Hello POI and Norm
This has been asked before.

The Imperial had a spacer between the cylinder and cylinder head.
This reduced compression and made for a cooler-running engine.

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=62325

Victa Zoom Outboard was 50:1 and I believe that ratio to be the best
for imperials.

But I do note that Victa did say the machine would run on 80:1.

----------------------------
Jack

Attachments
page_09.jpg (111.91 KB, 50 downloads)
2016_02_ebay_victa_01.JPG (34.39 KB, 52 downloads)
2016_02_ebay_victa_03.jpg (289.63 KB, 50 downloads)