1980 cb750k, few questions.

[Kha]

Hello, I just got that bike about 2 week ago for $750. Run and drive, shift good. Did a basic tuneup on it and it idling alot better than when I got it. Few bad things I still need to fix is the rear brake pedal is soft and stick when I press on it. Pedal wont return to normal position without help from my ft. Where should I start doing the diagnosing? Other problem is the charging system is not working. I know for sure the battery is bad. I plan to replace the rotor, stator and rectifier once I get some spare change =). The main thing I'm worried about is the rear brake. Can someone give me some pointers on where to start with the rear brake problem?

Thanks yall

[shinyribs]

Welcome to the forum.

It's a good chance your rear brake is sticking because of rusty linkage. I'd start by removing the pedal itself and taking a look at it's pivot. Since the bike has been sitting it's a good chance it just needs a good cleaning and a little dab-o-grease. That's where I'd start, at least.


As far as the charging system....eww. That can get pricey on these bikes.

I hate to not give you a straight forward answer on this part, but the best thing to do will be to download the FSM (Factory Service Manual) off the homepage here. It details how to check all that out. You can also use the search box on the homepage of the forum to look for more info. Type in something like "DOHC charging issues" and go from there. It's been discussed here alot.  It's the one weak point of the DOHC CB.

Good luck and keep us posted.

[Kha]

Thanks for the response. I'll check it out asap. I can start the bike for now with a battery charger so I'll prob cont to do it that way until I have enough for the charging system. Thanks

shinyribs wrote

Welcome to the forum.

It's a good chance your rear brake is sticking because of rusty linkage. I'd start by removing the pedal itself and taking a look at it's pivot. Since the bike has been sitting it's a good chance it just needs a good cleaning and a little dab-o-grease. That's where I'd start, at least.


As far as the charging system....eww. That can get pricey on these bikes.

I hate to not give you a straight forward answer on this part, but the best thing to do will be to download the FSM (Factory Service Manual) off the homepage here. It details how to check all that out. You can also use the search box on the homepage of the forum to look for more info. Type in something like "DOHC charging issues" and go from there. It's been discussed here alot.  It's the one weak point of the DOHC CB.

Good luck and keep us posted.

[icerigger]

Rather than replacing the whole charging system, figure out which parts are broken. Mike Nixon has a really good troubleshooting page.

http://www.motorcycleproject.com/motorcycle/text/DOHC_charge.swf

[Kha]

Definitely will check that page out. Thanks icerigger

My bike is able to start and run with a battery charger plug up to it and it will remain running after take the charger off. Wouldnt that mean my charging is working, kinda like a car? If i try to start it without the charger than it don't have enough power to start.

Anybody have any inputs? I know different bikes require the battery to start, but I can basically run it with no battery just connect it to a battery charger and start it right up. Hopefully my problem is just a bad battery. †praying†

[Re-run]

I am going to put this in our list of links. Some good stuff on here besides just the charging troubleshooting.

[sgtslag]

Couple of things...

Get some SeaFoam, run it through the fuel tank at least once per month, and you will never have fuel-related issues again (it dissolves varnish, stabilizes gasoline for up to 12 months, absorbs water, and some other stuff which won't matter one bit...); if you do this, you will never have varnish clogging up your carburetor jets again, period.  Run the SeaFoam through the crankcase (search this forum for "Seafoam", and you will get quite an eye-full of stuff to read) and transmission, to dissolve varnish, sludge, and carbon deposits within the engine and the tranny, left over from the SF-rated oil, used back in 1980 -- when the SeaFoam has done its work, the oil will turn black, change it, and the filter.  Modern, SN-rated oils, will not leave deposits behind, if you change oil on a regular basis (3,000 miles conventional, 5,000 miles for synthetic).

Check the spring on the rear brake pedal (FSM has diagrams showing its location).  Sometimes this is lost, or stretched, preventing the lever from coming back up to its normal resting position.

