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Discussion Starter · #1 ·
I bought a CB900c a couple of months ago and finally got around to working on it. I so far have changed out the old stator and rotor, as well as the rectifier and put a brand new battery in. The starter spins now and will continue to do so after I let off button, but I don’t think it’s getting a spark? You can hear the starter spin and engage but it just won’t turn the motor over. The previous owner said it killed the battery over last winter so I immediately replaced some components of the charging system. The old stator had no continuity between the wires and I figured I might as well do the rotor and rectified while I was at it. Any advice would be greatly appreciated!
 

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You can hear the starter spin and engage but it just won’t turn the motor over.
Just to be clear on the terminology: if the motor "won't turn over" it means the crankshaft won't turn, the pistons won't move up and down; in other words when the starter engages all you hear is a "thunk".

Is what you really mean that the motor cranks (turns over) but won't fire?

If the bike sat with gas in it over the winter and you didn't put any kind of fuel stabilizer in the tank then the carbs are probably gummed up. In that case you'll have to clean them, maybe by taking them apart. Search the forums for "varnish" or "gummed up carbs".
 

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Discussion Starter · #3 ·
I guess I was wrong about that then! I’m still new to this so I guess it’ll be hard for me to explain. What I’m hearing is a spinning noise as if the starter is engaging and spinning the crank. I don’t hear a thunk at all though, just something spinning. I have removed the starter and checked it out and all seems to be fine with the gear on the end of the shaft and inside the case as well.
 

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Discussion Starter · #4 ·
I have also spun the crank by hand with a wrench and it’s pretty easy to spin. Compression seems to be fine as well.
 

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Pull the plugs and spin the starter, if the engine is turning over you should get air puffs out of the plug holes.
 

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Does the engine turn over when you are pressing the start button, or does the starter just spin and then stop once it engages the engine, or does the starter spin without affecting the engine?

Have you verified that the engine is not stuck or frozen?

You can pull the plugs and verify it cranking by that air blowing in/out of the cylinders, and it should also crank the engine faster.
 

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Discussion Starter · #7 ·
I’m thinking it just doesn’t engage.. You can hear it spinning the gear inside of that casing, I’m not entirely sure what that is called as I’m still learning the terminology so my apologies! But I have already pulled the plugs as well and felt compression when I spin the stator rotor so I don’t think anything is seized up. Next step I’m assuming would be to check for spark?
 

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No the next step is to find out why the starter is spinning but not engaging the motor. Motor has to be spinning before you need to worry about spark.
 

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Discussion Starter · #9 ·
No the next step is to find out why the starter is spinning but not engaging the motor. Motor has to be spinning before you need to worry about spark.
Perfect, I have the manual for it but I’ve been sitting through it to no avail as I’m not sure what I’m really looking for. Any clue as to where I should begin looking?
 

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Most starters spin a centrifucal clutch which then engages the engine. Pull everything there, check and re-install.
 

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I’m thinking it just doesn’t engage.. You can hear it spinning the gear inside of that casing, I’m not entirely sure what that is called as I’m still learning the terminology so my apologies! But I have already pulled the plugs as well and felt compression when I spin the stator rotor so I don’t think anything is seized up. Next step I’m assuming would be to check for spark?
What do you mean, you felt compression? Puffs of air as suggested previously? If yes, then next would be spark or fuel. It isn't clear to me if you are getting fuel or not. Were the plugs wet when you removed them? If so, then yes, spark would be the next step. You might try giving a shot of carburator cleaner in the air filter and hitting the starter. That assumes you replaced the plugs of course and the engine is in fact spinning(turns over) when you hit the starter button.

Have you read the thread, My motorcycle won't start? One of the moderators(Miss Mercedes) put a lot of work into that thread to compile many threads relating to motorcycles not starting.
 

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Discussion Starter · #12 ·
I’m thinking it just doesn’t engage.. You can hear it spinning the gear inside of that casing, I’m not entirely sure what that is called as I’m still learning the terminology so my apologies! But I have already pulled the plugs as well and felt compression when I spin the stator rotor so I don’t think anything is seized up. Next step I’m assuming would be to check for spark?
What do you mean, you felt compression? Puffs of air as suggested previously? If yes, then next would be spark or fuel. It isn't clear to me if you are getting fuel or not. Were the plugs wet when you removed them? If so, then yes, spark would be the next step. You might try giving a shot of carburator cleaner in the air filter and hitting the starter. That assumes you replaced the plugs of course and the engine is in fact spinning(turns over) when you hit the starter button.
Yes, I felt little puffs of air when I took the plugs out. I didn’t notice any of them being wet, and I picked up carb cleaner last night to try it out today. I’m going to remove the plugs and check for spark as well just to make sure, and then I will try and spray some into the air filters.
 

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Discussion Starter · #13 ·
Update: I have gotten a lot of progress today I’d say. The bike is really trying to fully start, but it sounds like it’s getting to a point where the starter is slipping the gears after turning a few times. I don’t know if that means I need to shim it? I took the crank case off the left hand side of the motor and the starter out, which the gear on the starter has a little 45 degree or so chamfer in the tips of each spline. Inside of the crank case looks pretty clean, no metal shavings or anything. The spark plugs though, are completely dry when I pulled them again. Would that mean the jets in the carbs are plugged up? I bled them out of the bleeder tubes attached to them with the little set screw and I had a nice healthy flow coming out of each one.
 

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Discussion Starter · #14 ·
Another Update: I put some fuel on the spark plugs yesterday and had no luck. I sat the new battery on the trickle charger all night, and viola, it ran for about 10 minutes until I shut it off. I immediately was able to start it back up! So I shut it down again and put the seat back on and got it ready for a test drive, and now it’s back to not starting. I put the battery back on the trickle to fully charge it again so we’ll see. Not really too sure as to what would cause this, any suggestions?
 
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