Topic: 1981 cb650, engine seems to lock up until manually reversed?

[basketcase81]

So what I have is an 81 cb650 that I got from my dad. Been sitting in the garage for a while now and I decided to start working on it.

It is a basket case so right now I'm just trying to see if the motor runs. I hooked up the harness to the battery and charging system and jumped across the starter solenoid with a screwdriver and it seemed like it was trying to turn but wouldn't, just kinda clicked. I took off the alternator cover and the engine turned that way but seemed to get stuck at a certain point. Then I jumped the solenoid again and it started cranking, but if I stopped it would not crank again until I turned the crank backwards a little.

Anyone have a clue where to go from here. I'm  probably not going to mess with it if I have to do a tear-down.

Please give me some input, Thanks

[mystic_1]

That could just be a weak battery.  It doesn't have enough "oomph" to push the motor past the hump when one of the cylinders reaches highest compression.  Rotating the engine backwards gives it some room to "wind up" before pushing past the compression.

Repeat the test with a fresh battery (or wire a car battery in parallel).

mystic_1

[basketcase81]

Repeat the test with a fresh battery (or wire a car battery in parallel).

mystic_1

I did use a car battery.

edited: formatting

[Bodi]

Technically it's not a basket case unless almost fully disassembled with all the parts are in baskets/boxes. Yours sounds more like a typical "barn find" with most of the bike still together!

Try taking the plugs out and turning it over that way. Do not try to turn the crank with the "big nut" at the points plate end: you will just destroy the weak bolt that the big head is on. If the engine turns fairly freely forward all is well, if it hits a solid feeling obstruction don't force it... find out what is doing that.

Maybe your battery is weak, maybe something serious is keeping the engine from turning - like a stuck open valve that's easy (and cheap) to fix now but will be hard (and expensive) to fix if you force the engine over and bend it. Attaching a car battery might give the starter enough torque to do lots of damage.

[mystic_1]

I've never worked on a 650 but it looks to me like it'd be pretty easy to remove the valve caps and eyeball things up there.  With the caps off you should also be able to gauge the cam chain slack by comparing crank rotation to cam rotation.

Spinning the motor over with the plugs removed is a good call.  The only resistance then should be from the valve train.

mystic_1

[Don R]

 Maybe a little light oil in the spark plug holes for the rings too?

[basketcase81]

Technically it's not a basket case unless almost fully disassembled with all the parts are in baskets/boxes. Yours sounds more like a typical "barn find" with most of the bike still together!

Try taking the plugs out and turning it over that way. Do not try to turn the crank with the "big nut" at the points plate end: you will just destroy the weak bolt that the big head is on. If the engine turns fairly freely forward all is well, if it hits a solid feeling obstruction don't force it... find out what is doing that.

Maybe your battery is weak, maybe something serious is keeping the engine from turning - like a stuck open valve that's easy (and cheap) to fix now but will be hard (and expensive) to fix if you force the engine over and bend it. Attaching a car battery might give the starter enough torque to do lots of damage.

well it was the frame, engine and wheels. the rest was removed. Anyway,the battery may have been kinda weak but I also did not have the plugs out.  I am going to pick up some more tools today, hopefully I can get back on it tonight and Ill try to see whats going on.

gauge the cam chain slack by comparing crank rotation to cam rotation.



mystic_1

can you elaborate a little more on this?

Maybe a little light oil in the spark plug holes for the rings too?

noted

[mystic_1]

gauge the cam chain slack by comparing crank rotation to cam rotation.

can you elaborate a little more on this?

Rotate the crankshaft forward a little, then backwards.  The cam should move at the same time as the crank.  If there's any lag, that indicates slack in the chain, meaning you have to adjust the tensioner, or that the tensioner isn't doing it's job, or that the cam chain is too worn for the tensioner to take up the slack.

mystic_1

[basketcase81]

gauge the cam chain slack by comparing crank rotation to cam rotation.

can you elaborate a little more on this?

Rotate the crankshaft forward a little, then backwards.  The cam should move at the same time as the crank.  If there's any lag, that indicates slack in the chain, meaning you have to adjust the tensioner, or that the tensioner isn't doing it's job, or that the cam chain is too worn for the tensioner to take up the slack.

mystic_1

ok, I see.

Unfortunately I think I may be scrapping this project indefinetely. I discoved some more wires cut that I dont know where they go. I dont even have a manual. plus, any money that I spend on this thing comes out of my "new bike" funds. I appreciate the help and will update if I do decide to go further