• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

My project for the next few weeks (or years)

sunfish

CB750 Member
Messages
21
Reaction score
0
Points
1
Location
Denver
Hi everyone. I'm brand new to cb750.com, although I've been an owner for more than a decade. Im just getting started on a new project, putting together my old bike, and it seemed like the perfect time to join and introduce myself.

So this is my 1980 CB750C that I bought from my brother a few years back. It wasn't running at the time due to some carburetor issues which worked for me since I didn't know how to ride then. I have a background in auto mechanics so I had some base knowledge about carbs. The rest I filled in with a service manual. After learning to ride for a couple of summers back in Minnesota I decided to replace the leaking valve cover gasket. The long Minnesota winters turned that into a full head to toe gasket replacement. I managed to get the bike back in one piece but it took about two years (mostly because I moved away from where my bike was). I can't say why exactly but it never ran quite perfect after that. It did run though and I put a lot of miles on it. It stayed in MN after I moved to Denver about four years ago and I bought a cheapo CM400 to keep me occupied until I could bring it here. Finally flew there and rode it back for my first cross country trip. It made it just fine but after a couple of years the running issues started to get more pronounced. The symptoms include a vague rattle in the engine and some hesitation under load at mid range RPMs. at one point last summer the oil light came on but it had all the oil it was supposed to. I decided it was time to dig in and try to find the problem. I think that the original symptoms were probably mostly due to valve clearance issues but the oil light looks like it was caused by a clogged pick up screen. I don't know what it is that was clogging it but it was some sort of fuzzy residue. If it were that simple I would just clean it out and put it back together but there were also some chunks of broken metal in the oil pan. On a couple of occasions I've decided that it won't get fixed and I should just sell it in parts. Today I brought it home from a friends garage where I was working on it and I've decided to dismantle it and find the exact cause (source of the broken metal) before I make that decision.

So there's my full story. I feel like I'll stand a better chance if I post my progress on here where I'm sure to get advice and encouragement.

Thanks for reading and let me know what you think.
 

Attachments

  • 20140323_141414[1].jpg
    20140323_141414[1].jpg
    230.5 KB · Views: 138
  • 20140323_142101[1].jpg
    20140323_142101[1].jpg
    272.8 KB · Views: 141
  • 20140323_142251[1].jpg
    20140323_142251[1].jpg
    304.9 KB · Views: 145
20140323_184145[1].jpg20140323_184200[1].jpg
I cant find it right now because I just moved. I'll post when I do. These pictures show what I found today after I got the crank case apart. There Is some sort of pin or spring that came out of the pivot for the drive chain tensioner. That's not what I found in the oil pan. what I found was dark metal (probably steel) that looked like it came from some sort of support bracket like the one shown behind the tensioner, although that one is completely intact. I put it away for the night and I'll get back to it on Tuesday and dig a little deeper.
 
You also have one tensioner and guide for the two timing chains. The one in the head likes to break and get mangled up.
 
Had to leave town unexpectedly so I guess it will be a couple of days before I get to it.
 
20140329_180057[1].jpg
Well this isn't how this was supposed to go. Is it recommended that these bolts be replaced after they've been torqued? Where would I find replacements?
Still haven't found any major broken parts (other than the ones I'm breaking while disassembling). most of the important parts look like they are in good shape. This thing might actually run again! If I can stop breaking things, that is.
 
You can get new bolts from Honda. I use Service Honda for my oem parts. They have a very easy to use online micro fiche and there prices and shipping are the lowest I have found. No these are not torque to yield bolts so they dont need to be replaced, just the ones you break. May have been over torqued from previous repair?? If you haven't found anything bad yet possiblility that your debri is from previous issues that have been fixed.
 
Dirtdigger, you were right on the money about the chain guide. The one that sits in the front just came out in 3 pieces. this seems like a potentially hard to find part. The guide under the top chain (between the cam gears) seems to be pretty worn as well. any suggestions on where I might score some internal engine parts?
Good news is that today when I finally got around to extracting the stub of the broken bolt, it came out with out any trouble whatsoever. I was using a left handed drill bit with plans of using an extractor tool but the bit caught the bolt and spun it out before it could even bight into it.
 
Beings the broken bolt spun out free it indicates an overtorqued bolt failure. Always a good thing to start with reverse twist bits cause you never know when you will get lucky.

I have done some looking and that part is discontinued from Honda. I also checked on a UK honda parts site that usually has odd honda surplus and they are showing temporarily out of stock. http://www.davidsilverspares.com/parts/by-part-number/partnumber_15429/ maybe check back here later. So...best bet is going to be a used one from ebay ifyou can find one. Thats kind of the problem working on the DOHC motor because they just aren't that popular and no one is making much for aftermarket stuff.
 
Thanks for the link. When I went to it it said there was one available so I ordered it. Around $52 after shipping which is the same that I would have paid at the dealership if they could get it. I got the same from my local Honda dealer that they were discontinued. I did get 2 bolts ordered though. I think you're right about the overtorqueing Not sure how that happened though. I followed specs carefully when I last assembled the motor. I'm not sure what torque wrench I was using at the time but maybe it was out of calibration.
Next thing to explore for me is what things I can do, since the motor's in pieces anyway) to build some more power into it. I'll have to see what my budget will allow. Last time I was only barely able to get it back together. This time I may have some extra for some performance upgrades. I'll have to do some research but any suggestions on the low to medium cost side of things would be appreciated. I don't need to do wheelies but who doesn't want just a little more power.:bike:
 
The biggest problem with these bikes is the carburetors. Any performance mod you do to the motor you wont be able to tune the stock carbs worth a crap. On these motors I wouldn't do any performance upgrade except maybe exhaust without putting aftermarket carbs on and that really drives up the cost.
 
Just a quick update: the project is coming along slowly but it's still coming along. I've begun reassembling, stopping periodically to replace parts that are worn or bolts that ran away when the boxes of parts were moved. I'm taking my time but making progress. Should be running in a few weeks.
 
Back
Top