adding the second washer will lower compression slightly. Personally I wouldn't bother.
Is the engine cutting at high revs? or is it as if it is bogging down? If it's bogging it's too rich, but it sounds from your description as if it's leaning out.
My solution (presuming it's lean) would be a cooler plug, and richen the mixture a little. Are you doing a full warm up before the race start? if not then at 3 minutes your engine is probably only about 1 minute in to running at race temp.
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