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Author Topic: CB750 K6  (Read 1692 times)

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Offline ozzydog

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CB750 K6
« on: March 10, 2014, 03:22:41 PM »
Hi there , please forgive me if  I am not posting this in the correct place, I just joined and did a few searches but came up with no matches. I would like to ask a couple of questions if possible please to see if you think it may be possible for me to rebuild my CB750 K6.
It was given to me by a friend as he was going to scrap it, its complete and I got it running after it had stood for 10 years, although it did rattle an awful lot. Its done a lot of miles (140,000+) without any sort of rebuild and after cleaning it up a bit a few things have become visible that make me question if its even possible to repair.
At the front of the crankcase, where the engine is mounted to the frame, the crankcase has a crack in the alloy on both sides, its hard to explain but its sort of cracked on the crankcase behind where the bolt sits. I have no idea what caused this or if it can be fixed  :(
I also know it has a sardine can glued at the rear of the engine, near the sprocket, he's run it like that since 1986 when the chain snapped.
I would be very grateful for any advice please,
many thanks
Heather
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JWExperience

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Re: CB750 K6
« Reply #1 on: March 10, 2014, 03:45:45 PM »
Tough to say without seeing it in person but you could probably weld it up. It won't be perfect and may crack again over time due to the stress/vibration in that area but could prevent further cracking in the meantime. Is the sardine can covering the sprocket? Sounds like the chain damaged the original cover. You might be able to pick up a new or used cover on eBay as long as the threads are good for that cover. Might want to check them with a bolt before you order a cover and figure out if u have to re thread anything. Good luck!
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Offline martin99

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Re: CB750 K6
« Reply #2 on: March 11, 2014, 04:04:18 AM »
Hi Heather. That's a lot of miles without any prior engine work. Be prepared for a major and very expensive rebuild, I would hazard a guess at rebore, pistons, rings, valves, clutch, bearings, seals, chains and tensioners at a minimum. The cracked case is an additional problem, but the least of your worries. I suspect the sardine can has been stuck over another hole where the chain let go and went through the case - not uncommon. Don't overlook the cycle parts either. Wheel bearings, tyres, forks, cables, brakes will all need looking at and possible refurbishment.
If time and money is on your side, it sounds like a good project. Definitely worth more than scrap, even if you decide to part it out.
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Build threads:
77 750F2 Refresh Project http://forums.sohc4.net/index.php?topic=144075.0
TRIBSA http://forums.sohc4.net/index.php/topic,160296.0.html

1977 CB750 F2
1958 Norton Model 99
2011 Triumph Street Triple 675

Offline Johnie

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Re: CB750 K6
« Reply #3 on: March 11, 2014, 04:52:31 AM »
Welcome to the boards Heather...good job saving another from the scrap pile. I would agree with the guys 140,000 miles is getting up there and it is a tired motor. You said you did have it running that is a real plus. The "rattling" could be the clutch, dirty carbs or the carbs are out of sync. Did you do much to it before you got it running such as a oil & filter, check gas tank for junk, carb cleaning, points, condensors, timing, valve and cam chain adjustment? Once those are done a lot of the "rattling" may go away. If money is tight and you can get it running good by doing the above and add tires, new chain and sprockets to the list why not drive it for a while. May burn some oil, but you can ride until you feel it is time for a complete rebuild or another bike with less miles. Post up some pics when you get can...I think you need like 5 posts to be able to do that so post away and welcome. You will find the guys and gals here are more than williing to help. Might even find some people in your area to give you a hand too. Good luck with it and let us know how you come out.
« Last Edit: March 11, 2014, 10:30:53 AM by Johnie »
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1970 CB750K0 - Candy Ruby Red
1973 CB750K3 - Candy Bacchus Olive or Sunflake Orange
1970 Chevy Chevelle SS396 - Cortez Silver
1976 GL1000 Sulphur Yellow

Oshkosh, WI  USA

Offline lrutt

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Re: CB750 K6
« Reply #4 on: March 11, 2014, 05:13:31 AM »
Likely the cam and primary chains rattling big time with that many miles. Pretty major surgery required. Those would surely make a LOT of noise as I'm sure they are pretty loose.

Are you sure those are real cracks with oil actually coming out, or are they just casting marks? I'm having a hard time understanding how the case could be actually cracked there without throwing a rod. Even of the motor mounts were loose I don't know if it would actually crack.

