Starter clutch problem/ Honda Nighthawk 750

Hello, I have recently taken my Honda Nighthawk 750 out of storage
after it sat for a couple of years. I did all the recommend things
before putting it away so I was hoping for a relatively easy start.
However, I find that my starter motor just spins away without even
turning the engine over.

I took out the starter motor; the pinion gear looks fine. I know the
engine is not seized because I can get it to turn over (but not start)
by rolling the bike and popping the clutch. So I think I must have a
starter clutch that is slipping.

So, question A: why would the starter clutch start acting like this
after sitting for a couple years? It was working fine before I put
the bike away. Question B: how difficult is it to get at and replace?
My clymer manual is not so clear, but it looks like I would have to
practically take the whole engine apart!

please help if you can and thanks in advance.

Andy

 

It would be nice if you'd mention what year your Nighthawk is.

I looked at the 1982/1983 models. Fixing the starter clutch would be a
piece of cake on those models.

But the 1991 and on Nighthawks have a nightmare starter clutch on a
jackshaft that drives the crankshaft via a chain.

 

Bad Bike Pixies.

On the DOHC 16v 750, the one wih hydraulic tappets, you do. It's a
common fault on these lumps and a right PITA.

 

If your Nighthawk is a 1991 or later model, with a starter that has
planetary gear reduction, you might just have a problem with
the reduction drive unit. You can see two of the planetary gears on
the starter motor drawing at www.partsfish.com.

Yamaha Virago models were plagued with reduction gear problems.

The nightmare late model Nighthawk starter clutch design has some sort
of weird directional dog coupling device that seems to connect the
starter clutch to the sprocket that turns the chain that turns the
crankshaft. And the dog coupling thingamajig is preloaded by four
dished springs that are called "belleville washers".

Those springs are item # 6 on the starter clutch drawing. I have seen
stacks of preloaded belleville washers used as clutches.

The whole idea of the chain drive starter clutch on a shaft buried
deep inside the engine is to get the alternator off the end of the
crankshaft.

 

Thanks for the informative replies! I do unfortunately have a 1991
model nighthawk, which apparently has the starter clutch buried inside
the engine.

Now concerning the planetary gears, If you mean the large ring gear
with two smaller gears inside, that is inside the starter motor, I'm
pretty sure those are fine. I took out the starter motor and took the
end cap with the pinion gear off to reveal these gears and they didn't
appear damaged. So I dabbed a little fresh grease in there and put the
motor back together. I did not try applying current to the motor while
it was out of the bike to see if the pinion gear was actually
spinning. But I could spin the inner gear that connects to the
alternator shaft freely with my finger, which suggests to me that the
clutch was not engaged, i.e. not working. Though I don't understand
what actuates the starter clutch, so I don't know if that's a valid
conclusion.

After looking at the manual in more detail its clear I need
essentially disassemble the motorcycle enough to lift the motor out of
the frame, then disassemble the crankcase to even get a look at that
clutch. I'm feeling pretty overwhelmed and undertooled to attempt a
job like that. I'm sure I could get it apart but not at all sure I
could put it all back together!

Andy

 

That's what the engineers did to get the alternator off the end of the
crankshaft to make the engine narrower.

Typically, an operating planetary gear reduction system has a
stationary ring gear, and two or three planetary gears have to follow
the internal teeth of the ring gear, while they turn a sun gear
attached to the final output shaft.

The planetary gears have to be mounted to a metal "spider" and it's
usually the spider that breaks, spot welds come loose, or the
planetary gears' axles break off the spider.

You might want to google for some Nighthawk 750 forums to see what
problems other owners are encountering with the reduction drive
starter.

Automatic transmissions often have planetary gear reduction systems,
and the ring gears are allowed to spin freely while the transmission
is in neutral. Then the ring gears are braked to a stop by
hydraulically actuated bands and the planetary gears have to start
turning the sun gear, and the car goes.

Ah, yes. Starter clutches were invented by the psychotic Doctor
Frankenstein and his equally looney assistant, Professor Sprag...

Refer to the STARTER CLUTCH diagram on www.partsfish.com or www.bikebandit.com.

When the engine is running, #9 CHAIN turns #4 SPROCKET (21T). The
sprocket has splines, so it turns the
#7 SHAFT, ALTERNATOR.

#3 CLUTCH OUTER is also splined to the # 7 shaft, so the outer clutch
has to turn with the shaft.

