What's it for?

[steve pagano]

     Had this engine for five years and just noticed the screw in the backplate. Can someone tell me what it's for?

                                                           T.I.A.
                                                           Steve

[Bill Gruby]

 Steve;

  That screw is probably blocking the pressure tap. Take it out, if the hole goes all the way thru the back plate that is what it is what the screw is for. If it does not go all the way thru my educated guess is that they mounted a remote needle valve there.

  "Billy G"

[Jim Oliver]

Not sure which particular Fox this is, but the screw is being used to plug a hole in the backplate from which  crankcase pressure was used to pressurize the fuel system.  

Crankcase pressure is/was used on go-very-fast engines for combat, and speed type appplications.  The pressure was needed because the usually very large venturies had poor fuel draw............

Cheers,
Jim

[Just One-eye]

From the threaded holes in the exhaust stack, I'd guess it was used as a Class 2 Carrier engine with an early throttle of some kind, or twin needles, and an exhaust flapper, based on the 1955/56 "Series 1" Combat Special, but if there's any black paint hidden between the head fins, it could have been a Series 2 from '57/59.  As everyone else already said, the racing engines in those days used pressure tank fuel systems with a take-off from a small nipple that screwed into that hole the screw is in. 

[Chuck Feldman]

   Hey Ty.  You are so expert on the Fox history. Thats good.

Chuck
Stuart Florida

[James Lee]

Fox did  do some engines with those tapped holes inside the exhaust stack for early bolt on exhaust throttles...     The R/C Special 35 was on a similiar case but with a different head.
Given the shortened venturi and tapped pressure fitting boss, I would guess a previous owner 'fixed' it to suit his purpose.
Later
Jim

[minnesotamodeler]

Looks like a butchered Blackhead, top of belled venturi cut away.  Shameless, those were good old Combat engines as long as you could keep them together.  Take out that venturi insert, put a nipple in that backplate for pressure, run a 9x6 or 8x8  and and Fox Blast, and you'd have a screamer.