MB Doc, as always, you give great advice.

Today we did a fuel flow test and could fill a 1 liter bottle with gas in about 18 seconds so it's all good. We checked the timing at 1500rpm and at 3000rpm and it was within spec.

The coil was not a Bosch coil (instead one made in Mexico). As we had only noticed this problem while the engine was hot, we replaced the coil with one that I had lying which was taking from one of my SLC 5.0's and then took it for a drive.

Problem solved.

David S Poole also provide some telephone support which was also very useful. Thanks you both for your help.

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With best regards

Al