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MG MGA - Pings with vacuum advance

My newly rebuilt 1600 engine is bored out to 1622 + 30 thou and has MGB valves in the head. The dynamic timing is set at 12 degrees advanced at 800 rpm and 30 degrees at 3500 rpm, vacuum disconnected. The car runs great with lots of power and does not ping with the vacuum advance disconnected. When I connect the vacuum advance the power is the same but the engine pings under load especially climbing hills. I've checked the vacuum advance unit and it's advancing correctly starting around 7 in of mercury and advances 10 degrees and stops at 14 in of mercury. With the vacuum advance connected the dynamic timing changes to 15 degrees advanced at 800 rpm and 42 degrees advanced at 3500 rpm. Presently I'm running the car without the vacuum advance connected with no apparent problems. Would it be better for me to retard the timing until I get no pinging and connect the vacuum advance, or leave as is? Any comments or suggestions appreciated.

Andy
Andy Preston

Where do you have the vacuum connected? The venturi vacuum signal (bottom of rear carb) should be nil at idle speed, so timing should be same at idle with or without vacuum connected. By "nil" I mean the idle vacuum signal is below the threshold needed to start the vacuum advance moving.

Also check timing in the 2000-2500 rpm range with and without vacuum connected. You may have too much mechanical advance in that speed range if the bob weight pivot points are badly worn.
Barney Gaylord

Brown and Gammons advertise an all-centrifugal dizzy. They say its more suited to low octane / unleaded.
But you give me hope. My motor is the same as yours now, 1622+.040, B head and my vacuum unit is kaput.
Art Pearse

This thread was discussed on 03/06/2009

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