Honestly....Given it's a K6 and not worth all that much even in pristine condition, it would be hard to justify putting much money in the bike, sorry to say.
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06 Harley Sporster 1200C, 06 Triumph Scrambler, 01 Ducati Chromo 900, 01 Honda XR650L, 94 Harley Heritage, 88 Honda Hawk GT, 84 Yamaha Virago 1000, 78 Honda 750K w/sidecar, 77 Moto Guzzi Lemans 850, 76 Honda CB750K, 73 Norton 850, 73 Honda Z50, 70 & 65 Honda Trail 90, 70 & 71 Triumph 650s, 65 Honda 305 Dream, 81 Honda 70 Passport, 70 Suzuki T250II, 71 Yamaha 360 RT1B, 77 BMW R75/7, 75 Honda CB550K, 70 Honda CT70

Offline Stev-o

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Re: CB750 K6
« Reply #5 on: March 11, 2014, 05:35:50 AM »
Sounds like that bike has had a tough life.  Sounds like he "repaired" the engine case with the sardine can after the engine case broke from the chain break. 

After 140,000 mikes, I would say it might be time for at least a top end rebuild. 

Good luck. 
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'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Black 750K8

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Re: CB750 K6
« Reply #6 on: March 11, 2014, 05:59:36 AM »
It might be cheaper to locate another used engine.
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Offline Johnie

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Re: CB750 K6
« Reply #7 on: March 11, 2014, 10:31:43 AM »
Not sure if you know this or not, but you can download the service manual from this board.
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1970 CB750K0 - Candy Ruby Red
1973 CB750K3 - Candy Bacchus Olive or Sunflake Orange
1970 Chevy Chevelle SS396 - Cortez Silver
1976 GL1000 Sulphur Yellow

Oshkosh, WI  USA

Offline flybox1

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Re: CB750 K6
« Reply #8 on: March 11, 2014, 10:54:42 AM »
Quote from: Black 750K8 on March 11, 2014, 05:59:36 AM
It might be cheaper to locate another used engine.
+1
High mileage...glad a motor lasted that long, but If you're wanting a reliable rider for years...i'd find a used engine with solid cases, get a copy of Hondamans book and the shop manual, and start your rebuild with a good foundation, but keep this one till you're finished.  might be a part or two you can reuse. and a good 'test' subject.  open her up and take a look.
and welcome to the forum!  ;D


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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline ozzydog

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Re: CB750 K6
« Reply #9 on: March 11, 2014, 01:33:49 PM »
Wow thanks so much for all your responses and advice, it really gives me some hope  :D
Once I collected her from my friend all I did was clean out the fuel tank and carbs, oil/filter change and new plugs and she fired up. She did rattle but everything worked, lights etc I was pleased and really surprised to be honest. I don't have loads of cash to throw at her right at the moment but also have no urgency to get her on the road so i can take my time. I just really like the fact this poor bike has done so many miles with no work done to it and even after spending 10 years in a shed it still fires up and wants to go, I don't want to give up on it and think its time she got some tlc  :D
The cracks at the front are not leaking any oil, sorry I don't think I explained it very well, you have that bit of alloy that curves out and the bolt goes through it to hold the engine, well its cracked across as if that curved bit with the bolt in is trying to seperate from the crankcase and its on both sides. I will take some photo's tomorrow and if I can I will put them on here, I am not very good at explaining things and maybe it will help.
Thanks again for all your advice, I am going to spend a bit of time this evening looking round this site cause there is so much information to take in
best wishes
Heather
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Offline flybox1

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Re: CB750 K6
« Reply #10 on: March 11, 2014, 01:41:34 PM »
 :) OK....then you'll need to download the shop manual ...and read the maintenance section thoroughly.
http://forums.sohc4.net/index.php?topic=426.0   
You'll want to complete each and every one, in order, to see what state of tune the bike really is in.
none are difficult. most require tools you probably already have.  the ending vacuum sync of the carbs will be the challenge if you dont have access to manometer, but one can be made  http://forums.sohc4.net/index.php?topic=23038.0   or even purchased for about $50US

The FAQ section of the forum is a huge help. http://forums.sohc4.net/index.php?board=11.0
« Last Edit: March 11, 2014, 01:48:14 PM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline mickwinf

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Re: CB750 K6
« Reply #11 on: March 11, 2014, 02:23:50 PM »
Heather also check out the UK site www.sohc.co.uk and use same username.
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