Well, I see Richard already gave you the answer to the proper repair of worn
throttle shafts. Here's some "band-aid" techniques that sometimes do work.
Never tried them on an SU though.
One is to use shims to take up all the side to side end play of the throttle
shafts, thereby sealing the entry to the air leak. You can do this by selecting
different thickness washers and installing them on the throttle shaft between
the circlip and the main housing. You've got to be carefull to not create
bind! Ideally you want a perfect fit with zero play or drag.
Another is to use o-rings instead of shims or washers. This is an old Holley
carburetor trick. Same caviats as above apply.
My favorite in the sense of comical relief is what Ford now does for old
tractors like mine. The carburetor rebuild kits now include felt "air filters"
that fit on the throttle shaft. I kid you not. It doesn't fix the air leak,
but
it does address the grit that can come through there. Significant on
tractors which live in a very gritty environment.
Cheap replacement air filter? You can carefully cut out the paper and
wrap filter media around it yourself. That's what mine happens to run.
Crude but effective. The better the furnace filter you use, the better the
filtration. That would cost you less then a dollar.
I've seen lawnmower filters of about the same dimension, they might
work. Don't know about flow adequacy with them.
Throttle linkage can be fabricated using hardware parts from a hobby
store and a hardware store. Mine works just fine that way.
A Spitfire doesn't have true PCV, in that there is no fresh air intake
for the crankcase. It just blows off excess out the top hose in the
valve cover. So routing that to the back of the air filters seems to
work well enough, and is something I commonly see done. Mine
is currently simply in overboard dump mode.
You can connect the carbon can to the back of the filter if you wish.
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