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MG Midget and Sprite Technical - Pinking and 1098 timing values

Can anyone offer some advice on pinking when driving hard when engine is up to normal temperature. I have gradually retarded timing but having got to 22 deg btdc @ 1000 rpm and am wondering if I am going too far compared to handbook value of 8 @ 600 rpm albeit for leaded fuel, and the pinking is still there.
The figure above is with vacuum advance disconnected and plugged, although re connecting makes no difference. An accuspark electronic distributor is fitted and the rebuilt engine has covered about 1200 miles. Any ideas? Thanks.
Andy Potter

I think you need to pull the advance back pronto. Having as much as 22 degrees @ 1000rpm means your part throttle with vac connected and total advance will be well over 35 which is enough to melt a piston quite quickly. Adjust the timing to the recommended figure, then set it so that full advance stays between 28 - 30 degrees, and run it on premium fuel.
F Pollock

Thanks for the reply. I have tried super unleaded and/or fuel additive and still get the pinking. What rpm would you expect to get full advance value? Thanks.
Andy Potter

The 8 degrees at idle will be good up to about 1000 rpm when the centrifugal advance will begin to come in. 800 rpm is therefore a safe place to make the initial setting - but you may get a smoother idle at 900 rpm with unleaded. Don't let it idle faster than that. Centrifugal advance ends at about 3500 rpm, so you'll need to make a chalk mark on the pulley 28-30 degrees BTDC from the notch and rev the engine to 3500 rpm or till the advance stops. Adjust the distributor so that maximum advance stays on the 28-30 degree chalk mark, and tighten up the pinch bolt. Once you done this it's best to fine tune the mixture, once the engine is up to temperature and reset the idle to 900 rpm, so you don't get any running on.
F Pollock

Thanks for the explanation, that's much clearer in my mind now. Could anything else other than grade of fuel contribute to the pinking issue?
Andy Potter

The 22 degress advance you currently have will lead to detonation, pinking,running on,difficult starting, and rapid engine destruction. That is the basic problem.
Other contributors might be a C/R over 10/1,an incorrect distributor advance curve or wrong grade of plug, a weak mixture or an engine running too hot.
F Pollock

Have re-set timing to 8 deg btdc @ 900rpm and it is definately smoother, still has a little pinking though when pushing the engine. Also checked at 2000 rpm and got 25 deg btdc (last notch on chain cover), seems about right as handbook says centrifugal advance is 17-21 at 2000 rpm + 8 gives 25 to 29 btdc. Am I correct in thinking that's how the figures should be used? Will now pursue mixture settings. Thanks.
Andy Potter

Yes, that sounds much better. If you set the mixture one flat rich and fill with BP Ultimate or Shell V Power you should be OK.
F Pollock

Andy
Your calc is almost right - but you add the static book advance (5 deg) to the mechanical advance to get total advance, not the strobe figure, so at 2000 RPM it should be 22-26 deg.
Paul Walbran

If the pinking goes on with the right compression, mixture and advance the head can be cracked or the valve seats loose as can be seen below

Flip


Flip Brühl

Thanks for the pointer Paul, lots of good advice as usual everyone. Cheers
Andy Potter

My approach with points was always to nudge the baseplate CCW/CW until the highest possible revs was obtained on a fixed throttle - start at 3000 say - then tighten it up. Reduce idle speed from the carb screw from there on.
Mark O

This thread was discussed between 02/02/2013 and 05/02/2013

MG Midget and Sprite Technical index

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