Twin 750 Twin Starter Clutch Failures!

Both of my 750 twin starter clutches have failed. One dramatically, one silently. They sure do seem to be prone to it...

750 #1:

The starter clutch exploded and seized my engine just last month on my "first" 750 twin. I was riding along at about 30mph in town when the engine just rapidly seized and the back wheel locked up. I got a handful of clutch and rolled to the side of the road. I pushed the bike home, pissed off. A week later I got around to checking out why...

It had automatically backed 1 of the 3 mounting allen screws out (the other two were already loose too), dropped a roller out, jammed against the starter sprocket or something and cracked in half. I was lucky it wasn't worse. I just pulled the rotor, removed all the debris, got a replacement starter clutch from a spare engine, removed all the pins, springs and rollers (only kickstart now), and reassembled with RED Locktite.

Oddly enough, in the 5+ tankfuls of fuel that I've used since this problem, my mileage went up from an average of about 47mpg to consistently 50-51mpg. Weird.

750 #2:

So, today, I was working on my *other* 750 twin, because I swore I would pull the rotor to examine the state of the starter clutch before that thing saw the road this season.

This time, after I pulled the rotor off, I was looking at what looked like the backside of a starter clutch, still on the starter sprocket. Lo and behold, it *was* the starter clutch. The three bolts that mount the starter clutch *to* the rotor had ALL sheared cleanly, separating it from the rotor. No shattering, no other damage, just a completely free-floating starter clutch. It looks like one of the three bolts was a little worn like it had been grinding like my other bike, but only a little. I'm sure this would have caused damage in the future... Now I have to get the little bolt bits out of my rotor, and hunt for 3 new bolts!

Conclusion:

Please... if you own one of these beasts, please pull the rotor and check the starter clutch. It could be your life. It's actually pretty easy to do, and given the consequence of a failure, well, you know...

Question:

Though I re-installed my "first" 750's starter clutch with no pins, rollers, and springs, I'm wondering: can I just leave the whole starter clutch assembly out entirely for the second 750? I mean, is it an important spacer or balancer? I loose-fit it together, and it looks like it'll be fine without it.... Let me know what you think!

Thanks guys...

I can't answer your question on if you can skip installing the starter clutch w/out any problems.

But something got me to thinking (uh oh?).
The Kawi x-ref software shows the following:
13194-005 (subs to 13193-1001) = 74 KZ400. 77/78 KZ400-A1/A2, 75 KZ400D, 76/77 KZ400-D3/D4, and 75 KZ400s.
13194-006 (subs to 13193-1001) = 76/77 & 79 KZ750-B1/B2/B4.
13194-1001 (you guessed it, subs to 13193-1001) = 78/79 KZ400-B1/B2, 76~79 KZ750-B1~B4.

So since that starter clutch (current part# 13193-1001) shows as fitting all of these KZ400/440 and 750 twin models, it's probably designed to handle the higher torque the twins have, especially the 750's.
Perhaps the Kawi engineers realized this after similar prob complaints as you're experiencing and re-designed it, hence the NEW part number?
No clue as to what was changed in the design, but it could be as simple as factory applied loctite on bolts with a higher grade sheer strength. :huh:

It's perfectly safe to remove all the starter drive, clutch, sprocket, chain, and starter, you'll just have to plug up the hole where the starter was.

I bought a 750 twin out of a junkyard, the guy that owned the junkyard said the bike was ridden in. I got it home and stuck a battery on it to crank it over and the starter just screamed like it wasn't connected to anything so I pulled the stator cover off. It appeared that the previous owner had pulled the starter without removing the stator cover, and just stuck the starter back in. As I'm sure you know, when you pull the starter the starter sprocket and chain falls down inside the stator cover, end result was teeth ripped off the large sprocket, stator ripped to pieces, pretty much everything inside the cover destroyed, and the motor full of little pieces of copper wire.

A couple of the 750 twins I have bought had all three bolts attaching the starter clutch to the rotor sheared, so I'd say it must not be uncommon.

