78 CB750 First Motorcycle Project

[Zippy5150]

Hey guys, new to this site, got a 1978 CB750 super sport from a Craigslist deal less than a week ago. This is my first time working on a motorcycle but so far it's going well so that's nice.

Here it is when I first got it:



A couple of progress pics:



And here it is in its current state:


I think it's coming along nicely. I don't have as much time as I would like to work on it because of my job but it should be ready to either get sandblasted this weekend or get tearing into the engine, I haven't decided which yet. For the final look I want to go with a rugged but clean cafe-racer/bobber.

One thing I am going to do and am really excited for is painting the gas tank black with a thick off-White stripe and this off-White emblem intersecting the stripe like so: and then I'm going to cover it in matte-clear.

One question I have is would it be alright to shave off the attachments for the motorcycle-stand thing in the middle (not sure what its technical name is) and make it a smooth bar or would that maybe weaken it for something that won't even be seen?

 

[dirtdigger]

It is ok to remove the center stand brackets. Get a factory manul and do some research before you tear into the motor. They are fairly simple but there are a couple of things that will get you into trouble if you dont know about the 750 internals.

 

[Dave]

Excessive heat is what could weaken the frame so....

Don't use heat to remove the brackets AKA a torch or *grinder, cut them off with a sawzall or hack saw. Then do the final grinding a little at a time letting the spot air cool (do not use water) between grinds.

*You could probably get away with a grinder if you use a zip disk.

Your photos actually make me a bit jealous. I wish I had the funds to tear down and repaint/chrome my bike.

 

[Zippy5150]

Excessive heat is what could weaken the frame so....

Don't use heat to remove the brackets AKA a torch or *grinder, cut them off with a sawzall or hack saw. Then do the final grinding a little at a time letting the spot air cool (do not use water) between grinds.

*You could probably get away with a grinder if you use a zip disk.

Your photos actually make me a bit jealous. I wish I had the funds to tear down and repaint/chrome my bike.

Thanks! I actually just sold an 87 Honda dual sport so I could do this and the guy I bought it from didn't have the time or space to work on it and wanted to see it go to someone who would take care of it; just gotta find the right deals. I'm kinda addicted honestly, I feel like bikes are more fun and a bit easier to work on than cars

 

[Zippy5150]

Hey guys been workin on my motorcycle a bit, tore into the engine and its a bit rougher than expected. I had to either bore the cylinders or buy new ones so I went with the cheaper option of getting new cylinders and piston rings.




While the parts are in the mail I'm working on making the exterior look better and I was wandering how to get the springs off the shocks so I can clean them up.

Thanks

 

[amc49]

What 'new' cylinders? You order oversize pistons and rings and have to get cylinder block bored. If using the one you have you may well find it still will not bore out cleanly as that is a whopping amount of corrosion you have there. I'd be looking at carb that matches that cylinder, you may be replacing it too.

Luck with the boring, it's becoming a lost art and many are wrecked by incompetent people now.

You can cut the centerstand brackets off but how will you support bike if a tire needs changing? I leave mine on and keep centerstand and shaft as a tool quickly inserted to change tire.

Shock is junk if the rust is on the center shaft..............working rust area into shock body seal will destroy it pretty much instantly.

Hope you're marking where every single head part goes back, they pretty much become matched sets once run, especially valves in the holes. Mix them up and instant trouble. Heavy duty freezer baggy your friend there.

 

[Zippy5150]

What 'new' cylinders? You order oversize pistons and rings and have to get cylinder block bored. If using the one you have you may well find it still will not bore out cleanly as that is a whopping amount of corrosion you have there. I'd be looking at carb that matches that cylinder, you may be replacing it too.

Luck with the boring, it's becoming a lost art and many are wrecked by incompetent people now.

You can cut the centerstand brackets off but how will you support bike if a tire needs changing? I leave mine on and keep centerstand and shaft as a tool quickly inserted to change tire.

Shock is junk if the rust is on the center shaft..............working rust area into shock body seal will destroy it pretty much instantly.

Hope you're marking where every single head part goes back, they pretty much become matched sets once run, especially valves in the holes. Mix them up and instant trouble. Heavy duty freezer baggy your friend there.

I never said I got oversize pistons I said I got new piston rings. I know the corrosion is bad. Thats why I got new cylinders. The pistons are fine though and theyre cleaning up really nicely.

Thanks for the luck but I'm not boring them because I got new ones that I'll just have to hone.

They make motorcycle stands for a reason.

Thanks but that doesnt really tell me how to get the shock apart like I asked...

Yeah thats why the engine is on a piece of cardboard with everything labelled (pic 5).

 

[Zippy5150]

Another thing I was wondering is do I have to put the air box intake filter on;

(this clunky thing)

Or could I just use the K&M style clip on ones?



I've seen both done but I was wondering more specifically if there are advantages for one over the other because I would prefer to do the second option.

