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1981 honda cb750 custom

coty0912

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1981 honda cb750 custom

Im new to the bike world but Ive been wanting a bike for as long as I can remember and I finally picked one up the other day. I was told I got a steal ($400) and I believe it. Im the second owner. Its an 81 cb750 custom that had been garage kept its entire life other than the 3,500 miles its seen the road. Its in amazing shape. I was told when it was last parked (2014) that it ran fine and had no problems whatsoever. Just had a dead battery. Not gonna lie I don't know hardly anything when it comes to bikes. I know how to ride only from dirtbikes years ago. Anyway it came with a brand new battery still in the box. I had to fill it and charge it myself. Once it was charged I put it in to see if she would turn over. After I drained the old gas of course. When I turn the key on i have no power no lights nothing. So I checked the fuses by the handle bars. They all appear to be good. I checked the main fuse by the battery and there was no fuse but a wire in the fuses place. It was a little gage wire like standard speaker wire. Not having the correct fuse I replaced the wire with a heaver gage wire but still no power. I was told if I had a corroded ground wire or the wire to the starter was corroded that could stop me from getting power. I found the ground on the inside of the frame but couldn't tell if it was corroded. It didnt feel that it was but i couldn't see it so I left that one alone and ran a new ground wire.. Still no power. I don't know how stupid this may sound but I don't have a clue where the starter is.. Any help and or suggestions will be appreciated! I also did try a jump off a non running car with a good battery and still no power. Not sure where to go from here.
 
How long did you charge your 'new' battery? You need to measure the volts on it.

You can try to jump/run bike with booster cables and a car but DO NOT HAVE THE CAR RUNNING or you will toast your electrical.

Ignition switch itself could be a problem too.

Go buy (or get free using coupon) a Harbor Freight cheap voltmeter, you'll be needing it. $15 IIRC. Maybe even cheaper.
 
The new battery took about 5 hours to charge. And I did already try to jump off of a non running car battery. Still nothing. When I turn the key over all I have is a tail light no brake lights or anything else.

 
Your supposed to slow charge new motorcycle batteries (2 amps) overnight. It usually states this on the box or included instructions.

What do you mean "Turn the key over"? You do know there is a separate button to start the engine? It sounds like your turning the key to the park position.

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When I turn the key over I was turning it all the way to p but if I remember right I tried all the positions. I will definitely try when I get off work. Thank you.

 
And yes im aware its a different button to start it. I tried that and the horn and lights as well just to see of i had any power at all and it appears that I don't.

 
Start by flipping all the switches on and off a lot, this might rub off enough crud to make contact if that is the problem and replace those missing fuses, they are there to keep the wiring from melting and catching fire in case of a short.

You said the tail light comes on when the ignition is in park? That would indicate juice is making it to the ignition switch. The connector for that switch is probably under the gas tank. I'd test the switch first with a continuity tester.

After that, follow the wiring from the battery testing to see where the juice stops. At this point a simple 12v test lamp will do.

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The bottom of the ignition switch (white plastic part) often comes loose or apart to do stuff like that, even worse if someone has changed that plastic part, the Chinese copies use such flexible plastic they often distort to give issues even brand new. Why they are dead cheap. Often the problem is there.

Battery volts need to be up around 12.7-12.8 with it sitting, you really did not charge that one enough like said. Good way to kill it very early, the first charge on a new battery really needs to be a very full one. I go overnight as well.

Check for 12 volts at the red wire going into ignition switch, if you have it then you can jumper it from red to the black and you will have all bike power.
 
The bottom of the ignition switch (white plastic part) often comes loose or apart to do stuff like that, even worse if someone has changed that plastic part, the Chinese copies use such flexible plastic they often distort to give issues even brand new. Why they are dead cheap. Often the problem is there.

Battery volts need to be up around 12.7-12.8 with it sitting, you really did not charge that one enough like said. Good way to kill it very early, the first charge on a new battery really needs to be a very full one. I go overnight as well.

Check for 12 volts at the red wire going into ignition switch, if you have it then you can jumper it from red to the black and you will have all bike power.
I had a friend come over and he brought a tester to see where im losing power at.. In the main fuse block the two red wires have power but the two on the other side don't. Green and red and yellow and red. Should they have power going to them? And the red wire on the ignition has power going to it, even when the key is off.
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Well nevermind that main fuse question! On a good note im very happy. I went out messing with the ignition.. I turned it on and wiggled it for a bit and all power came on. As soon as I had power back to everything I put the tank back on and new gas in it. After a few attempts she runs. Hasn't started in two years and fired up with hardly no problem! While running the overflow or runoffs on a couple of the carbs is steadily leaking fuel. Does this mean the floats are stuck?

 
Yeah, the float needles probably have some crud keeping them from closing all the way. You can try whacking the carb bowls with the plastic handle of a screw driver while it's running, you might clear them. Tap them on the bottom, straight up, not on the side.
 
Okay I will give it a shot in the a.m. I was told I should put seafoam in the gas tank to help clean everything out. Is that a good or bad idea?

 
And I have to say thank you to everyone for the help and pointers. I wouldn't have made it this far without all the help. Very much appreciated! Thanks again!

 
Do what you want but I sold seafoam by the truckloads in parts and we considered the people who used it as suckers.................just sayin'. I'll say it cures brain issues far more than mechanical. If you want the carbs clean you take them apart and clean them.

I personally have never used additives or special 'cleaners' of any type in 45 years of working on engines. No need when you know how they work. Just use good grade fuel and oil.

I do of course thank all those who bought those additives by the truckloads for the incentives I got to sell them. Why I feel indebted to give back now, those people got suckered in my view.
 
Okay. Thanks a lot. Should a guy that has no clue what has doing attempt it? I watched a few videos seems pretty simple.

 
Just getting the carbs off and on the bike is the hardest part. Sometimes a 2x4 and 2 very large c-clamps are involved.

This is the spot you stop and do a compression check with a screw in type compression tester. My local auto parts store will rent tools like this, I don't know about yours.

If the compression is good then it's time to think about spending some cash.

If your going to put in money anywhere this is the spot. Have a reputable bike shop rebuild the carbs with new jets and do a complete tune up while they have it.

They should sync and adjust the carbs, adjust the timing chain, check and adjust valves if needed, new spark plugs, check the spark timing, oil and filter change, check and replace air filter if needed, adjust and lube drive chain.

Adjusting the valves on these is a pain in the ass even with the right special tools and huge box of shims, it's impossible without them.
 
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