I usually just kickstart the 750 twins, when they are properly tuned they will start in two kicks cold and one when warm, and it just seems like the "right" way to start the bike

steell wrote:

It's perfectly safe to remove all the starter drive, clutch, sprocket, chain, and starter, you'll just have to plug up the hole where the starter was....

Thanks for that. I think I'm just going to leave everything (starter, chain, sprockets) but the starter clutch assembly itself then. That way I can still hit the starter button and hear, "zzzzzZZZING!"

I usually just kickstart the 750 twins, when they are properly tuned they will start in two kicks cold and one when warm, and it just seems like the "right" way to start the bike

Same here. I haven't used the starter since I got the bike (it actually did work when I got it). I was always afraid of the starter clutches.... now I guess my fear was founded... :S

If you are going to remove the starter clutch, then you really need to remove the sprockets and chain also, the starter clutch acts as a spacer to locate the large sprocket. If the starter clutch is no longer there, then that large sprocket is free to move 1/2"-3/4" back and forth on the crank, and I don't think that would be good.
You could still leave the starter in place though, then you could hit the starter button and make the bike sound like a ZX6R at 16,000 rpm

steell wrote:

...you really need to remove the sprockets and chain also, the starter clutch acts as a spacer to locate the large sprocket...

That's exactly what I was wondering. I mean I dry-fit it together, and I almost cannot see how the starter clutch makes a difference, spacing-wise, but I wanted to be sure. I mean the starter sprocket seems to move about the same amount with the clutch assembly in there or without. But I think your idea is safer anyway, just less moving parts in there to fall apart, eh?

Thanks man!

Post edited by: Biquetoast, at: 2007/05/25 07:46

I haven't tried to install the sprocket without the starter clutch, so my assumption about the additional clearance may be incorrect, but like you say, "Better safe than sorry"
Although the parts involved probably only weigh a few ounces, I figure that the 54 hp motor can probably use all the help it can get

steell wrote:

...I figure that the 54 hp motor can probably use all the help it can get

*Alleged* 54 hp.... at the crank... On paper... how ever you wan to put it...

In the end, I got some great advice over on the KZ400.com forum about an oil journal that exists on the crankshaft right under the starter sprocket. Without the starter sprocket in place, oil pressure might be reduced, by how much, I don't know. Since I don't want *any* reduced oil pressure, I put it in.

So, in summary, here's what I effectively did to the two 750s:

Bike #2: Removed/replaced cracked starter clutch *housing* only, no pins, springs and rollers, with RED Loctite on the 3 bolt threads. So the final difference is that it's just missing pins, springs, and rollers. I did this one first, and thought I needed the housing as a spacer.

Bike #3: Left out the *complete* starter clutch assembly. I realized that I do not need the housing as a spacer, but I *do* need the starter sprockets and chain to hold the sprocket steady while running.

Now, all is well. And I can kick start them forever now.

Here are some pics for you to enjoy of the broken starter clutch assembly, the sheared bolts on the other one, etc. Make you cringe? They made me....

This is on Bike #2, immediately after removing the assembly from the siezed engine. Note the melted screw head. All 3 screws were loose and backing out. The pin and roller had freed themselves and wedged somewhere between the assembly, the starter sprocket and the engine case to cause the engine seizure...


Still Bike #2, I hope you notice the housing cracked in half, the missing metal chip (hole in housing in foreground) I recovered (in background),


This is on Bike #3, all 3 screws were sheared, and the starter clutch was effectively free-floating in there. How long before one of the screw pieces got free and made it's way toward the crank? Fortunately, we will not find out.


So is everyone going to go get a gasket, their rotor puller (or rear axle) and check them out?

Sorry if I sound like an evangelist, folks. But my engine seizing - even at relatively low speed - might have been my scariest moment in my brief motorcycle history...

:blink:

Post edited by: Biquetoast, at: 2007/06/15 09:01

mine did exactly the same thing, i was riding about 30 mpg and all of the sudden it just locked up.
when i tore it down i couldn't believe it, the screw backed out and siezed the engine. i was lucky not to be going any faster. i used locktite on it when i reassembled it, no more problem in that area