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First Post - 1978 CB750K - Project Start

zanthor

CB750 Enthusiast
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Des Moines, IA
So I'm new to this forum, new to motorcycles overall. Back in 1989 I owned a 250 enduro, in 1992 I bought a Kawasaki to restore and then got deployed to Germany, no title, so I couldn't keep the bike...

Fast forward to yesterday, my old boss IM's me and asks if I am still interested in his 78 CB750K... he rode it into his garage 7 years ago and hasn't touched it since. It had intermittent electrical problems but was a very reliable around town bike for him at that point, and I got it from him for $500...

Anyhow just wanted to introduce myself since I'll be kicking around here looking for information on the new project. Once I see the bike (haven't seen it in 7 years) again I'll grab some pictures and see what I'm working with!

--Will
 
Welcome

Good to see some one else from Des Moines getting into an old 750. I am in the process of finishing up a '78 Super Sport. Welcome to the forum.
 

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Nothing like a dark and rainy afternoon to go retrieve the new project! Kicks over fine so I assume that means nothings bound up. Any advice on things I should check would be appreciated, I'll be digging my way through posts trying to make heads from tails.
 
Nice.. What are your plans for it. Just get it running and ready to ride? I'd start with the basics... compression, change oil, then check for spark and go through the carbs and clean the tank.
 
Nice.. What are your plans for it. Just get it running and ready to ride? I'd start with the basics... compression, change oil, then check for spark and go through the carbs and clean the tank.

Two phases... first is to get it running, second is the longer term project that will involve totally refinishing it.

Making notes mostly to myself of things I need to do, and observations as I do them... figured may as well make/share them here...

First things first, the basics...

  • Test/Replace battery
  • Air up tires, they LOOK OK, but are both very squishy, it sat in a garage for 7 years so no weather/sun exposure.
  • Test Compression
  • Check for spark
  • Drain fuel/filter
  • Change oil/filter
  • Clean carbs

The bikes previous owner (PO) told me it had random electric problems. There are random visible wires in places I'm pretty sure there shouldn't be. As well as an exposed fuse panel on the left side, and a dangling bit with 2 of 3 wires plugged into it... So wiring work will need done.

The PO had a rag tied around the front brake cylinder, he said it had a VERY slow leak but would always drip on his way to work and hit his dress pants... the rag was the fix. I'll fix that proper...

I assume to do the wiring I'll have to strip the bike pretty far down, so I'll make the call if it gets a new paint job.
 
Sounds like you're on the right track. Summer is right around the corner. I'd get the kinks worked out (items listed above) and ride it for the summer. Then decide what you really want to do with it and work on it over the winter. You might want to think about getting a new set of tires. They may look OK and hold air, but they do tend to get hard and dry out over time. You'll be much happier and safer with a new set of sticky tires.
 
Took the battery down to the auto parts store and they tested it, said it was flat with 0 cold cranking amps but advised it should take a charge once I added acid... I've done that, and have it on the trickle charger now.

I drained the oil from the crank case and the oil tank, replaced the small o-ring on the bolt that holds the oil filter in, but the new o-ring for the oil filter housing was notably smaller than the old one... I put the old o-ring back in and added 2 quarts of 10-40. Do I need to track down a proper o-ring and replace it or should it be fine for the life of 1 oil change?

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At this point I wanted to turn the motor over and see if it was going to suck some of the oil down into the crank case from the oil tank, my manual comes next week but I believe it holds more than 2 quarts... Threw the battery back in, hooked up jumper cables to my car and turned the key on... I got headlights, turn signals, brake lights, all those worked... horn doesn't, don't know if it ever did. When I kick it into neutral, hit the start button and the lights dim slightly, but nothing.

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Any suggestions at this point would be appreciated. I suspect the solenoid, but that's just a wild guess!
 
Oil capity should be 3.7 qts. Did you hold the clutch in when trying to crank it? Also from my experience if the battery charge was dead and low on acid the battery will be junk but go ahead and try it nothing to loose. Acid batteries dont like to be dead or low on acid. Also with the electrical problems you said the bike has I would look over the wiring and try to figure out if any of it is related to the no crank. Check fuses then check to see if you get power through the relay when trying to crank.
 
Yea I held the clutch, same results.

I assume it will suck some oil into the crank case if I can get it to turn over?

The previous owner said the electric start worked before...
 
At the advice of a friend I crossed a screwdriver over the coil it cranks... The battery's been on the trickle charge for ~3 hours and ONE crank was enough to drain it entirely... you may be right about it being dead flat dead.
 
Its a dry sump motor most of the oil is always in the tank. The only oil that is in the crankcase is what runs out of the bearings when running before it is picked up by the scavenge pump and returned to the tank where is is sucked back up in the pressure pump and sent to the engine again.

Start by checking if you have voltage to the solenoid went pushing button. If you have power it is a bad solenoid or clutch switch. If you dont have power then you hae wiring issues. One side is a ground the other is a power. The clutch switch is on the ground side and the start button is on the power side. The button contacts do go bad so check there as well as the clutch switch.
 
I have voltage at the solenoid when I try to start it, but it wont throw. Now to see if I can find a suitable replacement locally, or wait for vintagecb750.com to ship me one.

I fixed it... though I will be curious if my fix lasts a long time or not. Pulled it out and gave it a solid smack end down on the bench a few times. Touched the leads to the battery and heard 'click'. Put it back in place and it cranks away... (on jumper cables.)

In other news, as Dirtdigger predicted the battery is shot. On the trickle charge all night and it was up to 8v before I cranked it, after cranking it it wont even bring up the lights.
 
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Three cylinders compress to 150psi, one to 70psi. Put some marvels mystery oil in at the advice of a friend and will test again tomorrow to see if that helps. Either way I assume I can get it running poorly at the very least. Pulled the fuel tank (BTW: Is this REALLY only held in by the rubber bits and gravity?) and the fuel lines cracked and fell apart. I need to drain the tank, buy new fuel line and a filter... then it's time to take a look at the carbs, which I assume are going to be in a bad way!
 
The fuel that came out of the tank was a lovely color of orange, looked a lot like gatoraide. Got it to fire when I sprayed carb cleaner into the carbs though.

 
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So hosed out the carbs with cleaner a few times, with sea foam spray a few times. Pulled the fuel petcock and did the same with it, though the reserve setting still doesn't work so I have to do some more work on that... but a new fuel line/inline filter and a bit of luck it's alive.

 
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