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MG MGB Technical - Good lord!

My car, as you know, has been running rough recently. It hasn't been revving cleanly and has been very rattly at idle. Today it was only running on 3 cylinders, and just barely that, when I was driving into work. I just about made it in. Anyway on my lunchbreak I removed the no2 spark plug to see what was going on and the insulator was shattered and the electrode bashed in, as if the piston had whacked it. It was like this on both no's 1 and 2. No's 3 and 4 were fairly OK actually. Anyway I got a set of N12YC spark plugs instead of the N9YC ones I was using and I didn't tighten them up as much this time. I leaned out the mix a fair bit (1 and a quarter turn) on the front carb and all of a sudden the idle stabilised, the engine stopped rattling and it revs much cleaner now. Now, what I would like to know is, is it likely that the piston did hammer the plugs or could this have been caused by detonation? This was only going on for a few days. Is it likely to have caused any lasting damage anywhere?
Ross Kelly

Ross, it sounds like detonation, you've probably got a carb problem on the front carb causing it to run real lean. With luck the damage is limited to the plugs, the colder plug will help, but you really need to take a look at the carb before anything else is damaged. What colour were the remains of the insulators? If they were very light brown or white then it's definitely a lean condition.
Bill Young

No they were black and sooty. The 2 back ones were light tan colour and looked fine. Since I wound in the mixture screw a bit it seems much better. I have a new set of carbs on the way anyway cause the ones I have aren't a matching pair, so that should sort out my problems.
Ross Kelly

Screwed in or out? On HIFs screwing in richens, screwing out weakens.
Paul Hunt 2

I screwed them in. OK so I richened it, but the car is running pretty much fine now so whatever I did it worked. When I fit my new carbs, Im going to get them close enough and get a professional to tune them properly.
Ross Kelly

Hello,

Just to pinpoint a little fact: N12Y are hotter than N9Y.
It is better to run with colder plugs.

Cheers,

JGC
Jean Guy Catford

You shouldn't need to run with either hotter or colder plugs than the standard N9Y in a standard MGB. If you do need hotter plugs i.e. to 'cure' fouling, it implies something else is wrong with the engine.
Paul Hunt 2

I dont know, maybe I tightened the plugs up too much cause the washer on the plugs were flattened. Is there really that little clearance in the combustion chamber that such a thing could cause the plugs to get whacked by the pistons? Its incidental that the plugs are hotter. I got them cause they were marginally shorter.
Ross Kelly

No, they are not that close, although I don't know what the clearance is in practice. Detonation shockwaves could cause the insulator to shatter, but I'm surprised the outer electrode was flattened on to the central one as you imply. I'd check the distance by removing the plugs and turning the engine so each piston is at the top and measuring, maybe you have non-standard pistons. Then you will need someone else to measure theirs :o) If it was whacked by the piston you may even see a dent in the crown. Then there is the quesation of what caused the detonation.
Paul Hunt 2

The only time I've heard of this was when something got dropped/sucked into the carb. Seeing as it's afffected two cylinders I guess you may be looking for two things or one thing in several bits. It would be interesting to know what the top of those pistons look like and if the car has air filters in place?
Steve Postins

Ross,

Even with flat top pistons there should be no problem with the right choice of sparks. We tried out different brands and it looks like the most promising were NGK for the B-Series engine, although some Bosch plugs gave very good results too.

With a stock engine and correctly set carbs and the right dizzy (abvance checked on a testbench) nothing should go wrong with the original Champion sparks. If the dizzy does not meet the engies requirements to the point (weak bearings, springs or isnsufficiantly working vac. capsule) and low ocatne fuel is used, severe damage to the sparks and the top of the pisons can be a result of this kinds of defects. It is not a fault of the MG engine, it is something of a 'tribute to age' and modern fuel formulations.

Ralph

Ralph


Ralph

I forgot to add that your comment about it being very rattly might also suggest something was bouncing around inside the cylinders?
Steve Postins

This thread was discussed between 09/04/2008 and 13/04/2008

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