Starter Clutch Failures

Both of my KZ750 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 on my “first” 750 twin first.  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/bolts 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.

Backed Out Screw

This is immediately after removing the assembly from the seized engine. Note the melted screw head at top right. 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.

Cracks in Starter Clutch Housing

Now from the other side, I hope you notice the housing cracked in half, the missing metal chip (hole in housing in foreground) I recovered (in background).

I just pulled the rotor, removed all the debris, got a replacement starter clutch from a spare KZ400 engine, removed all the pins, springs and rollers (only kickstart now), and reinstalled the *housing* only with RED Locktite on the 3 screws.  So the final difference from stock is that it’s just missing pins, springs, and rollers.  When you push the starter button, it just goes “Wheeee…”

Since I did this bike’s repair first, I naively thought I needed the housing as a spacer (of course, if I had it to do over, I would have left the housing out).  More on that below…

750 #2:

So, shortly after, 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 that 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…  So I had to get the little bolt bits out of my rotor…

Three Sheared Starter Clutch Screws

This is where 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.

For this bike, I left out the *complete* starter clutch assembly.  This is because (as I mentioned above) I later 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.

IMPORTANT: I got some great advice over on the KTOF about an oil journal that exists on the crankshaft right under the starter sprocket.  Without the starter sprocket in place on the crank, 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.  And if the sprocket is on the crank, I may as well have the starter chain to hold it steady, and the starter to hold the chain, and the ankle bone’s connected to the….

Conclusion:

Now, all is well.  And I can kick start them forever now.  But do the pics make you cringe?

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

On a related note… If you decide to repair rather than remove, it appears as though Parts N More offers a starter clutch rebuild kit (though I’ve never bought anything from them myself).

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…
😉

UPDATE, 20120602:  In the recent days I have removed the starter sprocket and chain from 750 #1.  I thought I was hearing a noise in there, and decided to take the cover off and inspect the start clutch bolts and such, just-in-case.  Given my past experience, I didn’t want to risk it.  I found that everything was as I left it, even after thousands of miles.  So I re-assembled, leaving the sprocket on the crank in place (to keep pressure against the oil journal in the crank), along with the starter clutch housing and 3 bolts (without pins, springs, and rollers of course).  I would have taken the start clutch housing off too, if I had a working torch on hand; I used red Loctite and the 3 bolts wouldn’t come out… Ironic, don’t you think?  😉

 

66 Comments

  1. Drago

    Good info. I’m looking at buying a 1980 KZ 750 T. Like the classic retro look of the bike. Needs work so I’ll keep an eye on your site

    Thanks

    Drago

  2. Chris Graham

    Hey there!
    We PM’s each other a few times from the kz website, and I was wondering if there were any documented failures that you might be aware of with the CSR’s in this regard (the starter clutch failures?). Just wondering if they were ok with this particular model or not necessarily so?
    Is the rebuild difficult should this be my course of action?
    Thanks!
    Chris

  3. Biquetoast

    @Chris
    Not sure if the CSR is affected like the earlier models. really, you should post that in the forum to see if any other CSR owners can corroborate or refute! But I would advise you check it anyway, if you have a spare weekend morning, and rotor puller, and a gasket. 😉 If rebuild were to be necessary, it is incredibly easy once you have it apart, and if you have a starter clutch rebuild kit (as mentioned in the post above)… But sometimes the problem is not with the pins, springs, and rollers, but with the boss on the sprocket itself being worn un-evenly. And really on a lathe or a new sprocket will fix that…

  4. Chris Graham

    Might you know of a step by step written and picture process of this rebuild/repair for the mechanically challenged minds…er….like mine????
    Chris

  5. Biquetoast

    @Chris

    Actually, yes! The procedure for removing the sidecovers and rotor and starter clutch are all very well outlined in the Kawasaki Service Manual. You might have to hop around the manual a bit, but it has everything you need to know. Of course, if you are still confused by that after reading it through, make sure to visit the KTOF and we’ll help you there.
    😉

  6. Tony Benusa

    So did you just leave the starter motor in place? Also, I’m wondering where you got your the rotor puller and if you happen to know the size/thread pattern?

