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MG MGB Technical - Ping Ping Pinging

I have a 1967 MGB recently rebuilt..timing is set to 12 degree BTDC, idle is fine, low end power is good when I hit 3000 RPM the pinging or rattling begins...i have adjusted the lifters several times..looking for ideas????

Retard the timing more?
Possible timing chain?

DW Don

Did you discinnect the vacuum advance when you set the timing? If you have a vacuum guage, set your timing to the highest vacuum reading as a baseline and try adjusting from there.

Jeff Schlemmer

Could be so many things...get yourself an adjustable timing light and accurate tach and then you can determine exactly what your distributor is doing over the full rpm range. The manual gives advance numbers for various engine speeds, try to hit these. You might be surprised what you learn (I was). Good luck! Cliff
Cliff Maddox

You stated the motor is recently rebuilt. What is your compression? Too much compression can cause the pre-ignition/pinging you are describing and at 3000 rpms the distributor is advancing the timing via the centrifugal advance and causing this problem.

Ray
Ray Ammeter

Could it be fuel starvation? I had gross jets installed and my car shuttered at higher rpm. Replaced with original needle valves and problem went away. Also, try 20 degress BTDC @ 800-1000 rpm. Hope this helps.

Paul
Paul Hanley

Hi all.

I also have this problem when the engine is warm, even on high octane fuel. There are some previous threads on the subject.

My investigations suggest that the problem may be one of centrifugal advance coming in before the vacuum advance has gone, but I haven't proved this yet.
Weak mixture at this engine speed / throttle position is another possibility.
The pinking will continue indefinitely if the throttle position is held.

New (stronger)centrifugal springs did not provide a cure.
I guess my head may have been skimmed more than once, so CR may be a factor, but the actual cylinder pressure is not all that high at part throttle, which is when I have problems... snapping the throttle open eliminates the pinking.

Removing and plugging the vacuum advance pipe OR setting static timing to 0deg does.
Spraying water into the air filters helps a lot.

I guess that Cliff's method, maybe combined with some observation of a vacuum gauge while driving, is the ultimate answer (short of a rolling road).
I have a vacuum gauge awaiting fitting.

Don
Don

Perhaps you can check your sparkplugs or try some colder ones.
Arthur

Weak or missing centrifugal advance springs can cause this, and adjustable timing light is very helpful in checking both the mechanical and vacuum advance curves. Whilst I have spent some time and effort compiling a table of original curves they were arrived at with the fuels of the day and with all engine components expected to be within a known set of parameters, and even then with mechanical systems the actual curves only approximated to actual requirements, and manufacturers erred on the side of caution. Certainly todays fuels are very different, and I would hazard a guess that the vast majority of engine components are also different, possibly not even the correct distributor etc. The end result is that setting the timing must be by trial and error unless you are prepared to spend a long time on a rolling road. Failing that the best one can do is to have the timing as advanced as possible without pinking or pinging at any combination of throttle, revs and loading that one normally encounters. Even then you can come unstuck if you normally drive in East Anglia (flat as a pancake) then take a holiday in the West Highlands of Scotland. Mine pinks at around 3000rpm on part-throttle on hills when set to the book values, so I have to have the timing backed off a bit. My particular symptom - increasing the throttle stops the pinking - indicates that there is too much part-throttle vacuum advance, so I fitted a capsule with a less aggressive curve which improved things a bit and has allowed me to increase the overall timing a little bit more hgence increasinmg performance and reducing running temperatures, but ideally it needs one that reaches its maximum advance even later, I'm toying with the idea of fitting an additional return spring.
Paul Hunt

Hi again.

I found that colder plugs helped a little, but made the car grouchy until it had warmed up.

Don
Don

Don,
depending of the coil,
normal coil : gap 0.6 mm (0.025 inch)
"hot" or sport coil (mor Kv) : gap : 0.9 mm (0.035 inch)
with the NGK BP6ES or Champion N9YC

Arthur

It appears to be an advance curve problem. The earlier distributors (non smog control) had much less centrifugal advance. The later had the initial timing set with much less advance (retarded) and as a result had to have more mechanical/centrifugal timing advance to get the timing up to the proper setting at mid engine operating speeds. Result: If you set those "smog" distributors based upon static setting for an early distributor you will have too much advance at operating speeds.
I messed with a smog distributor and got it to work much better. I put bushings around the pins to reduce the travel of the centrifugal weights and lighter springs to bring in the advance at lower rpms. In the end it turned out to be easier just to use a pre smog distributor.
The quick fix is to retard your initial timing. That will hurt your low end performance but work fine on the road.
There are also different vacuum advance cannisters. They can differ in the amount of advance/retard, and can differ in the amount of vacuum required to trigger retard. A mechanical/vacuum advance distributor can be tailored to give good economy and part throttle performance, along with max full throttle power.
Finally: How about an electronic ignition controlled by a knock sensor?
Barry
Barry Parkinson

DW Don: You didn't say in your original post what the octane rating is of the fuel you are using. The advice above is excellent, however, I would eliminate the simple things first before digging too deep. Running the best available fuel (I use Sunoco 94), retarding the ignition by trial and error worked for me. The 25D distributor has a vernier adjustment that will allow you to retard the ignition by small increments for fine tuning. I also find that the NGK BPR6ES work best. Finally, I also replaced my worn out distributor with a new one, which made a big improvement.
A.K. Blackley

There are a few issues surrounding this subject but we must first of all assume the engine to be in good mechanical condition and to original specification. Additionally it must not be in need of a decarb as this significantly increases tendency to pink. Make sure that engine operating temperature is not excessive as this too can have detrimental effects.

Having said this Paul seems to have hit the nail on the head. The condition of the distributor is vital and it is common for the advance weights/pivots to accumulate a fair amount of slop over the years. This together with weakening springs means that too much advance takes place too soon. This cannot be disguised by retarding the ignition as poor low down performance then results.

It is also most important to ensure that the correct distributor is fitted, confirming that it is a 25D is not sufficient and you must ensure that the correct part number is installed in order to ensure correct advance. This number is on the side of the unit.

Today's fuels are lower octane than those for which this engine was designed and in addition lead, which was also a good anti knock additive has disappeared from our fuels. Whilst the original setting was 12/13 degrees BTDC if the above is all correct you should be able to run happily on a 10 degree setting without loss of performance. Under these conditions the engine may pink very lightly if you move off quickly at too low revs, this is certainly not detrimental and really only indicates that your settings are optimised.

Over advanced settings will encourage engine run on especially if the temperature is high and the idle speed much over 800rpm unless of course your US spec car is fitted with an anti run on valve.



Iain MacKintosh

Knock sensing electronic advance would be the ideal, IMHO, as being a closed-loop system you would always be at the optimum timing for every possibly variable you can think of. The mechanical and electronics to do the actual advancing and retarding shouldn't be too bad, but the positioning of the sensors and filtering the signal would be a bit of a black art. I remember driving cars in the 80s that used this system, tromp on the throttle while cruising gently and you could hear it rattling like a biscuit tin full of bolts, rapidly reducing as the electronics backed off the advance.

Run-on is less of an issue, although is probably is associated with a tendency to pink (certainly on mine) but the plumbing for a system that actually works on a non North American car isn't that difficult.
Paul Hunt

This thread was discussed between 03/01/2004 and 05/01/2004

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