Briggs and Stratton carburettor

[Nick]

Not sure if it will help - I had a problem last year with my Honda mower which required a carb strip & rebuild - I used much the same techniques as you - strip, ultrasound, STP Carb Cleaner (wonderful stuff), replacement gaskets etc. However, the varnishes in the fuel had also 90% blocked the in-line fuel filter - it *looked* fine, even seemed to pass fuel, but was sufficiently blocked to render it useless when the engine was running - see below - cut open, it looks like its just got the usual crud (bits of grass etc.) but what you can't see is the varnish blocking the pores... the give-away was that blowing through it was a real effort - the whole fuel path needs to be free-flowing... (check the tap too...).

Replaced the filter and engine was *much* happier