Topic: Cb750 stuck motor, how to tell if it’s a valve

[Dingus76]

Hey everyone. First post here as I just bought my fist 750. I bought a crusty complete 750 for $375. When I looked at it I was able to turn the motor by hand. Although now that I think about it I probably didn’t turn it over THAT much. And I don’t know how they are supposed to feel compression wise because I never kicked one over before. But it wasn’t easy to turn over by hand. Anyways the other day I went to kick it over for the heck of it and it moved a little bit and then got stuck.

Anyways, I’m in the process of removing the last plug to get some oil down the cylinders. Just letting it soak some as the plug is pretty tight I didn’t want to mess it up. My question is can I tell if a valve is stuck open, locking the motor, without removing the cam cover? Am I able to tell just by looking through the tappet covers. I don’t want to try freeing the motor until I know it isn’t a stuck valve.

Thanks!

[HondaMan]

Well, first thing to know about turning this engine: DO NOT turn it using the big nut by the points. This will immediately bend the points shaft, if it doesn't break it right off in a seized engine. Instead, remove the alternator cover (left side of the engine, front) and use the nut there for trying to turn it.

Next: if it is stuck, it very likely will not run, either. It doesn't take much rust to ruin the rings & bores. It takes even less to make it stick. So, use penetrating oil down into those bores if you want to try to turn it, and let it soak for a long time. If you can't remove one of those sparkplugs, that is very likely the rusty cylinder, too. This usually happens when the engines sit for a long time with old oil in them, and with no airbox on the carbs or with exhaust pipes missing.

[bryanj]

1 you cant remove the rocker cover without removing engine
2 you need to be able to see all 8 rockers to guess at wether a valve is stuck
3 DEFINATELY DO NOT USE THE NUT AT POINTS SIDE

[newday777]

What year is this 750? Pictures??

[Dingus76]

It’s a 1973. I jumped on it because of the original exhaust. Ofcourse it has holes in it but it’s still very complete and had a title. It’s been sitting since 84 according to the insurance card under the seat.

[Dingus76]

What year is this 750? Pictures??

[willbird]

It’s a 1973. I jumped on it because of the original exhaust. Ofcourse it has holes in it but it’s still very complete and had a title. It’s been sitting since 84 according to the insurance card under the seat.

Amazing the exhaust survived, dad bought my K2 new and I don't think the OEM exhaust even made it 4 years.

[Stev-o]

Most likely a piston ring is rusted to the cylinder wall, very common.
Time to pull the motor....

[Dingus76]

1 you cant remove the rocker cover without removing engine
2 you need to be able to see all 8 rockers to guess at wether a valve is stuck
3 DEFINATELY DO NOT USE THE NUT AT POINTS SIDE

Yeah I deff won’t use the points side nut. I’ll take the cover off the other side and try with a pry bar. Yeah I’m trying to avoid taking the motor out but we’ll see if that’s possible

[newday777]

It’s a 1973. I jumped on it because of the original exhaust. Ofcourse it has holes in it but it’s still very complete and had a title. It’s been sitting since 84 according to the insurance card under the seat.

👍

[bryanj]

Pry bar not a good idea, use an impact socket with long bor on the rotor bolt, dont lever too hard you may well damage/break something

[HondaMan]

Wow, a fully unmolested K3! What a find!
Nice catch for $350: around here that same bike would be at least 3x that much, in the same (or worse) condition.

[Dingus76]

Pry bar not a good idea, use an impact socket with long bor on the rotor bolt, dont lever too hard you may well damage/break something

Noted. I’ll just try the socket. Made a little progress on the stuck plug today. Put the heat gun on it for quite a while and got the slightest turn out of it. Moved it back and forth slightly. Left it turned out to soak overnight

[Dingus76]

Wow, a fully unmolested K3! What a find!
Nice catch for $350: around here that same bike would be at least 3x that much, in the same (or worse) condition.

I thought so too! I’ve been looking for a 750 forever but didn’t want to pay a premium for a nice one. It should be a fun project and a learning experience.

[Tracksnblades1]

Pry bar not a good idea, use an impact socket with long bor on the rotor bolt, dont lever too hard you may well damage/break something

Noted. I’ll just try the socket. Made a little progress on the stuck plug today. Put the heat gun on it for quite a while and got the slightest turn out of it. Moved it back and forth slightly. Left it turned out to soak overnight

Yep. Just like a rusty bolt that you don’t want to twist off.. Back and forth, back and forth, and a lot of patience…👍

[rotortiller]

Rock it back and forth in 5th gear with the plugs out with the well oiled cylinders. You can also put it on the center stand and leverage the tire by hand back and forth in 5th. See if it frees up again.

[Dingus76]

Rock it back and forth in 5th gear with the plugs out with the well oiled cylinders. You can also put it on the center stand and leverage the tire by hand back and forth in 5th. See if it frees up again.

Is it bad to turn the engine in the opposite direction? I’ve read that somewhere I believe

[scottly]

Rock it back and forth in 5th gear with the plugs out with the well oiled cylinders.

1+ Also, be aware that all that oil (or ATF) may come shooting out of the spark plug holes when the engine does free up. I believe Stev-o still has the distance record.

[rotortiller]

1+ Also, be aware that all that oil (or ATF) may come shooting out of the spark plug holes when the engine does free up.

I ruined a brand new T-shirt, next time I threw a rag over the plug holes. lol

[Stev-o]

Rock it back and forth in 5th gear with the plugs out with the well oiled cylinders.

1+ Also, be aware that all that oil (or ATF) may come shooting out of the spark plug holes when the engine does free up. I believe Stev-o still has the distance record.

   
Yes, 17'!!

[seanbarney41]

Rock it back and forth in 5th gear with the plugs out with the well oiled cylinders.

1+ Also, be aware that all that oil (or ATF) may come shooting out of the spark plug holes when the engine does free up. I believe Stev-o still has the distance record.

   
Yes, 17'!!

ha ha my last squirt only made it 12 feet...but it was at the top of a 12' garage door!  The landlord made me climb up there and clean it up!

[Stev-o]

Rock it back and forth in 5th gear with the plugs out with the well oiled cylinders.

1+ Also, be aware that all that oil (or ATF) may come shooting out of the spark plug holes when the engine does free up. I believe Stev-o still has the distance record.

   
Yes, 17'!!

ha ha my last squirt only made it 12 feet...but it was at the top of a 12' garage door!  The landlord made me climb up there and clean it up!

That was your "shot" at the record?!!!! Ha