CB750 DOHC engine valve head problems new/uneducated builder

902skinny
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902skinny
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Joined: April 7th, 2018, 10:57 pm

July 11th, 2018, 12:13 pm #1

So quick preface, I have no idea how to build or work on a chopper, have always loved them and wanted to build one so I bought a cb750 basket case a year ago that supposedly just needed a new cam chain and was half tore apart so I figured it'd be an easy project to start on, hah. Anyways as stated I have no experience or really clue what I'm doing, just trying to fumble my way through the processes and learn some new things and practice to get better and more educated. That being said some of my questions may be silly or ridiculous but I welcome anybody who wants to help me learn some things and please keep in mind I have no idea what I'm doing so try to explain things like you would to a layman. I'm going to have to ask for tons of advice during this build but the first couple real hiccups I've found are below, any help or advice is appreciated.

Problem #1
I'm tearing down the cases to get them seperated and as I'm doing this I find lots of people always saying check the valve compression which I can't do because the cylinders were already off when I got the bike (more of that problem later). But  I did run some water through the valves and found out that more or less 70% of them are leaking. And after taking a good look at other builds and parts I can probably tell why, they look like shit and the seals are scary looking. I'm thinking that replacing valves themselves is probably a complicated job that might be way too advanced for a new guy like me but I'm not sure. So i'm pondering whether or not I should maybe just buy a replacement top end that needs less work. So the question now is after looking at the photos below, do you guys think the valves are likely totally shit or is it possibly something that could be fixed by a totally inexperienced guy like me for fairly cheap? Will cost me about $200 bucks to replace the whole top valve head.

Problem #2
Another reason I'm contemplating replacing the valve head altogether is once I took the top clamp off the I think they are called journals? The metal C clamps that hold in the cam shafts? Once I got them apart I checked them all and noticed 1 is quite worn inside and scuffed up and there is even some metal chunked up and missing towards the corner of the piece. Then there is another one that is also scuffed up some but far less worse. I am not sure if this is a big issue or not though or if its something that could be run anyways, tried to get the best photos I can appreciate any help and information.
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Joined: October 5th, 2007, 6:52 pm

July 11th, 2018, 12:27 pm #2

That cam journal was starved for oil and ran dry.

Best bet? Find a running motor.
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902skinny
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902skinny
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July 11th, 2018, 12:35 pm #3

Thats a possibility too for me though I'd like the experience of the building part, I do know a local guy selling a whole motor for $400, just that it isnt in a bike and I'm afraid to buy a lemon again. This engine also had a cracked cover which wasn't stated when I bought it, also the shim buckets were already removed and not marked where they came from. Overall I think that maybe where I'm at with no idea what I'm doing this engine is just maybe too much for me right now.
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Joined: October 5th, 2007, 6:52 pm

July 11th, 2018, 9:30 pm #4

pick up a compression tester
With the carbs off you can hook a battery to the starter
with jumper cables. check all four holes.
If it's got good compression, it'll run.
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902skinny
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902skinny
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July 11th, 2018, 10:58 pm #5

I've seen that done and I thought about that, I know the compression tester can be picked up fairly cheap, not 100% sure on how to hook the battery up to the starter though but can't be too complicated. That will tell me about the condition of the cylinders and the seals and everything though right? So if I have good compression the only other problems the motor could have would be like slack chains or worn bearings that kind of stuff. This might be the route I need to go, I also have a cb650 project I might focus on for awhile pretty bummed about this motor, mainly because my woman's been on my ass for months saying I'll never get it running and now she's right lol.
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902skinny
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902skinny
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July 11th, 2018, 11:23 pm #6

So after a little research I basically just need to touch the positive cable to the positive terminal on the starter and it should crank over? Looks like I'll be picking up a compression tester next, seems like a useful tool to have. Thanks for the advice Rake, I'll keep the busted motor for parts, still am going to split the cases on the bad motor just to see what else is inside but looks like I'm on the hunt for a less toasty motor.

Because those cam journals are scraped up does that mean more or less that the cylinder head shouldn't be used ever again?
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CHUCK DV
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Joined: April 9th, 2003, 9:46 pm

July 12th, 2018, 3:37 am #7

(I basically just need to touch the positive cable to the positive terminal on the starter) Yes but you need to Ground the motor to the ground of the battery.
(Because those cam journals are scraped up does that mean more or less that the cylinder head shouldn't be used ever again?) YES..
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902skinny
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902skinny
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July 13th, 2018, 4:07 pm #8

Thanks guys, foolishly I didn't really look too much into the top end because I thought all I had to do was replace the cam chain in the bottom but after closer inspection the top pieces of the clamps on the cam journals are all badly scratched as well.

So then I checked out the oil pan and filter closely and I can see tiny metal shavings and pieces of the tensioner which also broke. I haven't given the cam shafts themselves a good look yet but I'm assuming they would likely also be damaged. 

Kind of bummed the engine sucks but I'm definitely learning a lot about the function and assembly of the unit from all this. Once again appreciate your knowledge fellas.
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Joined: October 5th, 2007, 6:52 pm

July 13th, 2018, 8:28 pm #9

any time skinny
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902skinny
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902skinny
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August 8th, 2018, 2:16 am #10

So I'm making some kind of progress, got my hands on a cb900f motor that runs briefly but doesn't stay running, need to do some tinkering with it figure out what's going on, hopefully just needs the carbs cleaned maybe. I also have a cb650 survivor project I'm working on so been dividing my time between both but am slowly getting towards a running bike.
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In frame checking fit.
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