Change out the front fork fluid -- it likely is anything but cherry red in color (Dexron color; fork oil is light caramel color,when new), which is what it should be.  The FSM explains the process, but I did not disassemble mine, or remove them from the bike -- I just used the drain screws.  This is the most overlooked maintenance issue on any bike.  The good news is that Honda used Dexron III ATF in the forks, which runs around $5 per quart; fork oils run around $15 per quart!  The Dexron works very well, it is inexpensive, and it is available everywhere.

Support the engine under the front, with a jackstand; add 2 oz. of SeaFoam to each of the fork tubes before draining, compress the fork tubes by lifting the front wheel up/down, around 10 times, to circulate the SeaFoam inside the fork hydraulic systems, which will dissolve varnish/crud; remove the drain screws on the outside, bottom of each tube, and carefully collect the old fluid which will run out (read the FSM instructions, before attempting this modified method).  Compress the fork tubes again, to push out as much fluid as possible, then re-install the drain screws, and tighten just beyond snug.  Re-fill the fork tubes, as per the FSM, with fresh Dexron ATF (III-VI is all good, but the VI will be synthetic, and a bit more money -- but it will last twice as long).  Changing the fork fluid every two years is a good schedule -- it does wear out, over time, and miles.

The suspension is more than just comfort on the ride -- it keeps your wheels tracking the road surface, as opposed to flying above it!  Poor maintenance of the suspension system can lead to loss of control, and...  Cheers!

[Kha]

Was thinking about seafoaming. Did it on my old cars and it made a ton of difference. Just gotta wait til it warms up a bit so I can let my bike idle outside my garage. I just got a clymer and will download a FSM for the bike soon. Can't wait to fix all these little things before I start doing the body work. Plan to build a Brat style.

Still need few pointers on the issue I've pointed out above regarding the bike running normally after I start it with the battery charger. Bike won't start without on the battery alone though. Hopefully its just a bad battery rather than the bad charging system.

[shinyribs]

These bikes don't like to start with a weak battery. It's like when they are cranking there's no ''juice'' left over for them to fire. Healthy batteries are super important on these bikes.

[Kha]

Ill just go buy a battery and trickle charger and make sure it always fully charged. I'll test the charging system while I'm at it. Only thing left is to fix the rear brakes and I'm good to ride. Just can't wait to cruise down the winding highways. =)

[free2ride]

shinyribs wrote

These bikes don't like to start with a weak battery. It's like when they are cranking there's no ''juice'' left over for them to fire. Healthy batteries are super important on these bikes.

I've wound up having to replace my battery every second year. frustrating.

[Ba5sKing]

i cant tell you how right shiny is, however what makes you believe your charging system is not working properly? Have you measure the voltage when the bike is running on the positive terminal of the battery? If it is not higher than the battery is prior to the engine running then there may be some sort of an issue. However if you believe it is not charging, just because the battery does not have enough juice to crank the bike may be a false assumption. These batteries are small and the cold weather won't be helping you out if your up north like myself. They do naturally drain over time if their not kept in a climate controlled environment, and even then will still drain. Check your voltage on the positive terminal before you go wasting your money on parts that may or may not fix the problem.

Look into the characteristics of bridge/3 phase rectifiers on youtube and google and how AC to DC power supplies work before you start tickering. A theoretical base can add alot of new angles that you can try to diagnose your issues with.

Best of luck!

[shinyribs]

That is actually really good advice. And,WOW, I can't believe none of us thought to ask if you checked voltage to see if your bike was actually charging or not!

Good catch Ba5sKing!

Keep in mind that these bikes won't charge at idle. You have to get the engine up to at least 2,500 rpms or so before the charging system kicks in.

[Kha]

Yes I'll check the voltage out this weekend. Keep y'all updated

[Kha]

 Good news for me. I bought a new battery because the old one was dead. Took apart the charging system clean off parts with wire brush and checked all connectors. Put the new battery in and checked the voltage. 13v. The bike started and revved it up to 3k rpm. To my surprise the voltage got up to 14.55v. I was so happy. According to all the articles Ive been reading, that mean that my charging system is charging! Exciting find. I will keep it on the battery tender jr just to make sure its always full. I also fixed my sticky rear brakes. Turns out that it was just a sticky spring. Cleaned it off with wd40. Adjust the pedals and worked just fine now. Got my plates and ready to ride.