If the outer clutch is typical of many Frankenstein starter clutches
(1), there are three cylindrical rollers which are pushed away from
the protruding end of the # 1 GEAR by centrifugal force. The # 1 gear
sits motionless on the #18 BEARING (25X30X20) as the alternator shaft
turns at high RPM.

#18 bearing is a needle bearing with very small diameter rollers, and
it's probably getting plenty of oil from the engine.

(Gawd help you if that bearing seizes and locks up the starter clutch
while you're riding down the highway at speed. The starter clutch will
try to backdrive the starter at very high RPM and the rear wheel may
lock up as the starter refuses to turn.)

When the engine is NOT running, three little springs push the
cylindrical rollers in the #3 outer clutch down onto the protruding
end of the #1 gear. If you hear a "whiz ching!" sound when you turn
your ignition key off, that's the sound of the rollers dropping down
onto the end of the #1 gear.

(It's possible that your starter clutch is slipping because the
cylindrical rollers are stuck in their ramps. You might try putting
the transmission into high gear and rolling the motorcycle back and
forth a little bit.)

When you push the starter button, the starter turns its own little #1
GEAR (shown on the STARTER MOTOR
drawing. The small #1 gear turns the larger #1 gear, and since that
gear is in contact with the three cylindrical rollers, the #3 clutch
outer has to turn the #7 alternator shaft.

(1) Another diabolical alternative to the starter clutch with
cylindrical rollers is the Sprag clutch, which looks like an ordinary
roller bearing. The devilish twist is that the rollers don't roll,
they slip when the bearing is turned one way, and they jam the inner
and outer races together when the Sprag clutch is turned the other
way.

The dastardly effect of Sprag clutches is that they are slipping when
the rider isn't trying to start the engine and they wear out *when not
in use*!

I sympathize with you, Andy. I never would have bought my Yamaha
FZR-1000 if I had known about the buried starter clutch. It's the
first thing that goes into the crankcase, and the last thing that
comes out, after tearing the engine down!

 

Find a breaker and buy a complete engine. There are loads of Nighthawks
out there.

Do an engine swap. A decent used lump can't be more than a very few
hundred dollars, and you could easily spend that on gaskets, fluids and
a few special tools.

It sounds silly, just for a small fault, it's probably the easiest
solution.

 

You said that it does push start. If the starter was working when the
bike was last used the problem could be that the caps on the springs
that push the rollers are stuck, and that getting the oil hot while
running the engine for awhile might clear up the problem. You would have
nothing to loose if you wanted to push start and go for a 10-20 mile
ride and maybe the starter would work afterwards.

 

Thanks everyone for the replies...this a great list! I have not gotten
it to push start yet, just rolled it up and down the driveway with the
plugs out. After it's been sitting for awhile it usually takes some
cranking to start it... so my plan is to get a couple of guys to push
me up a really big hill and try to get it to start on the way
down...I'm HOPING that bruce is right, the hot oil and vibrations
might do the trick.

Andy

 

Well, I have tried push starting while rolling down a hill, and can
get the motor to turn over plenty of times and can smell the gas
flowing. No sign of ignition however. So I am assuming I have no
spark. However, I don't know if that problem is related to the starter
clutch problem. I get no spark when just spinning the starter motor
with the ignition on. I don't have a convenient way to see if there is
any spark while rolling the bike downhill. Is it possible the
alternator isn't spinning because of the starter clutch being
disengaged?? Or is it more likely a bad coil or broken wire?

 

It sounds to me like you have no spark too.... but a simple test would
let you know definitely. remove a pspark plug and hold it so that you can
see if you get a spark while you try starting it (assuming you have a push
button to start the bike). If no spark, then you need to trace it back to
see if you are getting spark to the coil(s). If no, then keep going back
until you find spark, then replace the component that does not give you
spark during testing.

Hope this helps..

Fwed

 

Thanks for taking the time to explain all that.

The battery is brand new and was fully charged, so I'm pretty sure
that wasn't the problem. Main fuse has to be fine if the lights work
and the starter motor spins, which they do.

I can inspect and clean the kill switch and sidestand switch and
connections everywyere, but it's impossible to really troubleshoot
them with the bike rolling, which is the only way I could get the
crankshaft to turn and activate the pulse generating
mechanism...right? I will also check the the voltage on the ignition
coils with a multimeter.

If only I had a kick-starter! or one of those cranks, like on an old
model T.

Andy