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'81 K - Starting troubleshooting

JMC

CB750 Enthusiast
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Location
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Hello fello wrenchers,

I've been trouble shooting my '81 CB750K auction find for a couple weeks now.

I replaced the wiring harness with a new one from Vintagecb750 mainly because the one on it was "highly customized".

My current issue is getting the engine to turn over. The original starter relay was just clicking when I pressed the start button (and smoking a bit). I replaced it with a new one, but the issue remained.

I followed the ground wire and noticed the connection to the starter was extremely corroded. I have cleaned that up. Now, the starter will "whir", but only if it is not seeded fully/properly. If I seed it to where it should be, it tries to turn, but it seems like something is seized.

Question, should I replace the starter, or is it something else?

Thank you!

JM
 

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Also, that big wire that you cleaned the connection on is the positive from the starter solenoid/relay NOT the negative. The negative connection at the starter is achieved from getting its ground from the engine, so, the starter has to be bolted tightly in place to get its ground. Those bolts are loose in your picture. You can check that ground connection with an ohm meter, one probe on the battery negative and one probe on the starter case or starter bolt.
 
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remove your battery and put it on a charger.
while waiting for the battery to charge, reinstall the starter motor.
once battery is charged, use a pair of heavy duty jumper cables to jump the starter directly from the battery. (black to ground, red to wire terminal post on starter)

if you do not see an improvement, your problem is likely the starter. they are internally brushed DC motors and sometimes the brush holders will corrode causing the brush to get stuck. you could try to disassemble the back cap off the starter to see if the brushes are in fact stuck. if you do this, remove the armature from the motor housing and clean up the commutator bars with some scotch-brite. Then take some WD-40 and spray the brushes and try to push them in and out of the holder to break them loose. sometimes you can buy brush kits for starter motors like these and are not terribly expensive. just be forewarned that when you remove the armature and brush cap, there will likely be some wave washers on the bearing ends. do not lose these.
 
Have you tried turning the engine via the crankshaft? If the engine is seized the starter won't turn it
 
I pulled the spark plugs out and tried to turn the crankshaft with the bike bike up on the middle stand. It doesn't turn whether I'm in neutral or in gear.

I'm guessing that equals a seized engine?

I have poured oil into the spark plugs holes in hopes of loosening things up (the spark plug spark ends looked atrocious).
I've also popped the case off of the top (I know I should remove the engine if I'm going to work on it :) ) hoping to see more. It doesn't look promising. This is after cleaning it up a bit. There is definitely rust inside, which is surprising. The oil is super clean.
 

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Also, that big wire that you cleaned the connection on is the positive from the starter solenoid/relay NOT the negative. The negative connection at the starter is achieved from getting its ground from the engine, so, the starter has to be bolted tightly in place to get its ground. Those bolts are loose in your picture. You can check that ground connection with an ohm meter, one probe on the battery negative and one probe on the starter case or starter bolt.
It's a new battery and I used a Noco Genius 5 to charge it (the thing is ALMOST idiot proof).

I picked up a voltmeter and will double check what the battery is putting out, but not getting the crankshaft to turn may unfortunately be what the issue is on this rat bike. It was a $90 auction buy, so I guess I can't complain too much for this blessing of a now much-more-complicated project.
 
A cheap scope might be of value to look in the cylinders. If you have stuff in your cylinder(s), oiling won't help. Pulling the engine to get the cylinder head off is probably the right move here.
Side note: if the bike has been sitting a long time, the clutch plates might be stuck together and not releasing. This would prevent the engine from turning over even if the transmission is in neutral. Opening the clutch would be a quick check before taking out the engine.
 
A cheap scope might be of value to look in the cylinders. If you have stuff in your cylinder(s), oiling won't help. Pulling the engine to get the cylinder head off is probably the right move here.
Side note: if the bike has been sitting a long time, the clutch plates might be stuck together and not releasing. This would prevent the engine from turning over even if the transmission is in neutral. Opening the clutch would be a quick check before taking out the engine.
I am extremely glad you mentioned the clutch plates. I have noticed some strange behaviors with the transmission when shifting gears - sometimes it shifts through all of them, sometimes it only seems like 2 or 3.
I'll will check there before really digging into the engine. Thank you!
 
I am extremely glad you mentioned the clutch plates. I have noticed some strange behaviors with the transmission when shifting gears - sometimes it shifts through all of them, sometimes it only seems like 2 or 3.
I'll will check there before really digging into the engine. Thank you!
With those shifting problems.........are you 100% sure that you are in neutral when trying to rotate the engine?
 
seems like you are dealing with a potential seized engine at this point. if the rear wheel spins in neutral and you do not have the clutch pulled in, then chances are that your clutch plates are not stuck. at this point, it would be a good idea to remove your spark plugs and turn the engine over by hand. with the spark plugs removed, it should turn over very easily in neutral. if it seems like you have to put a lot of effort into turning the engine by hand, you likely have bigger mechanical issues.
 
I think the clutch plates could still be stuck if the rear wheel spins in neutral, but if you're unable to turn the engine via the crank bolt in neutral, you do likely have a seized engine.
 
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