    I saw the poor man tool post and I think it’s an awesome idea, I just don’t want to use my rear axle if I don’t have to.

    Thanks!

  7. Biquetoast

    @ Tony

    Yep, starter is still there, as is both starter sprockets and chain. The rear-most starter sprocket is there due to needing to maintain oil pressure over the oil journal beneath it on the crank. The chain is there to stabilize the rear-most sprocket while running. The starter sprocket is there due to the chain. The starter is there so i didn’t have to plug the hole, and due to the rest behind it.

    I got my rotor puller bolt from z1enterprises.com. I’m not sure now which size it is, but feel free to call them if their website doesn’t make it obvious, and they can help…

    I hope that helps!

  8. Bret

    Been there, done that. Funny how my oil pressure light came on at idle in hot weather right after removing the whole starter clutch/gear from my crank. I’ll have to go back in there and look for the oil journal. Thanks.

  9. Dave

    I have a question about that (stupid thrust washer in your pictures) if you don’t mind, since you’ve done this more than once? I have a 77 b2 motor I’m using in another model. I had to replace the stator as it was dead. So I was of course checking that nasty starter clutch to make sure no bolts were lose or sheered off like you had.

    However even with the manual I own, I can’t for the life of me see if that Thrust Washer is suppose to go between the flywheel and starter clutch, or behind the entire assembly against the block.

    It appears by the manual to go between the flywheel and clutch assembly, yet every time I put it there and torque the flywheel bolt down to it’s 51-57 ft lbs it’s locking the assembly agianst the block and won’t let it roll over… my logic tells me it belongs behind the entire assembly agianst the block..

    So would you mind helping a doh brain out? The manual mentions the washer twice, yet both descriptions and pictures don’t really show it in plain English I can understand.

  10. Biquetoast

    @ Dave

    Ah… I understand your confusion… but it’s actually pretty simple… The washer goes between the starter sprocket (that sits on the crank over the oil journal) and the rotor. It keeps the sprocket separated from the rotor so it does not bind.

    If you look at the last pic in the post above, you can actually see it sitting on the sprocket right where it goes… It stuck to the sprocket as I removed it, and I left it there for the picture.

    I hope that helps!

  11. Trey

    So if you get down to the starter clutch and find the screws are loose but otherwise everything is still in good shape, is it sufficient to put a little locktite on the screws and tighten them back down?

  12. Biquetoast

    @Trey –

    I think that would be fine.

    …Of course, you’ll probably want to inspect those bolts, cause they likely have seen a bit of abuse on the threads while they’ve been loose like that…

    I actually put the bolts back in on one of my starter clutches. I wish I hadn’t now, but I did clean them off carefully and used *red* loctite…. I pity the future-owner (someday), but hey, that’s not my problem….

    😉

  13. Chris

    Hi there, I have been hunting high and low for the large sprocket that covers that oil journal on the crank with no luck at all, my bike is the first model 1976 and the shaft is a different size (larger) than later ones so the 400/440 stuff does not work. My next option is to make a sleeve, make it a press fit, that is too tight so you have to heat it to push over, when cool it won’t move. I have thought of just tapping the hole and putting is a grub screw but the idea that is might work loose one day didn’t appeal to me.
    All that said if anyone has that sprocket I would be happy to hear from you….. thanks…chris

  14. john sparks

    I purchased a ’76 kz 750 this winter….27,000 miles, I think.
    The starter motor ran, but did not engage. I took the clutch
    apart and found a cracked rotor. Purchased and installed a new
    one but the starter still did not engage. Everything looked ok,
    so I reasoned that the 3 little springs that hold the rollers against the shaft had gotten weak and lost tension. I found (in
    my junk box) three smaller springs that would fit inside the
    original springs. Inserted them (sort of “double-springed” it)
    and it worked great. Anyone see any long term problem with my
    solution?

