[quote= JimF ...
" If I take the head off to inspect it, "
____ Have you yet first simply looked-through the spark-plug hole with a bright light shown inside there (to see if there's any chucks of carbon gone from the piston-crown, or anything else that's odd seen inside there) ?
__ I guess I'm left to assume that you didn't have a baffled muffler to check the input-side of, for any possibly captured foreign-debris (which had to've worked-through the combustion-chamber). (?)
" what if anything should I do in advance to insure I don't break anything in the bevel tower? "
____ Before pulling-off the cyl.head, you should first check that all-4 head-bolts have still been left torqued-down to
at-least 20-ft/lbs, - (it may take well over the
recommended 28-ft/lbs to
break-loose those 10mm-bolts, so-therefore take your torque-readings right-
after the bolts have broken-loose),, that-way, you'll then have had a chance to realize whether or not your compression-issue might've possibly been due-to air-leakage between cylinder & head.
__ There shouldn't be any way you could "break" anything,, however you still should-
not do the same procedure as common newbie-wrenchers are most apt to do,, and rather-instead, leave the head and tower-shaft & housing-tube all together and left entirely connected as a
complete-cyl.head-unit, (just exactly as it always exists when installed) ! _ Cuz that whole entire cyl.head & bevel-tower assembly comes-off the cylinder-jug & motor-case all-together in one-piece !
So there's no such screwing-around concerns to-be worried about ! _ (Although you might care to let the motor rest towards a rightward-tilt for some time beforehand,, so as to get the head drained of oil, ahead of time.)
" What can I do to make sure that I don't screw up the timing of the bevel gears while the head is separated from the engine? "
____ This worthy concern has been covered a few times before, so you now ought-to more thoroughly realize the gear-timing prep situation after this following head-removal preparation refresher-review ! ...
__ With the bevel-cover removed from the cyl.head, turn-over the engine until the timing-dots (near the teeth) on the ('upper-pair') of bevel-gears become aligned. - (With spark-plug removed,, it's more convenient to rotate the engine by gently wrenching-on the camshaft-nut, rather than attempting to use the kick-start lever.)
This logical preparation will conveniently leave
all the timing-gears preferably positioned '
in-time', for rather quick reassembly of top & bottom ends (without any need to check/reset the gear-timing within the bottom-end) !
However afterwards (with head still removed), if the still
timed crankshaft-position later happens-to become disturbed, [even greatly !],, it's really not an actual concern as most-all newbies believe it to be ! _ As there's a relatively
simple fix-procedure
* to get the timing of the top & bottom ends properly reset back-together (without having to screw-around with
any-other timing-dot alignment checking !). _ So if perhaps you'd like to kick-over the bottom-end workings a few times (while the cyl.jug is securely held-down), so as to take a look-see at the piston as it reciprocates within the cylinder (and-also witness the rate of oil-flow from the oil-passage/nozzle),, then go-ahead & do so, as we can rather
simply straighten-out the then lost gear-timing, later.
(
* viewtopic.php?f=3&t=1606&p=12198&#p12198 )
__ Then once the four head-bolts are removed, (you then probably better place a piece of rubber-sheet or something soft atop the head [to protect that nice red-paint your M1.frame has]), then-next lift-upward (possibly along-with a slight back-&-forth shaking-motion) on the cyl.head-assembly until the bottom of it's tower-housing/tube lifts-out &
slightly clears above it's tube-base/female-flange -(still-left attached to the motor-case), and-then slowly/carefully horizontally-twist
* it clockwise (
* around the tower-shaft center) as you continue slowly lifting (
lifting more-so,
after fully clearing the frame-tube), whilst proceeding to remove the entire head-assembly away from place, towards the right-side of the motor (& away from the bike).
__ While it's
possible to remove a complete cyl.head-assembly from a n-c.250-motor & frame with carb & ex.head-pipe still left fully attached,,
with a SSI-carb however, it's better for a 'newbie' to remove the carb.body off-from it's manifold-flange first, (before the head-bolt removal step).
____ Now next,
please ask any other newbie-type questions
which ya may now have.
Hopeful-Cheers,
DCT-Bob