Engine Locking up after combusting (1985 cb650sc)

[Cr20]

Hey everyone, I was wondering if yall could help me out. My engine spins freely with the starter motor, but as soon as I add starter fluid it combusts, then locks up. thanks!

[Bob H]

What do you mean "locks up"?  Siezed??

Your question lacks a lot of info, including what bike.
History, what is your diagnosis?

[mollusc]

Bike is in the topic headline:  1985 CB650.

[Cr20]

What do you mean "locks up"?  Siezed??

Your question lacks a lot of info, including what bike.
History, what is your diagnosis?

Yes, the bike seizes up and the starter motor can barely move the engine at all. I dont have much history on the bike but i replaced the exhaust camshaft and the head.

[mollusc]

Does it "unlock" at any point?  Did you check for free rotation as you were assembling?  Can you directly rotate the engine with a wrench?
It sounds like you might have some sort of mechanical interference.

[Cr20]

Does it "unlock" at any point?  Did you check for free rotation as you were assembling?  Can you directly rotate the engine with a wrench?
It sounds like you might have some sort of mechanical interference.

Yes it does unlock, and becomes free with a wrench, My original thought was that it may be a timing issue, but i was ant sure because it does spin freely after i get it unlocked.

[Bob H]

...the bike seizes up and the starter motor can barely move the engine at all. ...i replaced the exhaust camshaft and the head.

... My original thought was that it may be a timing issue, but i was ant sure because it does spin freely after i get it unlocked.

I think you were right on the timing being the cause.
You could have an exhaust valve contacting the piston - force it through that - spins free until next exhaust cycle.
I have a single camera borescope. You would need a 2 camera (one aims out the side) to hopefully see the valves. This cheap one is $26 right now.
May be able to borrow a better one from Auto Zone.
I would watch some youtube videos to see if a borescope may help you.
Or triple check the timing marks.

[Cr20]

...the bike seizes up and the starter motor can barely move the engine at all. ...i replaced the exhaust camshaft and the head.

... My original thought was that it may be a timing issue, but i was ant sure because it does spin freely after i get it unlocked.

I think you were right on the timing being the cause.
You could have an exhaust valve contacting the piston - force it through that - spins free until next exhaust cycle.
I have a single camera borescope. You would need a 2 camera (one aims out the side) to hopefully see the valves. This cheap one is $26 right now.
May be able to borrow a better one from Auto Zone.
I would watch some youtube videos to see if a borescope may help you.
Or triple check the timing marks.

Yeah I will check the timing marks, would a timing issue allow it to still spin freely until combustion though?

[Bob H]

Yeah I will check the timing marks, would a timing issue allow it to still spin freely until combustion though?

Visualize one of your pistons coming up while a valve is open.
I'm not saying that's what happened. Just guessing from 2000 miles away.

In the following video he talks about what can happen if a timing chain (or belt) breaks or skips a tooth.
"If" your timing is off, and "If" cranking it beyond the point of resistance bent a valve stem - that would suck.

[mollusc]

I think the timing can be off by several teeth and still not make contact between the valves and the pistons.  True for the 700S anyway.  It does sound like a good place to start though.  Just pop the camshaft cover off and watch the marks as you turn the engine.

[Cr20]

I think the timing can be off by several teeth and still not make contact between the valves and the pistons.  True for the 700S anyway.  It does sound like a good place to start though.  Just pop the camshaft cover off and watch the marks as you turn the engine.

My intake valve seems to be advanced about 1 tooth.

[draser]

Can you feel a hard spot when rotating by hand? I'd also try to correct camshaft timing and see if it makes a diff.