Have you re-jetted the carbs for the added compression and displacement?
Really it's hard to tell just what your problem is. I've heard that you can also purchase retarded ignition modules that will retard the ignition even more that the stock units will. The pistons will work it's just that you need to work out the fuel and ignition requirements.
My old 87 horse 944 SSie would rattle a bit when I'd roll on the throttle at 60mph on the freeway in top gear while riding on a 106-degree day. A simple downshift took care of the problem every time.
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