Author Topic: Starter Motor spins freely? (Read 1153 times)

bagatyr · « **on:** March 18, 2019, 09:42:33 AM »

Hey,
I've finally got some wiring done on my bike, redoing it from scratch with an M unit and Revival cycles starter solenoid. When I try to start it, the starter motor sounds like its spinning freely. A high speed whine. I took off the points cover, and the bolt at the end is only moving intermittently, mostly not moving. I took off the alternator cover, and the big gear (flywheel? or starter clutch gear?) spins freely when I crank the starter. I've replaced the springs and caps. Battery is new and has been on a tender between starts.
I don't have any gas in the carbs as of now because I wanted to see if the starter worked at all. Bike is new to me, and I've only been kickstarting it. It has sat for a couple of months while I mated a new front end/worked on other parts. So I'm not sure if the starter ever worked.

I've read some other posts and it seems like my alternator bolt may be loose? I haven't taken off my cover to check yet, wanted to post first and ask if there are other ideas I should try

Thanks

BenelliSEI · « **Reply #1 on:** March 18, 2019, 11:23:29 AM »

What year and model SOHC? Makes a difference......

bagatyr · « **Reply #2 on:** March 18, 2019, 11:25:58 AM »

Ah sorry, it's a 1978 CB750K.

Bodi · « **Reply #3 on:** March 18, 2019, 11:37:12 AM »

750?
If so then the alternator rotor (AKA flywheel) may be spinning on the crankshaft. Remove the side cover and have a look. With the cover off, don't spin the crank too much with the starter, definitely do not start the motor. An oil gallery plug is held in by the cover, and there's an oily surprise in store for you if you bring up the oil pressure.
If the rotor is tight, and the starter motor sounds like it's going, but the engine does not turn over... a few possibilities. Starter reduction gears may be stripped (inside the starter motor). The output shaft gear on the motor, or the driven gear on the starter clutch, could be stripped. Most likely is the starter clutch itself. By some weird magic the rollers, springs, and spring caps can escape from the assembly, also the body can shatter so the thing does not work.
So first step is to see if the alternator rotor turns when the starter is engaged, and go from there. I have never seen a rotor come free on any SOHC4 engine, as a rule they need some powerful persuasion to get them off. The engine would run like complete sh1te without a flywheel as well, so it's hard to ignore... I doubt if anyone could ride one with it loose: this is not a problem where you might just say "oh well, there's a kickstarter, I can do without electric start".
Not a 750? Then the rotor is not part of the starter drive so look elsewhere.

bagatyr · « **Reply #4 on:** March 18, 2019, 11:44:38 AM »

Thanks Bodi, I will see if the flywheel is spinning. I took the cover off yesterday and checked the reduction gears, they are in great condition and everything is working there. It just seems like something is slipping. The bikes got 50k miles on it, so I'm guessing the flange on the starter clutch gear is slightly worn/pitted. I have replaced the springs/caps/rollers a couple of months ago but never used the electric start as the bike wasn't wired for it. It was a basket case.
I'll update later tonight/tomorrow, thanks again.

BenelliSEI · « **Reply #5 on:** March 18, 2019, 11:53:09 AM »

+1 on evything Bodi says...... when you have the alternator cover off, the large gear behind the rotor should spin freely in one direction and lock in the other. If it spins both ways, starter clutch parts need replacing.
The spur gear cluster (on a shaft between the small starter gear and the big rotor gear) can be removed. All the teeth present? Also, watch for a large, hardened flat washer that goes between the cover and the spur gear cluster (it slips on the shaft AFTER THE GEARS/SHAFT and BEFORE installing the cover). It often “sticks” to the cover, goes in the wash tank and then disappears! The last two bikes I worked on were both missing this washer and it is NLA! Without it, the spur gears are misaligned and damage easily.......

bagatyr · « **Reply #6 on:** March 18, 2019, 12:00:50 PM »

Thanks Benelli, I'll check for that plate. I don't remember seeing it the last time I pulled all of this apart.

BenelliSEI · « **Reply #7 on:** March 18, 2019, 12:12:21 PM »

Good luck..... let us know what you find.

bagatyr · « **Reply #8 on:** March 18, 2019, 07:29:20 PM »

Ok, so the flywheel IS spinning when I run the starter.
The large gear only spins in one direction, it's locked in the other. Do I just need to tighten that alternator bolt? Ive got an impact gun with a 73lb torque stick, that should work right?

scottly · « **Reply #9 on:** March 18, 2019, 07:35:49 PM »

What's a torque stick?? Yup, tighten that bolt.

bagatyr · « **Reply #10 on:** March 18, 2019, 07:39:19 PM »

It's technically called a "Torque Limiting Extension", a bar that limits the amount of torque that an impact wrench will produce.

scottly · « **Reply #11 on:** March 18, 2019, 07:46:01 PM »

Hmm, never heard of such a device!?? Is it basically an extension with a clutch set to slip at the desired torque?

bagatyr · « **Reply #12 on:** March 18, 2019, 08:09:57 PM »

It's a pretty great little addition! Basically an extension bar that flexes at certain torque thus preventing torque past a certain #. So I've got two at 65 and 80. I had this torqued to 65 previously. But at 80 the flywheel is no longer slipping and I can hear the engine actually trying to start instead of the high pitch whine

scottly · « **Reply #13 on:** March 18, 2019, 08:21:48 PM »

In case the alternator rotor starts to slip again, the spinning rotor on the crankshaft may have left the surfaces on the taper less than perfect. The wedge fit of the taper is all that locks the rotor to the crank. The fix is to use valve lapping compound to lap the rotor and crank together. It should be used sparingly and thoroughly removed before final assembly...

Don R · « **Reply #14 on:** March 18, 2019, 08:24:40 PM »

My buddy that ran my old drag bike chucked the end off of the crank running a stocker flywheel. He warned me to lighten up or go without.

bagatyr · « **Reply #15 on:** March 18, 2019, 08:36:58 PM »

Ok so this is after tightening, here's a video
Does that seem right to everyone? It seems like it engages and disengages properly.
I'm holding the little (reduction?) gear with a socket extension so it doesn't fly out

scottly · « **Reply #16 on:** March 18, 2019, 08:43:29 PM »

Sounds good to me.

Button it up..

scottly · « **Reply #17 on:** March 18, 2019, 08:49:10 PM »

Quote from: Don R on March 18, 2019, 08:24:40 PM

My buddy that ran my old drag bike chucked the end off of the crank running a stocker flywheel. He warned me to lighten up or go without.

The older rotors were 1.234 pounds heavier than bag's late rotor, and judging from my K1, were unbalanced.

bagatyr · « **Reply #18 on:** March 18, 2019, 08:49:57 PM »

Thanks for the confirmation scottly! Thanks everyone else for the quick and easy turn around. You guys are great. I'm glad this was a minor issue.

scottly · « **Reply #19 on:** March 18, 2019, 09:00:39 PM »

Thanks for the tip about torque sticks!

BenelliSEI · « **Reply #20 on:** March 19, 2019, 05:39:55 AM »

Curious....... was the large washer on the outside of the spur gear shaft?

News:

Author Topic: Starter Motor spins freely? (Read 1153 times)