Specifications

Post 7: Sealing Air Leaks
Locating Air Leaks
Air leaking into the intake stream leans the fuel/air mixture. Symptoms include popping
in the exhaust under engine braking and excessively high idle once the engine is fully
warm. To check for air leaks, spray brief bursts of carb cleaner on carb mount joints and
throttle shaft ends. When the spray hits an air leak, the idle will rise or fall; allow a
second or so for response.
Mounting Boots.
If a leak is found at the back of the mount, tighten the clamp and retest. If unsuccessful,
remove the boot. If a leak is found between the boot and the engine, check torque on
the mounting screws, but don't overtighten (at best, overtightening only deforms the
flanges under the screws, leaving the rest of the sealing surface unaffected). If
unsuccessful, remove the boot.
Examine the boots for cracks, cuts and other damage, and for flatness of the sealing
surface. If they have been overtightened and warped (see above), it is sometimes
possible to relieve the area under the screws and get them to seal, using fresh gaskets
and sealant. Examine the screw holes in the cylinder head; these sometimes become
extruded from overtightening and must be flattened. If this is the case, seal the intake
port to keep grit and debris out; then use a flat file (one you've checked for flatness) to
carefully level off the intake flange area and follow up with 400 grit paper.
Throttle Shaft Seals
BS38 (70-79) and BS34 (80-83) throttle shafts have a rubber seal at each end. Before
attempting to replace the seals, remove the throttle shaft springs and check the shaft for
play. If it's wobbly, new seals may work for awhile, but the pivot area in the throttle body
is worn. Replacement is cheaper, but motorcyclecarbs.com remanufactures to spec.
Mike's XS lists seals for the BS38 but excludes the BS34 from the application list. For
the purposes of this article, I dismantled a BS34 from a 1982 machine. Comparing its
seals to a new one sold by Mike's for the BS38 showed no visible differences--same
seal type, depth, and diameters. The reason for the exclusion may simply be that for
some inscrutable reason the seal was not shown in XS650 parts books from 1980 on,
and Mike's has followed that lead.
To remove the throttle shaft for seal replacement, the throttle plate must be removed. If
you open a throttle and look at the back of the throttle shaft, you'll notice that the tips of
the screws protruding from it are cross-hatched. They've been staked--that is, deformed
slightly to prevent them from backing out, entering the intake port, and tearing up more
stuff than you even want to think about just now. They're brass, so it's very easy to
destroy the head, and removal is usually troublesome.
(Edit, 10/01/08: I've noticed that Mike's XS has changed
the application list to include the BS34. You're welcome, Mr. Lalonde.)