I've only ever run a Fox 35 once and that was only because someone asked me if they would run on zero nitro fuel. My engine is quite worn (someone gave it to me) but it started first flick, tuned easily and seemed to run just as well as any other engine of its type. Every time I'd stop it it'd start again with one flick, hot or cold. I can only guess that, for some reason, your fuel is bad.