"cackling" properly described usually indicates detonation or pre-ignition. One or more of the following usually causes this. Too hot a plug (try switching from OS #8 to a A5 or #10); overcompression (add a head gasket); too much nitro (10% should not be the problem in your case); or running too lean (overheating-same effect as too hot a plug). Suggest you try or or more of these. Worst case effect would be a pounded out rod end.