 

[dirtdigger]

Did you get new sleeves or a new to you used cylinder? I hope you were able to find the correct rings, the super sport motor uses a specific set of rings different from the k motors and they are very hard to find, hoping you just got lucky and found a place that had them. Also hoping you verified you have a F2 motor and not just a k and the correct parts got ordered. Be very carefull and make absolute sure you get the oil passages in the cylinder and in the head clean and verify the oil orfices are clean in the head. DO NOT use rtv on any gasket on anything on the 750 motor, use hondabond or three bond if you feel you need to use a gasket dressing. Use new gaskets and orings, dont reuse orings or you are asking for leaks. Also see you broke the cam chain, are you planning on splitting the cases? I highly suggest you do at this point. I do not recommend using the masterlink chains, yes people have used them with no problem but cam chain is something you dont want to take a chance with and I suggest a heavy duty unit anyway as the chain is an issue in the 750. If timing chain rollers are hard they must be replaced. With the way the top end looked you should dissassemble the bottom end anyway to thoroughly clean it anyway and inspect bottom end bearings. As far as the air filters yes you can use the pod filters with these carbs, you WILL have to do some tuning to get it to run correct.

 

[amc49]

'Thanks but that doesnt really tell me how to get the shock apart like I asked...'

No need to take shock apart if that center shaft is heavily rusted. Waste of time. Might use the springs on another though if you find some. Probably a pin somewhere there you overlooked or the top screws off like a bolt nut. Top of shaft threaded.

You call the part a cylinder block, that ensures us knowing what you are talking about. You can get separate cylinders but that requires much more machine work. If another block coming then you risk it being worn, EXTREMELY rare to get one that is relatively untouched and pistons from another motor matched to an unknown block are one of the biggest problem makers there is. Can be done but not nearly so easy as most people think, and why so many rebuilds are dogs powerwise. Of course everybody selling tells you block is in good shape.

Uh, any piston in that rusted hole should be pretty much junk too, the water leaks around the rings to eat into the lands. Never seen one that bad that didn't, do what you will of course. Pic the piston.................I could be wrong but lots of water damaged motors through my fingers say no.

They make centerstands for a reason......................so you don't have to buy a separate stand.

 

[Zippy5150]

Did you get new sleeves or a new to you used cylinder? I hope you were able to find the correct rings, the super sport motor uses a specific set of rings different from the k motors and they are very hard to find, hoping you just got lucky and found a place that had them. Also hoping you verified you have a F2 motor and not just a k and the correct parts got ordered. Be very carefull and make absolute sure you get the oil passages in the cylinder and in the head clean and verify the oil orfices are clean in the head. DO NOT use rtv on any gasket on anything on the 750 motor, use hondabond or three bond if you feel you need to use a gasket dressing. Use new gaskets and orings, dont reuse orings or you are asking for leaks. Also see you broke the cam chain, are you planning on splitting the cases? I highly suggest you do at this point. I do not recommend using the masterlink chains, yes people have used them with no problem but cam chain is something you dont want to take a chance with and I suggest a heavy duty unit anyway as the chain is an issue in the 750. If timing chain rollers are hard they must be replaced. With the way the top end looked you should dissassemble the bottom end anyway to thoroughly clean it anyway and inspect bottom end bearings. As far as the air filters yes you can use the pod filters with these carbs, you WILL have to do some tuning to get it to run correct.

Thanks for the pointers! I got a new cylinder block, sorry for the confusion. I saw that the supersport motor piston rings were rare/hard to find and I think I got the right ones (the ad said they fit 77-78f motors so I hope they're correct). And the chain cutting was a shame but it had to be done unfortunately. This weekend I'm going to build a stand to put the motor on to make it easier to undo the bolts, split the casing and clean out the inside thoroughly especially since some cylinder rust fell into it plus I'm going to have to get the new cam chain on.

 

[amc49]

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[Zippy5150]

Theoretical question: could a CB750A cylinder block be put on a CB750F motor? Why or why not?

 

[dirtdigger]

You can only if you have brass sleeves installed in 4 of the cylinder stud holes in the head. If you dont you will have an oil leak. The k/a cylinders use a different oil return passages then the f2 cylinders. They physically fit but you will have oil leak issues. I have had the sleeves put in my f2 cylinder heads so I can run them on k bottom ends. I have a couple of pictures in one of my photo albums on this site. APE and Cycle x do this job but its going to cost you $130 parts and labor not including shipping. Would be far less work if you use f2 cylinders.

 

[Zippy5150]

Little update, some stuff I did. If you have questions or comments I'll respond but its kinda self explanatory so here ya go:

 

[madmtnmotors]

The stock shocks that you have pictured are not rebuildable.


Here is a photo of the underside of the cylinder blocks showing the difference between the K and F cylinders:

The A cylinders will be the same as the K cylinders

 

[Zippy5150]

The stock shocks that you have pictured are not rebuildable.


Here is a photo of the underside of the cylinder blocks showing the difference between the K and F cylinders:

The A cylinders will be the same as the K cylinders

Thanks. I ended up getting a new engine, I'll post updates soon.

 

[Zippy5150]

Updates. Frame, tank, and swing arm off to be painted black with gold pearlescent. Made the seat for about $30.

 

[Zippy5150]

Question: Can an F3 clutch assembly go into a K4 motor? If yes, how and what needs to be modefied? If no, why not and are you sure nothing can be done? Thanks.