Specifications

Post 4: Slides and Floats
Slide diaphragm function can be tested as follows. If carb top screws were removed
above, reinstall them. Then look at the backs of the carburetors. At the top of each
intake bell there is a curved hole which opens into the chamber below the diaphragm,
exposing it to atmosphere. If you raise a slide, cover the port firmly with a thumb, then
release the slide, it should fall very slowly. If it falls quickly, the diaphragm is breached,
and the slide/diaphragm assembly must be replaced. Member donmurray has noted
that there is a passage from the diaphragm chamber to the enricher chamber which is
opened if the enricher cover or enricher plunger is removed. The passage must be
covered to test diaphragm integrity.
Remove the carb tops and springs and push the slides up, removing the needle
retainers and needles with them. Free the diaphragm gently. Use marked plastic bags
to keep the slides paired to their carburetors. Remove retainers and needles. Examine
each diaphragm visually for pinholes against a bright light.
Note that in addition to the hole for the needle, there is a second hole in each slide.
When the throttle is opened, air passing though the venturi (central part of the
carburetor) into the engine sweeps past this hole, creating negative pressure in the
upper part of the diaphragm chamber, which causes the slide to lift. The hole must be
clear. If it is obstructed, clear it with compressed air, probing with a wire if necessary.
Bag the marked slides and set them aside to prevent damage or contact with solvents.
To reinstall, note that each diaphragm has a "tab" that fits a matching recess in the carb
body. Be sure to align the diaphragms correctly.
The floats hang from a pin passing through two posts cast into the carb body. Care
must be taken not to damage the posts. A broken post can be repaired by carefully
drilling small matching holes in the post and the carb body and pinning the pieces
together, using JB Weld as an adhesive; but this is a touchy procedure, and damage is
entirely unnecessary. Don't try to beat the pins out!
BS38 carbs (70-79) use straight pins that fit flush with the sides of the posts. The pins
should push out. If they do not, apply a bit of spray-type carb cleaner and try to free
them. As a last resort, set a small nut against one post to relieve the end of the pin,
drive a tack through a thin piece of carpenter's shim so that the point protrudes, and use
a small C-clamp to gently press out the pin. As soon as you have movement, apply
more solvent and extract the pin with pliers. Remove the floats and float needles. Keep
float needles paired to their carburetors. If needle seats are varnished, spray with carb
cleaner and clean with Q-tips. The seat is threaded; turn CCW to remove. If needles or
seats show wear or corrosion, replace them.
BS34 carbs (80-83) use pins with flat heads and ends that protrude slightly from the
posts. They are snap-fits, and must be pressed out. Relieve the head of the pin with a
nut, press the protruding end flush with the post with a small C-clamp, and work the pin