    John

  15. Biquetoast

    @ John –

    Personally, I don’t think the double-spring thing is a problem, as long as it works. It think the only initial concern is that they all may not resist equally (tension) if they aren’t seated exactly the same way or if they collapse upon each other differently, and then your starter clutch may not grab well.

    I think the reason it didn’t engage when you re-assembled depends on a couple things… it could be that the springs you used were old and had lost tension, and/or maybe the surface that the clutch is grabbing on the starter sprocket is no longer perfectly round (I’ve seen it get almost invisibly bumpy from use) and the rollers no longer grab under normal use without added spring tension.

    I’ve read that some folks have put the starter sprocket on a lathe, and take a tiny bit of material off to bring it into round…

    😎

  16. David Lambert

    I have the 83 750 belt and the starter and clutch problems exist there also. Still worked but springs were weak or missing, housing was cracked, and bolts were loose at 19000 miles. Found a website selling rebuilt starters for $150, and a replacement rotor and starter clutch from a KZ440 on ebay. Everyone should keep in mind that both sprockets and the chain are not moving while the engine is running.

  17. Biquetoast

    David –
    Thanks for that report! I rarely hear that the later year models have the problem.
    I actually used a KZ400 starter clutch housing on my first 750 too.
    As for the sprocket/chain movement, you’re absolutely right. A couple points there; when the thing starts to com apart, I have a theory that it starts to jam up against the sprocket and spin the starter backward. I left in place, it will do who-knows-what to the starter, but it explains for me why mine *rapidly* slowed the engine to a stop at low-speed, rather than an abrupt, instant stop.
    During normal use, there is oil forced out of the oil journal in the crankshaft under the held-still sprocket.
    – ‘Toast

  18. David Lambert

    An easy bolt for removing the rotor is an oil filter bolt for any of the Honda fours made in the seventies or an after market chrome replacement available from JC Whitney. I use an impact air wrench on mine but imagine a long wrench and a sharp whack from a dead blow hammer would also work. Upon reassembly be sure to use thread locking compound (red) on the rotor bolt or the first time you start it the starter will unscrew the bolt. Always thought the engineers made a mistake making that a left handed thread as the engine rotates clockwise from the rotor end!

  19. Robert Luedeman

    Just got done with my ’76. Proper rotor holder for the earlier slotted rotor was readily available as was a starter clutch repair kit from Z-1 Enterprises. I noticed that the surface of the large sprocket where the rollers ride on was not entirely smooth and not having a lathe I smoothed it out with some emery paper. The clutch works fine. I would not recommend red loctite for anything you might want to take apart in the future, unless the factory recommends it. What I did with all the affected bolts and threads was to clean them with acetone and give them a good shot of compressed air. That way, the locking compound (blue) will do what it is supposed to. If they’re not clean it doesn’t work. I should get a first run off this week.

  20. Biquetoast

    @Robert Luedeman
    Thanks for that. I’m glad to hear that Z1enterprises is carrying the starter clutch rebuild set now! It was so hard to find, even just a year ago. For everyone else, here’s the link at this time of writing:
    http://www.z1enterprises.com/Starter-Clutch-Rebuild-Kit–Kawasaki-KZ650-KZ700-KZ750-KZ1300-ZN700-ZN1300-ZR750-ZX750-4361.aspx
    I have the rotor holder for earlier models but it won’t work for my later bikes, unfortunately.
    You’re right that a lathe is the best way to get the boss smooth and correct, but I like that you did whatcha gotta did to git it dun… 😉
    You are right about the red Loctite. In my case, I wanted it to never, ever, ever come apart on me again, and I’m pleased to say it’s working (after a recent check). But otherwise, I completely agree with you about the blue.
    Thanks again Robert, and make sure to post a follow-up someday.
    -‘Toast

  21. Robert Luedeman

    I had the bike running for the first time since I bought it in 2010 and probably for a few years prior to that. Pulled a good tablespoonful of rusty crap out of the petcock and put new fuel lines on. It fired right up and aside from the usual Mac header exhaust leaks it runs very well, nice quiet transmission that shifts well through all gears and good oil pressure. It sounds good, too-nice 2 cylinder rumble. I got the obligatory burn on my arm from getting too close to the exhaust.

