Topic: my plan to un-freeze a 1974 cb750 motor

[Stev-o]

Thanks for all the info on here on this, as I picked up a K5 week before last with a seized motor.  Tried the acetone/ATF trick, and damn!  6 days is all it took on a bike with 7 years outside and an 836 kit.  Mostly stock other than that. 

Two cylinders at 140-150, and two at 65psi.  Soaking them again to see where it gets me...

That compression may come up on the two low ones after you run it.

[MoMo]

That's great .  If you are able to get it started the compression will come up on the other two.  Further dousing should help.  I usually use Seafoam to unseize motors but that cocktail seems to work better to free up after years of sitting.  Every bike that I have managed to get unstuck was not taken apart and are still running...Larry

[dirtmagnit]

Tried the Seafoam last night, but nothing in the bores when turned over today.  That seafoam is pretty thin.  Put A/a back into it today for a day or two. 

I must say, being the first 750 I've played with, I love the springs on the float bowls.  After 3 550's, 2 400F's, a 350F, and a couple GL1000's, I love popping them off to check things out with the rack still installed.  Carb's are plugged solid, but a cool feature nonetheless. It'll run on ether (just to check) so I guess I know what needs to be done.  Not looking forward to the carbs...

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[faux fiddy]

50/50 ratio of ATF and acetone is the ultimate unsticker. Try and see if you can rotate the bolt on the points side first, don't hammer on it too much though. If it doesn't budge on that side pour in the 50/50 mixture and let it work it's magic for a day or two then repeat.
 

+1 on the ATF, never heard of mixing it with acetone, but that should cut some grime.

Heat never hurts, but seems impractical in your situation. I agree with the stuck rings, etc. You'll probably want to pull  the bores off eventually.

I  used a piece of delron (a machinable dense plastic) and a hammer for beating on the tops of the pistons to loosen them in the bores.

[terecha]

hi
I have small question. I have a problem with stuck piston. I didn't remove the valve cover and I did turn the kickstarter shaft using long wrench, the kickstarter shaft turned very hardly. I did 2 full clockwise turns. Later (after I removed the cover) I noticed the piston is completely 'welded' to the cylinder (no chance to move and didn't move when I turned the kickstarter). So when I moved the kickstarter shaft, could I cause serious damage inside? The bike was on neutral then.

[goldarrow]

I will try that method. Why don't you think it will work for me?

SO, did it work for you?  just freed up my K2 motor and kick starter won't move. 

[Stev-o]

I will try that method. Why don't you think it will work for me?

SO, did it work for you?  just freed up my K2 motor and kick starter won't move. 

I don't remember trying kicking the kickstarter to the rear.  Shorty after that, I bought my K4 and put the K7 on the back burner in the Batcave where it's been ever since.  I wouldn't doubt it's stuck again. 
I'm working on the title process (have VT reg) and plan to pull it out this fall. 
Kick it to the rear!

[goldarrow]

Couldn't wait for your reply, so I went ahead, and hammer it it rubber mallet.  It came loose. Now I'll just have to open the head and see the bores as I hAve a feeling the rings scrapes the walls as I turn it.

[Stev-o]

Couldn't wait for your reply, so I went ahead, and hammer it it rubber mallet.  It came loose. Now I'll just have to open the head and see the bores as I hAve a feeling the rings scrapes the walls as I turn it.

Good to hear you got it free.  Don't want to try to fire it up? I would...

[goldarrow]

The motor doesn't turn smoothly, and feel like something rubbing,binding in there...

[Stev-o]

The motor doesn't turn smoothly, and feel like something rubbing,binding in there...

Not good, ya, it has issues.  Well at least your a Pro at pulling those motors quickly!

[goldarrow]

The motor doesn't turn smoothly, and feel like something rubbing,binding in there...

Not good, ya, it has issues.  Well at least your a Pro at pulling those motors quickly!

Lol. Will see if I have time to do it this weekend.  And the title for this bike just came in

[Oldtech]

When I've tried everything and the motor is still stuck so that I'm sure it will have to come apart I use a much more gentle method to free the pistons then beating them with a block of wood and a hammer. Pull the tappet covers and see if one of the cylinders has both valves closed. If not the cylinder head cover has to be removed. I have a collection of spark plugs with the centers knocked out and threaded to take a grease fitting (grease zerk). Thread in the modded spark plug and put a grease gun on it. A grease gun can generate really high pressure so keep pumping until the cylinder fills and the grease starts putting pressure on the piston. This hasn't failed me yet and I've managed to get several engines apart that only needed a light hone and new rings afterwards.

[MCRider]

When I've tried everything and the motor is still stuck so that I'm sure it will have to come apart I use a much more gentle method to free the pistons then beating them with a block of wood and a hammer. Pull the tappet covers and see if one of the cylinders has both valves closed. If not the cylinder head cover has to be removed. I have a collection of spark plugs with the centers knocked out and threaded to take a grease fitting (grease zerk). Thread in the modded spark plug and put a grease gun on it. A grease gun can generate really high pressure so keep pumping until the cylinder fills and the grease starts putting pressure on the piston. This hasn't failed me yet and I've managed to get several engines apart that only needed a light hone and new rings afterwards.

That's a great idea. IF it works as well on pistons as it does on brake calipers, it would be fantastic. I looked it up and a simple pump handle grease gun generates 5000+PSI in short, safe bursts. It would sure be a more elegant way of popping a piston than some of the medieval methods described here.