Motor won't turn

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coffee_brake · Sat Mar 28, 2009 8:20 am

Rebuilding a CB750, when I got it the top end was apart and I'm putting it back together.

I got the cylinder block on no problem, but I put the head on and the motor won't turn. I'm supposed to be turning the motor by hand with a wrench on the right side to line up everything for the timing.
I had no problem turning the motor by hand before the head was on, now it won't budge but maybe 12 degrees of the rotation and locks solid.

Tranny is in neutral. Back wheel is off the ground anyway.

After trying to put it in gear and (gently) rock it with the back wheel, I took the entire head assembly off again and could turn the motor. So I don't think the cam chain is binding up in there. I kep tension on it while I put the head back on, so it wouldn't ball up and kink in the cases, but now the head's back on and I still can't turn the motor more than 12 degrees or so.

Any ideas what I've done wrong? Looks like I've put the runners for the cam chain in by the book, but could that be wrong? The front one only has one notch to sit in on top of the cylindler block, and it's in there. The other end of it just sits down in the cases, as best I can tell and doesn't have a notch to go into.

I'm really stuck here and would appreciate any ideas from you folks....

#2 Unread post by **Wrider** » Sat Mar 28, 2009 9:02 am

That's a tough one. Usually when a motor locks up like that it's because the cams are set wrong and the pistons are hitting the valves.
In your case I'd bet the rings aren't all the way in the cylinders. Are you installing them with all of the pistons at equal heights or with two high and two low?

If you're sure all of the rings are in, then I'd bet it's compression stopping it from turning. It can seem like it's locked up but keep working at it and it'll go.

coffee_brake · Sat Mar 28, 2009 9:11 am

The camshafts aren't installed yet. Just the head with the valves in it, the hydraulic tappets are sitting in their holes. Not even any sparkplugs in it.

The motor has two pistons high and two low, so I could manage compressing the rings during install. Also what the Clymder's said to do. I was pretty sure they all cleared. But I can check that again, lots and lots of room for error if I'm the one in there!

The crank bolt on the right side is weird, it is almost inpossible to get a grab on it. It is not shaped like a nut, it is shaped like a rectangle. A box-end won't fit on it but an open-end will, but only about halfway. I can't get a grip on it, the wrench slides off because there is a sloped shoulder with a very shallow area where the wrench will fit.

The flywheel with the timing marks is on the left. I know you're not supposed to move the motor from that side, but it has a real bolt that I can put a box-end wrench on it, could I just rock it back and forth from there to maybe get it to move? Or will I damage the motor if I try that?

Sure appreciate the help....

#4 Unread post by **Wrider** » Sat Mar 28, 2009 12:42 pm

That is strange... I don't know why it'd be like that.
You may want to wait for someone else to respond, but I don't see why that would damage the motor to turn it from the flywheel side. (MMI taught us on modern engines where you can only turn from one side.)
If there aren't any responses by tomorrow morning I'll call up a buddy and get a straight answer for you!

coffee_brake · Sun Mar 29, 2009 1:04 pm

Cause I sure didn't sprout any ideas overnight (sometimes I do, but not this time.)

I sure would welcome any ideas....

#6 Unread post by **Wrider** » Sun Mar 29, 2009 2:18 pm

Heya I'm back. Called up a buddy who loves older bikes and asked him for ideas. He said the ring idea as well, but then also to check the free-play in the piston/con-rods/crank, because apparently those go out relatively often when a CB has been sitting a while.
Oh, and feel free to turn from either side, shouldn't hurt it!

coffee_brake · Mon Mar 30, 2009 11:53 am

oooh...found it.

Bottom oil ring on cylinder #1 hung up. I don't know how it did that or what I did wrong, but I had to drive the cylinder block off of that piston. There's a deep gouge where the ring cut into the cylinder wall, and that one oil ring is quite non-circular now.

I don't know if it's ruined. The gouge is below the bottom of the piston's travel. This bottom oil ring only gouged where it won't ever travel, anyway. The piston is not badly damaged at all and looks like it will run.
I can buy a set of rings for this one piston but not another cylinder. Can't afford to bore it out and buy all new pistons/rings either.

If the gouge makes this cylinder ruined, then I have some Nighthawk parts for sale....

Crap.

#8 Unread post by **Wrider** » Mon Mar 30, 2009 12:10 pm

If it's below the travel of the piston I wouldn't worry about it in the least. Unless you plan on regularly pulling the cylinders off, get the new rings, put the head back on, and run it!

Tennif Shoe · Mon Mar 30, 2009 4:09 pm

below BDC dont worry about buy a new ring and put it back together

coffee_brake · Tue Mar 31, 2009 7:23 am

Not TOO pricey of a lesson, mechanic says it's good to run. Showed me some aluminum props to hold pistons steady while the cylinder goes on, and even said I should borrow them. But I could just make some from wood in an hour.
So ya'll are right, throw a set of rings on that one cylinder and keep going with the assembly.

What a relief....