    So the next thing is to drive it around the block a few times to evaluate the running gear.

  22. Biquetoast

    @Robert Luedeman

    Congratulations! Including your victory burn… 😉
    I think the “next thing” is actually the *brakes*, but that’s just me…
    😎

    -‘Toast

  23. Mischief5140

    Hi boys! How can you tell if the starter clutch on my KZ 750 is warped. I know it sounds stupid, but knowing what to look for when rebuilding a starter clutch before tackling the project is always best. Thank you guys for your time.
    M.

  24. Cindy

    Hey, I need some help please. I am trying to replace my starter clutch and don’t know how to pull it off with the tool. Realizing that it is a counter clockwise removal, I need to know how to hold the assembly from turning with the torque I need to remove the initial parts. Can you help?

  25. Biquetoast

    @ Cindy –
    You’ve posted a few comments, so I’m consolidating responses a bit here…
    – For my “poor-man’s tool tips”, see here: http://kz750twins.com/?cat=8 …this includes rotor holding, etc., but remember I’m not actually recommending you do the dumb things I do, I’m just sayin’..
    – First, I hate starter clutches. Second, I double-hate them on the KZ750. If you have any heft to you, I say tear the thing off and use the kickstarter. I *think* I’ve covered all the advice you need to live without the starter clutch in my posts, but if you have any more questions, drop me a line.
    Good luck!
    -‘Toast

  26. Jack

    Thanks for all the great info. A few more Questions: I have just taken the starter clutch assy apart on my 76 kz750b1, and considering leaving off the clutch assy, as you did with bike #2. However, I do notice the clutch housing has a fair amount of weight to it, and that weight becomes essentially part of the flywheel when bolted to it. If I leave this part off, wouldn’t it change the way the bike runs since the flywheel is now lighter? Did u notice any difference? I suppose it may increase throttle response so maybe not a bad thing…

  27. Jack

    And one other question…I notice small metal shavings sticking to the various magnetized parts near the stator…it seems like something is rubbing within the stator or starter clutch assy but I can’t figure out what it is….hate to think of metal shavings contaminating my engine….has anyone else here noticed the same?

  28. Biquetoast

    @ Jack –
    I have to say, I did not notice any difference. Perhaps there is one but it must be to subtle to notice, for me at least. But I guess I’ll try and notice now that you mention it. I rode Bike #2 to work just moments ago, and it’s still purring along like always… It does have rather quick throttle response, but boy, I’m just not sure I can attribute it to the reduced flywheel… So there you go, about as honest an answer as I can give, albeit a muddy one…
    😉

  29. Biquetoast

    @ Jack –
    Regarding the shavings… That is exactly what I noticed in my bike too, due to the stator bolt scraping against the starter sprocket. The shavings seemed to be mostly from the head of the bolt, rather than any material from the sprocket. Yet another reason I wanted that thing out…

  30. Jack

    Hmmm…. My bolts weren’t scraping, And all the pins springs and rollers looked okay as well… But I still had shavings and starter clutch still wasn’t engaging…anyway, I got it all back together and left out the starter clutch assembly like your bike #2. Tried to started up this morning and now it barely idles / backfires a lot and dies when I give it any throttle. Normally I would say that’s a carburetion issue But the bike ran fine a couple days ago before I did this…. Now I’m really stumped…. Do you know if the rotor has to be positioned in a certain way? (I.e. For ignition timing?)…. I wouldn’t think so…. May be a dumb question

