@bpepperb:
B&S is on it's very last legs RE: running their tooling and to comply with EPA stringent requirements on OPE emissions. With that most of NA no longer sells OPE flathead engines with plusa-jet carbs (non-adjustable, runs rich, etc.) They are now onto at least 625 or 675 series and upwards and couple them with Walbro clone LMS sidedraft carbs to keep up with emissions compliance.
I know B&S has ohv OPE engines out now but too new for me to acquire one and learn from them (tearing them down).
back to the subject RE: B&S flatheads: here are my 2c's worth:
(a) they do not need high octane gas for they have low compression ratio to begin with (I recall B&S site mentioned something like 73+ octane rating for their typically 7:1 compression ratio flatheads.
(b)typical combustion chamber carboning issues on B&F flatheads come from either slightly oil burning engine (oil seeps past rings into combustion chamber), engine inherently running a tad too rich (typical of pulsa-jet carb types), or rancid gasoline with lots of gum and varnish in it.
So, to keep your engine long and healthy: (a) run fresh 87octane gasoline, regardless of E10 or not; (b)for new/fresh engines, regardless of combustion chamber/head design, consider changing oil regularly (1st 5hrs, then every 20 or 25hrs); (c) change/replace/clean air filters on a regular basis.
adding fuel stabiliser such as sta-bil, etc. into gas will help in keeping the gasoline fresh longer, which, in-return, keep the combustion chamber carboning issue at-bay.
Q.