Topic: Back with new project 1978 cb750k

[Can550]

This platform is an addiction and to maintain my presence I had to get my hands on to something new. Have had 4 cb550ks and never had a 750. So I got a decent looking bike with about 10,000 kilometres on it.

I have no idea where to take a start. Motor turns over. I'll list down info from PO

last ridden was 1999.
Was started once or twice a year till 2009
Second last time it was started in 2015 and it fired up on 3 cylinders
Last time earlier this year and started only on 2 cylinders.

Now my observations
Tank was empty but rusted
Filled with dish detergent and hot water plus a gallon of vinegar. Left it for about 6 hours. Drained and rinsed (terrible smell) I would say about 90% of rust/ gunk/ varnish came out. There were tiny bit rust spots, filled with vinegar and have left it to come loose.
Master cylinder had fluid in it but less than half. On the top and neck threads, fluid has crystallised, calliper seems to be stuck. Rotor is rusted and pitted (looking for a replacement rotor)

Thinking of removing MC to clean it, brake cleaner is enough or there's a recommended cleaning method. ?

Vetter fairing has been removed and now I have to remove front forks to insert fork ears to mount stock headlight plus install stock turn signals (PO sent me a box of all stock items that were removed to mount fairing).

I also need a new battery as existing battery did not charge after filling distilled water and being in charge for overnight. What is recommended sealed battery ?

Side and top cover have spots of oxidation, are these covers factory polished or painted? How to get rid of oxidation ?

Should I spray some penetrating oil in cylinders before I crank it ?


Here are couple of pictures.





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[calj737]

Don't leave vinegar in that tank for too long. It's an acid and it has no discretion about when it should stop eating rust or metal. And, be certain to remove the rust from the underside of the top of the tank.

David Silver Spares has repro MC units that are a simple bolt on and go. Not worth trying to salvage yours. The caliper and piston sit tight on. Once you have a new MC, you should be able to drive the piston out, replace the seal, and restore/replace the piston as necessary.

The rotor can be sent to Tom @ Truedisk for Blanchard grinding, or have it done locally by a machine shop. There should be ample meat on the rotor to permit resurfacing for use.

Fully clean and reset the carbs. They are no doubt varnished up and preventing the bike from firing on all 4 cylinders. A sealed AGM battery is a perfect match for your bike.

[b1jackson]

First thing I'll do fuel related on a project bike is a carb overhaul.  And if you are expecting fuel supply issues from a cruddy tank, rig up a bypass tank (old oil container with a nipple, etc) to supply fuel to the carbs. I just revived a 77K that had been sitting since the mid 80s.  Turned the key and it ran on all 4.  Squirted some oil down the plug holes and kicked it over a few times first to lube the cylinders.  I typically start them on fresh oil as well with the expectation that I'll be changing it very soon.

[Can550]

Thanks guys
I have auxiliary gas tank
Haven't been well for few days. Went for cystoscopy
Today had some time to dismantle carbs
Mixture screws, only 3 have O rings, 4th one seems had an O ring that came out in several tiny pieces but no metal washer in either of the carbs.


Any thoughts what could be done as alternative unless only washers and O rings are available. Buying whole kit is $$$

May not be able to work on bike for few days.


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[Can550]

So today I started my k8 and was very pleased with. There are few concerns

1- tightened cam chain tensioner by bringing to 15 degree past TDC but still sounds like loose tensioner

2- idle sounds better when clutch is pulled in

3- there was smoke coming out of headers 3&4 when noticed closely, it seemed leaking (some leaking sound) tightened a bit gone but would like to know if I should put new copper rings ?

4- charging voltage (@ 4500 and higher RPMs )go to 15+ volts, is it normal or any attention needed

5- short test ride and chain seem to be making some knocking/ clicking sound too tight or too loose ?
Thanks


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[calj737]

So today I started my k8 and was very pleased with. There are few concerns

1- tightened cam chain tensioner by bringing to 15 degree past TDC but still sounds like loose tensioner this could be attributed to your exhaust leak, or possibly carbs needing a vacuum synch.