  31. Jack

    And maybe another dumb question…. Does it matter which way the sprocket goes on? I.e. Which side of the sprocket faces outward? I have the service manual but some of these things were unclear when I read thru it…. Thanks for your thoughts

  32. Biquetoast

    @ Jack –
    I don’t think it’s dumb at all. Always doubt the effects of the latest change when troubleshooting. While I am certain the starter clutch and rotor change is not the *direct* cause of the new problem, you may have done other things in the process, like rattle some debris loose into your float bowl, etc.
    Backfiring is pretty much always leanness (carbs, fuel restrictions, etc.) or timing. There’s nothing wrong with checking these things again! My rule to myself is to always doubt my own assumptions, until it’s running right…
    😉
    The shavings… Could have been anything… I wish I could help there, but without seeing it, you know, I can’t be much good…

  33. Biquetoast

    @Jack
    …Ooohh… I believe it does matter which way the sprocket goes on…. It’s been a while now, but I seem to recall that the starter sprocket only lined up correctly to the chain in only one direction! It’s been so long, but if I were you I’d check. If it’s dragging, that could very well be binding, and maybe even spinning the starter sprocket. Also, that could cause blockage in that oil journal in the crank. Yeah, be sure on that one…

  34. Jack

    Ok thanks again…but the rotor doesn’t need to be in any particular position on the crankshaft right?..I guess I’m going to order another gasket, pull it apart again and reinstall the clutch assy….I have the rebuild kit but didn’t use it the first time around, but maybe I’ll put it all back together and the new springs will help the thing engage properly…I really got a workout this morning with that kickstarter!!

  35. Biquetoast

    @Jack –
    No, the rotor is fine however it fit.
    Oh, and about the gasket.. I finally got sick of paying $10-$20 per gasket from eBay folks and started making my own from a sheet of material you can get from the auto store.
    My 750’s have started with one kick almost every morning for the last many years, except when either sits for a week or so (which is rare), then maybe two-three kicks.
    If it runs right, that’s all it should take.
    Also, starter clutch rebuild kits are not always the fix… Sometimes you have tiny, almost invisible, imperfections on the sprocket surface where the starter clutch grabs. I’ve read that some guys over the years have put it on a lathe just enough to scrape the abnormality/bumps off, then did not need the rebuild kit.
    If I were you… I’d focus on getting it to run perfect. Of course the starter clutch is up to you, but as you know I’ll never have on in there again…
    😎

  36. Jack

    Biquetoast – thanks again for all the insight. I will definitely have to subscribe to this feed. Not sure which way to go now…I really don’t want to pull it apart again…I agree its possible something could have clogged within the carbs, but just surprised since it ran great just before I did this, and I don’t recall jarring the bike in any way during the process. I’m thinking my sprocket probably has the imperfections you mention, because everything looked fine in there otherwise.

  37. Biquetoast

    @Jack –
    Glad to help… Your question about the sprocket being on the wrong way keeps running around in my head…
    And here’s another tip for you that might be too late, unfortunately… Next time you assemble a part with gaskets, use axle grease on the mating surfaces. It does 2 things: holds it in place for putitng it together, and keeps it protected if you need to disassemble immediately after re-assembly in situations like this.
    -‘Toast

  38. Jack

    Good idea with the grease. I assume it won’t lead to leaks if you’ve tried it. Certainly would make cleaning the gasket surface easier…unfortunately I used gasket sealant (albeit only a very thin coating)….unfortunately, now I don’t know whether to pull the side cover back off the bike or to pull the carbs off….frustrating

  39. Jack

    Another thought…I recently bought this bike and admittedly have no idea what the carbs look like inside…all I know is I rode it around my neighborhood, then filled the tank and added some Seafoam, and started doing the starter clutch work after that….maybe the seafoam dissolved some petrified sediment in the bowls and now my jets are clogged…never know

  40. Biquetoast

    @Jack –
    Ahhhhh… That’s interesting. Yeah, it’s HIGHLY likely that Seafoam would have taken effect in some way, but what I’d expect to be the case is that it should have broken down some of the gummed-up gas in various locations, causing you to run *rich*… Why rich? Because most people will just adjust their pilot screws or float levels to a richer setting in order to compensate for years of gummed fuel. Then, when you add Seafoam, it’s like you’re re-setting the passages to factory settings, so you may need to un-do years of previous-owner minor adjustments…
    -‘Toast

  41. Jack

    I could see that happening. Probably need to pull the carbs off in any case just to know what’s going on in them and eliminate possible causes.

