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1977 CB750 Slip noise when starting - Help?!

ViceCube

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Hi all,
Newbie question was hoping you could assist with identifying the issue. I just picked up this '77 CB750 and seems to run good if it starts - however, just tried starting today and get this odd noise when trying to start (for the life of me cannot get it to start today) - sounds like some sort of slippage. It sounds like it may be coming from the area that holds the stator or the area housing the starting motor - pulled the cover off the stator shell area and didn't see anything that looked odd. I'm not a mechanical guru by any means (yet) - picked this bike up to learn more and have as a first project bike. I recorded and put on youtube (click below) for assistance in identifying. Any advice/help greatly appreciated - thanks!!
https://www.youtube.com/watch?v=9lBDDIXAV2s
 
one way starter clutch is slipping, either has broken springs or the rollers and or the race on the gear is worn. It is common and getting more common as you get more miles. They make roller kits but if the parts the rollers ride on are worn you have to look for a better used piece as they are no longer produced. The clutch is under the stator behind the rotor, the rotor and the big gear are part of the clutch. You will have to get a puller to pull the rotor off to inspect the parts. Ultra slippery motor oil can cause them to slip also, even the synthetic motorcycle oils seem to make them slip.
 
Thanks dirtdigger - How do you get to the springs to inspect? I pulled off the cover and saw the big gear (along with two smaller gears that coupled to the starter motor). I didn't see the springs - do I need to remove the big gear to get to those? What is a puller?
I did just change the oil a few days ago (had it running afterwards) - just used regular 10w-40, non synthetic...
 
You have to pull the rotor off to get to the clutch, it is built into the back of the alt rotor. You have to get the rotor puller to get it off of the crankshaft, the rotor and big gear will come off together. The puller threads into the end of the rotor. Get a cheap one off of ebay or elsewhere. Automotive can cause slippage...did it work before and now slipps since the oil change?
 
Started up and ran great after the oils change - took it out for about 50 miles or so with a few stops. It sat for a few days now this is happening and just won't start up - that said, I had heard this noise before the oil change too but it would engage/start after a few tries previously.
 
Ultra slippery oil can mean most car oils now, they replaced the zinc with really slippery stuff and it doesn't have to really be synthetic to do it. I'm thinking the 'friction reducers' they replaced the zinc with are synthetic in nature. Why the starter clutches slip so much easier now.

I always used car oils in my DOHC and never a problem but other guys now have fits with it on them.

Could simply be worn rollers or springs too, they don't have to wear hardly at all to give issues as the lockup requires all three to work perfectly or no lockup; it slips loose.
 
I'm having the same issue. Waiting on a rotor pulling tool to inspect parts. Is is it normal for there to be oil inside the alternator cover? Also should the large starter gear inside the rotor have end play? It slides in and out about 0.100" maybe a little less?
 
You will need to get a rebuild kit for the starter clutch, more then likely it has 1 or 2 broken springs. Also need to inspect the surface the starter clutch rollers ride on when you get it apart. Yes on the sohc the rotor is in oil and yes side play is normal.
 
Mine has been doing this (sporadically) for more than 20,000 miles. Over 50,000 miles on it now and it doesn't seem to be getting any worse. Once it slips I just let off the starter button and try again. The sound of the starter clutch slipping is indeed a terrible sound and sounds much worse than it actually is. I replaced the springs and rollers on one and it still slipped occasionally which meant that the surface that the rollers ride on was worn. For me, I just live with it. By maintaining the engine in a good state of tune the time spent cranking the motor with the starter is minimized.
 
yes the problem is the area on the big starter gear where the rollers ride. On the ones that still slip after new rollers and springs, if you look there will be small chatter marks...problem is the gear is no longer available. I have done several and everytime if you find a good used gear that doesnt have chatter marks, the clutch will no longer slip. Im convinced if new gears would be available the issues could be fixed. Oil type is also very important on these...if the clutch is marginal, synthetic will make it slip about 95% of the time. If you switch back to mineral oil and run it a couple times the clutch magically works again almost 100% of the time. I have tried this several times and get consistent results. On my big bore street motor I have gone back to mineral oil to keep the starter clutch reliable. In my drag motors I run synthetic as I am not worried about the clutch not working 100% of the time, plus the big bore and high compression dont help the clutch any but I have other ways to start it if need be.

I agree if your tuneup is spot on you keep the crank time to a minimum and lessens the chance the clutch will slip. I dont think the slip and loud clunk will hurt much if it only happens intermitently. I havent noticed any metal flakes in the area when dissassembled, I just worry about breaking parts with the sudden shock when the clutch catches, which is why I keep trying to find a decent fix for the issue. If only the gear was available....
 
Oil type is also very important on these...if the clutch is marginal, synthetic will make it slip about 95% of the time. If you switch back to mineral oil and run it a couple times the clutch magically works again almost 100% of the time. I have tried this several times and get consistent results. On my big bore street motor I have gone back to mineral oil to keep the starter clutch reliable.

I tend to favor mineral oil as well. It also seems to smooth out the operation of the transmission and transmission clutch too.
 
FYI, the slipping is a huge problem on the DOHC as it has bigger engines and the problem is worse with them. Any slight kickback as the engine begins to light off and they slip. There is a Yamaha R6 part fix that replaces the 3 roller crap part with a multi-section true sprag like an automatic trans uses and it's bulletproof and you only need to drill a few holes to make it work on the DOHC. Wonderful part and problem is gone forever. .

https://www.youtube.com/watch?v=sYJSUiXDim8

That 3 roller clutch is a joke, all 3 rollers have to lock at the exact same time and what are the odds with some wear? One slips in the least and the other 2 let go instantly. The R6 part locks solid as a rock as there are so many locking members there not all of them have to lock and it STILL works.
 
I have no idea whatsoever..........but the working diameter of the hub the rollers (sprags in the case of the R6) lock against is 1.660" or very close. That should determine a possible or absolutely not scenario. After that it would get down to the ring fitting in the back recess of SOHC alt rotor and the ring OD is 3.500". It looks like it could be turned down on a lathe a bit if needed.

The DOHC has the starter on left side of bike, important to make the clutch lock the one way correctly.
 
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