Topic: 1978 cb750k questions

[Mikehin750]

Hi I just picked up a 1978 cb750k for $200.00 Just have a few questions for now. Im sure Ill have a lot more later I need to know all the info I can about what kind of plugs oil etc. Also what is a good link to look for tires? Oh and the bike has just the headers no mufflers or baffles. I do have some pipes im gonna put on just to extend them back some more. Is it ok to run this bike without mufflers or baffles? I dont want to mess it up? any and all info is greatly appreciated. Thanks!

[martino1972]

welcome to the site,you will find that it's one big family here,and we are all friends...
first thing you have to know is this:WE LOVE PICTURES!!!!!!!!!!!!!!!!
on the right top of the page you will see a search option,when ever you have a question,dig in the search and you will find a ton of good info,or drop a line here and you will get an answer too....
especially the oil topics are highly appreciated here..

running the bike without muflers/baffles would not harm the engine IF YOUR JETTING IS CORRECT....
if the bike is jetted for muflers/baffles it will run lean,and thats a no no..

tires,depence of what kind you want,ebay is a good starting point for them,depending where you live..

[eldar]

Well for plugs, the standard is ngk D8EA plugs. Make sure there is no R as that denotes resistor.
Oil, for the time being, till you get everything going AND to clean things up some in the engine, get rotella 15-40 from walmart. it is a diesel oil and cleans well.

For the pipes, if jetting is too rich, it can burn the exhaust valves. The longer the pipe, the less likely this will happen. You can get slip on mufflers, you might be able to find a kerker or supertrapp, or you could go the Mac route. Look around and see what you find.

As for tires, this is up in the air. You might be able to find them cheap on line but then you have to mount them. Shops generally charge more to mount if you did not buy the tire from them. Once you add the tire cost, shipping and mounting; you may be best just checking a local shop.
you do not have to get an expensive tire. A popular low cost tire is the bridgestone spitfire.
I run them  and they work pretty good. See what you can find though.
Some shops will mount for low cost if you buy the tire from them and you may actually save cash.
Take the wheels off though and take them in. That will help save AND you can then pull the axles and bearings and get them all cleaned up and greased.

[goon 1492]

well said eldar, you hit all the bases with that one, and like he said use the search function it is a great tool. Don't forget to add checking the chain and sprockets to the list too, a quick search here will help reccomend what kind you have and or need. Jc whitney has a section that tells what kind of sprocket you had stock.

[Mikehin750]

I tried to start it today and it wouldnt even fire. I pulled a couple of the plugs and they werent even wet the slightest bit. the carbs seemed to be leaking out the overflows at the bottom of the bowls. Any Ideas why fuel would not be getting to the cylinders? and yes the petcock valve was on

[martino1972]

you wanne type "carb cleaning" in the search option on top of the page...

[eldar]

You have a 78, you WILL need to clean carbs. No way around it.
Disconnect the throttle and choke cables and loosen the clamps and off they come. Pull the bowls off and remove the jets. the main will unscrew, the slow jets just need to have a piece of rubber wrapped around them and pulled out with a pliers. You may need to twist while doing this.
Remove the floats and needles there. Clean them but also clean the needle seats otherwise you will forever leak gas when not running.

Spray them with carb cleaner and then use a strand of copper wire or you can get a jet cleaner from honda, and clean the holes out and spray again.

pull the 3 screws out of the accel pump and clean all that out. Make sure there are no holes except in the ears of the diaphragm.

Pull the slides out and clean the gunk out of there. Remove all the old rubber fuel lines and clean those up.

Re assemble the units and close the idle knob till #2 carb(the one with the pump) has a 1/8 in opening, then adjust the other 3 to the same opening.

Put carbs back on, taking care to get good seals on the intake manifold side. Connect fuel and turn gas on. Carbs should fill ok. Make sure you have throttle and choke connected before this step.

Twist the throttle and watch the little brass tips at the throat of the carbs, they spray gas. If not, make sure the pump arm is being moved by the throttle tab that moves the slides. Gap there is easier to check with carbs off. It should be .008 inches.