2- idle sounds better when clutch is pulled in very normal for 750s

3- there was smoke coming out of headers 3&4 when noticed closely, it seemed leaking (some leaking sound) tightened a bit gone but would like to know if I should put new copper rings ? if the sound and smoke is resolved, don’t worry about changing the copper o-rings. If it persists in leaking, then certainly change all 4.

4- charging voltage (@ 4500 and higher RPMs )go to 15+ volts, is it normal or any attention needed Yes, you need to resolve this. Probably the BLACK wire for the Regulator is reading a voltage drop due to too much resistance in the circuit, thus the Regulator sees a lower voltage than the battery has, and sends too much voltage to the battery. Allowing the charging system to continually overcharge the battery will damage it.

5- short test ride and chain seem to be making some knocking/ clicking sound too tight or too loose ? clicking is too loose. Knocking sound might be the carbs, or primary chain.

[Can550]

Thanks Calj
Had vacuum synced carbs earlier but then had to remove them to replaced damaged diaphragm nothing else was done to carbs. Still no harm, shall do it again tomorrow. Glad I had bout Morgan carbtune

How do I go about electrical investigation on black wire ?


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[Can550]

Thanks Calj
Had vacuum synced carbs earlier but then had to remove them to replaced damaged diaphragm nothing else was done to carbs. Still no harm, shall do it again tomorrow. Glad I had bout Morgan carbtune

How do I go about electrical investigation on black wire ?


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Forgot to mention that I converted headlight to LED H4, would lesser draw of current cause this ? If so, guess will go back to stock headlight


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[calj737]

No, reducing the draw would actually lower the charging voltage. Using your Meyer, investigate the voltage seen on the Black wire, and the resistance on it, through the key switch, the KILL switch etc. These are the common locations for resistance to creep in and cause voltage drops and over charging.

[HondaMan]

The most trouble-free battery I have used on the CB750 has been the Yuasa "maintenance-free" battery. It comes with the acid-fill pack, which you plug on top of the filler holes and let it sit for 2 hours to drain and soak. I'm on my 5th year with the one in my own 750, and all of these I have installed on other bikes are still in use since 2011 when I bought the first one. That's a really good record, for this bike.

[Can550]

Agreed
Bought myself Yuassa and it came filled and charged. Price wasn’t bad 109CAD


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[Can550]

No, reducing the draw would actually lower the charging voltage. Using your Meyer, investigate the voltage seen on the Black wire, and the resistance on it, through the key switch, the KILL switch etc. These are the common locations for resistance to creep in and cause voltage drops and over charging.

Little investigation for bad ground on turn signals (which is yet to be found) but found an other issue. Upon turning left or right, when clutch cable pushed on to ignition switch connector, it cut off. Pulled the switch out and it passed bench test. However, when  male connector (from main harness) is wiggled, it cuts off. Now will have to find if overtime a wire is internally broken/ damaged?

Would this loose connection increase resistance for higher charging voltage ?



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[calj737]

It certainly doesn’t help 

[Can550]

So took the switch off, checked wires/ connections and all fine. Opened up switch copper contact points (where they are riveted) were a bit loose and moved around a bit. Soldered them there and put the switch back. Wiggled it all directions and it’s perfect contact now.

Put the led H4 back on, @ idle 12.20 volts on battery increasing with RPM 14.8-15 volts top and start to go down from 15volts. Is it acceptable?


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[calj737]

Sounds good now.

[Can550]

Sounds good now.

Thanks Calj
Shifting from 1st to 2nd gear is a bit rough. At times it’s like grinding or some resistance is noticed all other gears shifting is fine. What should I look for/ at ?
Unless it is normal.


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[calj737]

clutch cable adjustment for starters.

[Desert-SOHC]

Good progress

[Can550]

Thanks everyone for sharing knowledge and helping to give my K8 a second chance to enjoy to road worthiness

Absolutely love it.


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