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Is there any significance to the engine being painted yellow? Higher spec or cast iron bore etc.
Hi Paul and all
Briggs 'Yellow' was offered as a factory colour - best guess - in the late 1970s.

Rover took up the offer to use yellow in the early 1980s - but for one model - the Model 29 Baron.
The Baron was the intended replacement for the Model 26 Challenger.

Traditionally, Rover had used red for smaller hp engines; white for larger hp ones.

Thus, yellow was exclusive to the Series 9, 3.5 hp vertical-pull spec.
It was a higher spec in that it had Hush-tone muffler, dual element air-cleaner,
and an extended dipstick.

Later, Rover would use yellow on their Model 81 Edger 6 Series (previously a red engine).

I have not seen yellow on any other Rover of the period (1980s).

Cheers
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Jack