If you have gas there, then try to start. you may need to turn the idle knob to get the bike to idle.

[dusterdude]

eeeh,its a k8 who cares...btw welcome

[eldar]

You have to forgive duster. AS of yet, he has had no luck finding a k8 of his own. It shows in the amount of jealousy he has!

[eurban]

Nice write up Eldar! . . . .  The suggestions to do a search are right on the money as your types of questions / issues come up all the time.  One thing though, you don't want to search from the within this thread because it will only search this thread.  Search either from the top of the "Tech Forums" page or the "SOHC/4 Bikes" page.  Also, do all of us a favor and download and read the factory manual for your bike.  Here's a link for the down load:
http://www.mediafire.com/?5ttzdg9yyuz

[cdwicks]

 Welcome to the forum!!  I have a bike just like that and am going completely through it at the moment.  In fact, I just finished swapping frames on it.  As for the not firing, check your points.  Mine wouldn't fire when I got it and I realized I had to check the points.  Turned out they were rusted/welded together!  I unstuck them, filed them down a bit and she fired right up!  Of course I changed them ASAP but it cranked!  PM me if you have any other questions or just post here!  These guys are geniuses on these bikes!

[Mikehin750]

Thanks guys. I really appreciate all the help and posts Well a little after my last post I went out and figured what the heck why not try again and put put VROOOM! She fired. I forgot to mention that the carbs were dry for a year before I just hooked up the fuel to them. Maybe the floats were stuck from sitting so long? but any way I got her running but only with the choke on. as soon as I pushed the choke back in the engine died. If I tried without the choke on it would fire but as soon as I gave er gas it would stall out. I drove it down the road with the choke on and it seemed to run good. But with the choke off would just die. Is this symptomatic of any certain problem? A guy I work with seems to think the main jets are clogged. I work at a new Keihin factory that just went up here in Michigan. There is this japanese fellow who I work with that use to build the same carbs that are on my bike 30 years ago. He is a bike entusiast so I might talk to him tomorrow about it also. Now as for the strait pipes on it is there another way to make it run richer other than new jets? I searched for 5 hours last night on here and founds some interesting post but its just nice to get direct help on certain things so thanks again guys I really appreciate it  thanks for the link to the manual that should help alot! Ill be talkin to you all soon. take care

[joeyboy]

I suggest learning the links on line as explained by others, but also buy a book.  Get out to your bike and just look at stuff and understand as much as you can.  When you are out on the road, you don't have the internet to help you out, so learn your bike.  I rebuilt mine completely, so I learned a lot more than I thought I would.  But reading these forums and reading a manual exclusively on your bike will greatly contribute to the repair, and care of your ride.  Not to mention safety, cause making a mistake with brakes could cause great shakes.  Take care and welcome, ask many questions.

[eurban]

Reread Eldar's post and do exactly as he suggests.  Your slow jets are in all likelyhood plugged.   After your carbs are actually functioning properly, worry about tuning them to your exhaust.  Main jet size, main jet needle position (adjusted by shims in your case) and idle mix screw settings will be the areas where you can fine tune the carbs to your paticular setup.

[goon 1492]

I would also do like you said and talk to that Japanese guy, if he used to build them he will know all the ins and outs of those four little devils.

[Mikehin750]

Hey its me agian. Allright here is the scoop. I drove the bike today and seems to run good once warmed up. good throttle response and so forth even at high speeds. But wants to stall when cold. put new plugs in and the bike is definitly running lean. prob from no back pressure. Is there any adjustments I can make so it runs richer with out new jets? I dont have the money for new exhaust right now but would like to drive the bike in the mean time. Maybe adjust needle valve height or air mixture screws if I did any of this would I have to have the carbs synced? Thanks

[eurban]

Clean and properly functioning carbs are must before you start the fine tuning. You have a 30 year old bike with symptoms of an impaired idle circuit (lean looking plugs for instance), so start with a thorough cleaning and baseline adjustments of the carbs.  Eldar has told you how to do this.  Can't really say it any more clearly than that. . . .