I think Grumpy was on the money with the smoothness argument.
Yes, I'd agree. Power, which was not needed for the task at hand, was sacrificed for smoothness.
Interestingly, they recommend BP Outboard Zoom (50:1).
- runs smoother
- runs cooler
- longer life
- needs less oil
Some interesting 'selling points' in that lot!
Availability of BP Zoom 50 would have been a bit questionable through mower shops, though. AFAIK it was usually [but not always] a bulk product, sold 'at the pump', by boat marinas and a very few BP service stations. Whereas Zoom 25 was usually mixed from concentrate, at the point/time of delivery from BP, in mower shops.
It may be of historical interest to describe how this worked, so here goes.
Our shop kept a 44 gallon tap drum of BP Zoom 25, on a wheeled stand. We also had a 12 gallon drum of Zoom Concentrate on hand. When the 44 was getting low, we would use a BP-supplied aluminium dipstick to measure the remaining contents, and calculate the required top-up volume [IIRC the volumes of Concentrate and petrol were read directly from the dipstick markings]. Then it was a matter of phoning the local BP distributor, to place the order.
When the fuel tanker arrived, we would pour the appropriate volume of Zoom Concentrate into the 44, then wheel it outdoors. The tanker driver would set his discharge meter to deliver the corresponding quantity of Standard grade petrol, and pump this into the 44.
After 40 years, I can't recall the exact mix ratio used; but it was NOT 1:25, rather it was a lesser proportion of petrol. That is, the Zoom Concentrate was pre-diluted to some extent. I'd say this was to ensure that pumping the petrol in on top of the concentrate, provided sufficient agitation to achieve thorough mixing.