  42. Jack

    Biquetoast – is your 750 #2 a 1976? Doubt it makes a difference, but from your photos above it looks like the stator is configured differently than mine and maybe for that reason yours runs ok without the starter clutch assy and mine doesn’t….I know I’m reaching here. My ’76 has windings on the outside of the rotor ( I. e. the rotor spins within) as well as a set of windings that sit within the spinning rotor….lots of copper in there..

  43. Jack

    Correction..,not shown in photos above, but thought I saw a photo somewhere….maybe not yours

  44. Biquetoast

    @Jack –
    Ah. Both my 750s are ’78’s, and you’re correct; the stator changed with the ’78 B3 model. One of the few changes. The stator should be the same between the ’76 B and ’77 B2 models. By the way, I believe the ‘B and B2 used the same stator as my ’75-77 KZ400s did, for reference… At least visually they were same upon inspection…
    -‘Toast

  45. Jack

    Ok. Can’t imagine it makes a difference but maybe I will put the clutch mechanism back in just to test before I pull the carbs off…I have the flywheel puller and Kawasaki “special tool” (love how they call it that in the service manual!) to hold the rotor, so not too hard of a job…guess I should use blue loctite after reading above…

  46. Jack

    Ok last question (at least for now) I promise :). Is there a recommended torque setting for the case cover screws? I can’t find it in the service manual…maybe not since the oem screws are Philips head…I replaced with stainless Allen heads

  47. Carlos

    Hey, Bique

    not sure if anyone already suggested it or if you’ve done it, but you can take the actual motor out of the housing and leave the housing and sprockets in place. saves some weight.
    my entire starter assembly failed this weekend and now I’m on the quest to fix it. I’m going to try to make a contraption to hold that flywheel today out of a c-clamp. I’ll probably post it at the KTOF, but I’ve been studying your post here.

  48. Biquetoast

    @Carlos –
    Well, that’s clever… I hadn’t thought of that.
    But wait, if you do that, you lose a little entertainment value… My son loves pushing my starter button and hearing the “WWHHHZZZzzzzz!”
    Yeah, my flywheel/wrench trick is so ghetto… redneck… backwoods… Polish (yes, I’m Polish I can say that)… I’d regret posting about that too, if it wasn’t so funny.
    😉

  49. Carlos

    Bique,

    that’s certainly true; I do love that noise. I got rid of all that stuff on my bike, though. Probably chopped too much stuff off >_>
    how did you get the actual flywheel off? the bolt was easy, I just put a wrench on it and gave it a good whack with my rubber mallet. if you have a better way of getting the flywheel off I might abstain from the redneck engineering!

  50. Biquetoast

    @Carlos –
    Ah! I’ve got one for ya… I can’t remember if you mentioned what year your bike is, but here you go…
    You can buy a flywheel puller from Z1Enterprises.com for a few bucks, or just use a bolt. Or if you have a ’78/79, you can use your rear axle. The ’76/77 had a different puller bolt size, so you can’t.
    When you got the left-hand thread bolt off, you see the outer right-hand threads in there? You just gently screw in a bolt that until it is firmly hand-tightened. Give it a pop with a hammer, and you should hear a loud bang when the press-seal gives loose. Then be ready to grab it, because it will slide off fast and it’s heavy! It’ll hit the floor if you don’t grab it in time.
    I still remember that bang. Surprised me, it was so loud.
    😎

Leave a Comment

Your email address will not be published. Required fields are marked *