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MG Midget and Sprite Technical - Pinging at Idle

Hey folks,

Just wanted to throw this out to the masses for a quick brain storm...

Started car up today for the first time in a few months. Been working on tidying up a lot of the wiring, getting the dashboard properly installed etc.

Tried to dial in the carb (DCOE) a little better to get it idling OK. At about 1200 rpm everything sounds OK. Just the clatter clatter of the valve gear.

Winding down the throttle stop a bit to reduce idle I get to 1000 - 1100 rpm I get a very different metallic "tinkle tinkle" sound. Pinging/pinking I guess? No amount of additional fuel on the carb screws seems to stop it. Rev the engine and it is OK.

Pretty sure timing is OK, although didn't get chance to check (will check next time I get a helper), so I am thinking manifold leak.... right? Anyone have a good method of testing this?

Or should I be checking something else first?

Cheers,
Malcolm
M Le Chevalier

Malc.
tappets, timing then carb...

But if you do have a manifold leak, spray something volatile around and listen for changes in engine speed. I think WD40 works but easystart would be better.

Have you had the engine apart? Could it be a timing chain or tensioner?

Best of...
MGmike


M McAndrew

Just a thought M.....
As pinking (pre-ignition or detonation) is often, but not always, a symptom of ignition timing over-advanced and you got it when you reduced the revs; might it be the vacuum advance not backing off when the revs drop?
IE: could it just be a bit sticky from it's few months rest? Simplest thing to check for.........
RS Hughes

Tappets were checked.

Will double check timing. Mike, you have reminded me that I did have the front apart to try to sort an oil leak. I can't remember rechecking timing once reassembled, so maybe that is the problem after all...

Hmmm... lose track of what has/hasn't been done!!

Malc.


M Le Chevalier

Probably carbs are set to lean, needs richening up

I always keep a timing gun set up to check timing as I adjust the carbs, the more rich and faster the carbs are at idle the more advanced the timing becomes....the opposite for lean carbs

Prop
Prop and the Blackhole Midget

Malc. Do you own a mechanic's stethoscope? Even with the vacuum advance stuck full on (a quite uncommon failure as the common failure is no vacuum advance due to ruptured diaphragm) there should be no audible pinging at idle. At idle, or any other steady state operation, there is high manifold/carb vacuum and the vacuum advance should be operating fully. Hence, you would, always when working properly, have full vacuum advance at idle. This is why, when setting the initial ignition timing, you disconnect the vacuum line at the carb and plug it. I would look for timing chain slap, which sounds like a tinkling sound similar to pinking/pinging, and is most commonly heard at idle as the engine noise drowns it out at higher engine speeds.


But, I would check the initial ignition timing regardless of what you may find/hear. It is the starting point of any accurate tuning.

Les
Les Bengtson

OK, thanks for the other suggestion Les. Will look into it.

Chain is definitely new. I am pretty sure I changed the tensioner at the same time, will check the photo archive... Whats the cure for chain slap then?!

Malcolm
M Le Chevalier

Malc,
chain slap is fixed by a new chain and/or tensioner. If these have been changed and the tensioner installed correctly (they are a simple affair but a right bugger on the 1500) all should be okay at the front end of the engine.
It's very unusual to get pinking (pre-detonation) without the engine being under load unless the timing is way out, in which case it would be a bitch to start. Double check the timing and the vac advance is working correctly then try to pinpoint the source of the noise.

Best of...
MGmike
M McAndrew

Sometimes the starter Bendix gear will ping (rattle).
Sandy
SANDY

Thanks for the suggestions one and all.

Starts fine (once warmed up), fires on first turn. Needed a bit of nursing whilst cold, but choke isn't hooked up yet.

Another thing I thought I need to check is the crank case breathing set up. I run the breather to the manifold via a PCV valve. Perhaps the PCV valve is dodgey, rattling, or malfunctioning causing leaning of the mixture. It does only take it's feed from one or two cylinders so maybe it is clicking on/off with the engine pulses. Didn't sound like that though....

Vac. advance isn't currently attached. Vac from the two DCOE inlet tracts is attached, via a tee piece, to a vac. gauge at the moment. This was for scientific interest/diagnostic purposes and the intention is to hook up the vac advance to this eventually instead of the gauge.

Have a good idea of what to go searching for and check, but won't get chance to look into things until the weekend though. Boo!

Cheers,
Malcolm
M Le Chevalier

In fact... does the vac reading tell us anything more. It was about 12 - 15 inHg, flickering around that level, never stationary. Should it be more steady?

Malcolm
M Le Chevalier

hmmm... sticking valve? but why only at lower speeds? and why did I not notice it when turning engine over by hand to check valve clearances...?
M Le Chevalier

Malcolm,
don't forget to also check the basics before moving on especially if you can't remember what's been done/checked before (I know that feeling very well) check/adjust in this order - tappets, CB points plugs, carb(s)

also check for condition and fit rotor arm, dissy cap all HT leads

IIRC your car isn't road ready(?) so different in your case but I'm not a big fan of just starting a car up without driving it as often just driving the car on a reasonable length journey can iron out or help discover faults and causes, sometimes a few journeys are needed to do both or either

I don't know but could it also be that a DCOE isn't as suited at idle and lower revs as the SU(s), or it doesn't like the PCV or its set up, or you're just noticing more now with initially the lack of choke
Nigel Atkins

whoops - I missed out timing (after plugs and before carb(s))

so - tappets, CB points, plugs, timing and mixture in that order and if you have to adjust any item in that chain then you’ll also need to check, and adjust if required, all the items that follow it in the chain
Nigel Atkins

Maybe a stupid question, but are the bolts holding fan and pulley to the water pump tight? That makes a pinking-like rattling noise too.

Otherwise pinking happens because the timing is too advanced and/or the mixture too weak. Weak mixture can be caused by an inlet leak or insufficient fuel delivery.

Are you sure that the mixture screw adjustment is actually doing anything at all? I know nothing about Webers so beyond that I can't help
dominic clancy

Opps...

I missed that... your using a weber?

Is it a new weber off ebay, if so I hear they are crapola, there made in china with high china standards, best to use the old classic weber
Prop and the Blackhole Midget

Right...

Valve clearances OK. Plugs sooty, so surely not lean mix.

Couldn't check ignition timing as no key turning helper this evening.

However, Sandy may be the winner...

Gonna start another thread!

Malc.
M Le Chevalier

At low idle, there's not so much 'oil-splash' and a worn little-end can sound just like pinking - but with slightly less treble. Hope it's not that but it has been known.
Nick Nakorn

This thread was discussed between 08/02/2015 and 09/02/2015

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