ORIGINAL: robear1053
...Synthetics also remove the glaze that forms on the iron piston in old style engines... that glaze helps protect the moving parts. An old Fox may have great compression and with one lean run on all synthetic fuel loose compression so badly as to be hard to start.
Bob
Actually, I believe a relatively small percentage of synthetic will remove the castor seal just from normal running. The varnish seal can be restored by using all castor fuel.
One of the selling points for all-castor Fox Superfuel was to form a varnish seal on iron/steel engines in order to regain some compression on "over the hill" engines.
In another thread Jayseas referred to the Foxes as old. I'm not sure if he meant years, run-time, or both. Also, this castor varnish seal forms on porous iron but not on modern engines with high-silicon aluminum pistons.
Or at least I think that's the way it works.
George