91 Yamaha FZR600 transmission issues

My '91 FZR600 has had transmission issues ever since I bought it. When
accelerating fairly hard in second gear, it will sometimes pop out of gear
and into neutral (which is a big shock when you are expecting a speed hit
but it jerks forward, out of gear). What is the likely cause? If it is an
internal issue like a stripped gear or something, do the engine cases
require being opened to get to the tranny? Any insight I can get is
appreciated.

 

Register at www.partsfish.com so you can look at the TRANSMISSION fiche
and see what I'm talking about. There will also be a fiche for the
shifter mechanism.

It's just about impossible to strip the teeth off of a gear in a
motorcycle transmission, because all the teeth are meshed all the time.
Actual engagement of any given gear is accomplished by sliding one of a
pair of meshed gears *sideways* so projecting metal pegs called "dogs"
slide into slots on an adjacent gear. Yamahas have small dogs on the
gears, go easy on the powershifting.

If you abuse your transmission, power shifting at high RPM, you can
wear the shifter dogs so they are rounded off on on side, and the dogs
disengage by themselves.

The fix for this is to remove the engine, split the cases and install a
new gear. But I have heard of motorcycles where it was possible to
remove the screws from the plate behind the countershaft sprocket and
slide the countershaft out of the transmission. I dunno if you can do
that on an FZR600...

Some racers will take a grinder to the dogs and undercut them at an
angle so they will remain engaged under power.

The mechanism that slides gears sideways is called a shifter drum.
There's a linkage that rotates this cylindrical drum that has strange
patterns of grooves in it. Pegs on the ends of the shifter forks have
to follow the grooves, and the forked end of the shifter forks engage
slots on the sliding gears.

There is a detent mechanism on the end of the shifter drum which
engages a star-shaped cam. The detent mechanism HOLDS the shifter drum
in the correct position for the gear you want to be in. The detent
mechanism may be a spring and ball bearing, or there might be an arm
with a spring and a roller.

It's possible that the detent mechanism is not engaging the star-shaped
cam correctly.

The star-shaped cam is symmetrically shaped, each of the peaks is the
same spacing as the valleys that the detent ball or roller must go
into. One of the valleys is neutral, the other valleys are each for one
of the six gear positions.

Since the valley for neutral must be between the valley on the
star-shaped cam for first and second, this means your foot must move
twice as far to shift from first to second as it has to move from
second to third and so on...

So, your popping out of gear problem might just be because you're not
moving you foot far enough to get the detent ball or roller to drop
into the valley for second gear, maybe the detent is still on the peak
of the cam and the spring drives the shifter drum back toward
neutral...

Try moving your foot further when you shift from first to second...

 

Worn selector dogs. Common prob on the FZR600.

Engine and gearbox strip time. Not cheap.