom and gloom merchant and as you mentioned coolant topped up - and other other suggestions are much more likely on your car but on a very off chance, keep your eye out for signs of HGF, complete wild card though
Nigel Atkins

***** I have set the distributor body by simply rotating it until the best rpm was achived on a high idle*****

OH gez, ...That does NOT sound good...

doing this method, did you also blink your eyes really fast to see the timing marks, or did you use only your ears to time the ignition ?

bottom line.... you need a timing gun pure and simple, otherwise your just wasteing your time and trashing your engine...even a decent basic timing gun isnt all that expensive...maybe $30

part 2...

do you have the "123" or the "123 tune"

remember... higher octan burns slower, meaning a longer burn time... meaning a longer detonation time and a more complete fuel burn, meaning your dissy dosnt have to advance as high or as fast to avoid detonation

Vac leaks....that makes a hotter engine temp and makes faster explosion

but considering your timing the engine thur magic ear hearing technic, im guessing its a timing issue... to much ear wax can distort the engine sound at various rpms

prop
Prop and the Blackhole Midget

Mark
Traditionally, there was no vacuum advance at idle, so the engine was always in retarded mode - so setting it by turning the distributor to get highest idle RPM always results in too much advance everywhere else. (It took me a while to figure that out when I was a young fella, so the crank handle I used to start the car - broke student, dodgy battery - always threatened to break my arm with kickback :-))

Later on, for example rubber bumper B's, the vacuum was taken off the manifold so at idle it was on full vacuum advance and the turn-the-distributor method could probably be used reasonably accurately (I never found out, by that time I had enough money to buy a timing light!)

Where this is leading is that if your 123 vacuum advance is set up like the early distributors then who you have set it will always give way to much advance and could damage the engine under full power.

Because the engine is much more sensitive to wrong advance when under full power, it is best to set the timing at maximum advance. This involves using a timing light, but compared with the cost of a 123 it's only a bit extra and it will get the full potential of the 123.


Paul Walbran

I disconnected the vacum of my hose of my 123 while under full load and yes no pinking. Be careful retarding is a cure, advance is not always the cause. As Growler said a local heat spot can cause pinking (valve, valve seat, coal or crack). But you have pinking at low refs and full load, this makes an early ignition at low ref likely.

Flip


Flip Brühl

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prop
Prop and the Blackhole Midget

Paul

Thanks for that - much appreciated. To clarify, I rotated the distributor body to achieve max revs with vacuum disconnected at high idle - by high I mean about 2000 rpm. I think was is a throwback to old motorbike points methodology, but I appreciate it is far from ideal.

I think I will reset the static advance according to 123 instructions and as you suggest, invest in a strobe...

obliged as ever

Mark
Mark O

Mark,

I would definitely set the valve clearances to the cam supplier's recommendations and also make sure that the timing has a total mechanical (vacuum disconnected) advance of no more than 32° at 5,000rpm. I would do both before driving the car again.

Are you using standard rockers? If you are using 1.5 ratio rockers, then the valve clearances need to be wider - 0.012" -> 0.014" and 0.016" -> 0.019".

At 2,700rpm (when it goes quiet) you are getting towards the 'high speed detonation' zone - you can't hear the pinking (detonation) any more, but it can kill an engine very quickly.

Richard
Richard Wale

Mark,
I think you'll need to check/reset the other stuff you done before

check/set tappets, (plugs), timing and mixture in that order on a tank of 99 if that's what you're going to run on

you may or may not need to change the plug (rating) (I can't remember how far you went with these)

as for hotspots that's why I like to keep the coolant/heating and oil systems as clear and clean as possible to help matters
Nigel Atkins

Wow
Every post here seems good! Lots of knowledge and all useful! How come that doesnt happen when I ask a question?
Steven Devine

Chaps

Will reset the static timimg on the 123 as suggested in earlier post. At the risk of incurring great wrath from various professional engine tuners on this forum, is there a relatively easy way of establishing best dynamic timing - without the use of strobe and fddling about with timing lights?

Being aware of the discrepencies between what should be and what reality demands, I still maintain a deal of faith in if it sounds, feels, performs OK, then it probably is OK. Thus if I rotate the distributor to relate to some sort of performance indicator - such as going uphill majestically with no pinking, popping or overheating, then I may not be that far out.
Mark O

static timing is just to get the engine started after a rebuild or replacing the dissy...beyound that it dosnt do or mean anything...if your car starts, you dont need to bother any further with static timing

I dont understand what the resistance is to timing lights... it both common and comical from so many new midget owners, its easy to use, fairly accurate, trouble free, and not very expensive

any chance you would consider selling the car instead of destroying the engine...try a pawn shop, they have tools cheap, maybe a rental tool store....im sure there has to be someone close by that you can barrow one for a few hours

this is such a sad thread... I feel for you, when you blow up the engine, its going to be a sad day and yet was so easily preventable

seriously... if your not going to use a timming light, there is really no point with any further fiddleing ...just drive it as is....there is no advsntage to be gained

as they say... experiance is the best teacher

prop
Prop and the Blackhole Midget

btw mark...

most aftermarket dissys have a dwell angle of 35 to 45 degrees...the factory lucas dissy is set at 60 degrees of dwell angle

I dont know what the dwell angle is for the 123 / 123 tune

but you will need to account for that dwell angle differance when timing the engine by ear...I dont have a clue how thats done...

good luck

prop
Prop and the Blackhole Midget

Prop

Dynamic timing reference points are an ideal. I don't have one for my modded engine, although I appreciate that dynamic at 32 deg at 5000 mechanical as kindly advised by Richard will probably be adequate.

Thus in the light of not having such reference points for my particular engine, and in seeking to compensate for a non-ideal situation, my question in seeking an alternative method to set dynamic is entirely valid.
Mark O

you dont have a timing mark pointer

im at a loss for words....that is not good.

are you sure... the pointers are on the timing chain cover on the bottom side of the cover...I want to say 5 spikes

they do make a pointer kits that allows you to see the marks from up on top....peter may has them over there

in my opinion, with this engine being high performance....it really should be set up correctly

I really think you should invest the time and set up timing marks if they are not there

if your not, id run high octan with cold heat range plugs, and retard the timing severally other wise you can burn a hole in the top of your pistons or screw up the cly head and the very least blow out a head gasket....I definatly would NOT race this engine until you get timing marks installed and ignition properly timed with a light

hang in there
Prop and the Blackhole Midget

This thread was discussed between 15/04/2013 and 17/04/2013

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