1981 CB750K4 Street Tracker Project

[ty_deschaine]

First and foremost, I'd like to formally introduce myself. I've been creeping on the forums for some time now and figure now that I'm actually digging into my project bike, starting a project log to help catalog my progress and get help when needed was in order. There seems to be a pretty tight knit group of knowledgeable people on here.

I recently picked up a 1981 CB750K off Craigslist in non-running condition for $500. The bike was mostly complete and with a quick pull of the plugs and a rolling drop of the clutch showed that the motor turned over (didn't have a battery). He said it had some sort of electrical issue, and wasn't getting power anywhere. Here is a photo of how it looked the day i picked it up...




So after getting it home, I picked up a battery and started to test continuity and run through the process of finding where it all went wrong. This eventually led me to the ignition switch... After replacing the switch and adding some fuel...


http://youtu.be/Gk6DZJxy8tQ
SHE FIRES! Not great, but hell, it sorta started...

So today I wanted to try to figure out why it was running so rough. I am new to motorcycles, so I'm working on the learning curve to figure out the issues. I do love how straight forward a carb'ed engine is! I mostly deal with Diesel engines, so its a pretty fresh change in work flow. After sorta starting it today in the garage, I noticed that the number 1 and 2 cylinders exhaust was hot, but 3 and 4 were ambient. I pulled the plugs to check for spark and they both checked out. I also checked the drain valves on the carbs for fuel and both carbs seem to be getting fuel. Could this be a gummed jet? The last time the carbs were serviced was in 2003. So I'm sure they need a rebuild.

Also, the starter seems to be slipping. It will turn the engine over for a second and then give a terrible rattling sound. Is the starter clutch bad? I will try to take a video in the next day or two..

Overall, I'm pretty stoked to dig into this project over the next year. The bike is in pretty rough shape. It needs quite a bit of TLC to bring it back to life. This winter I plan to strip it down and rebuild the motor and eliminate/hide as much as possible.

Here is a photo of where all the inspiration began...

And a link to more photos... 

[LukeM]

Not bad for the first start.  I'm pretty sure your carbs will need a disassembly/cleaning/rebuild.  There are several threads on that topic.  You can't get the carbs too clean.

Also, do a thread search on "carb sync".  You'll see the terms "bench sync" and "bike sync" used.  Bench sync is doing them on the workbench.  Done properly, it'll get you close.  The final step, after everything else is done (timing, plugs, wires, points/condensor for the SOHC models, etc) is to sync the carbs on the bike with meters and screwdriver.  

I'd get your bike running good, and see if you have any other issues like burning oil, oil/fuel leaks, or clutch issues, or stuff like that.  Get the bike running good as she sits, and then figure out what you want to do with it.

Welcome to the forum...I hope we can help you more.
Luke M

[ty_deschaine]

Got my new bars today from VintageCB750.com!


Out with the old...


In with the new!!!




Quite a bit wider...


I was planning on tearing into the carbs tonight, but my lady finished her internship, so a celebration was in order. So the carbs will have to wait until next week.

Also, had to get the Jetta prepped for a weekend on the peninsula.



Also, a side note. Does anyone know if the CB750F tank will fit my bike?

It appears that it will, my title says my bike is a CB750K, and I have read that they are interchangeable. Regardless, I plan to do the swap. If anyone has a tank they want to get rid of let me know!

[LukeM]

It should. Unless I"m mistaken, there's no difference in the frame between the K and F models.

Luke M

[TOOLS1]

I think there is a little difference in the rear mount. You might need to fab something up for the mount.
TOOLS

[shinyribs]

I thought the frame on the C's wouldn't let an F tank lay all the way down. Not sure on this, just seems familiar.

[ty_deschaine]

So I was able to dig into the carbs tonight. Most weren't too bad. #4 was definitely the most gummed up.



Couldn't do it without my old friend Trailhead ISA


Did a quick bench sync to get the carbs in the ball park and then got everything back together. I couldn't get it to start due to the starter slipping... I pulled the cover off again and found the 3 bolts that hold the gear to be loose. I figured that was causing an alignment issue. Got it all back together and still sounds like this...



Any clue as to what might be going on. I'm thinking the actual starter is shot.

[rich]

Sounds like the stater clutch will need more work. Here's another thread on the topic 


Here's the video from the other thread:

[ty_deschaine]

Taking apart the starter clutch and really tightening those 3 bolts did the trick! Glad it was an easy fix. I'm hoping to get a idling video posted in the coming days... Just gotta find some time.

[ty_deschaine]

Threw some new plugs in and was able to get it run again, although still the same situation as last time. Its running pretty rough. I was able to get it to run on all cylinders for a minute (smoked like hell out of the 3 & 4 exhaust for a minute) though I can't seem to keep it idling without the choke. If I turn the choke off it will stall out. Is this a timing issue?

I have a feeling a full carb rebuild is in order...

[TOOLS1]

Sounds like the pilot jets are plugged, or a vacuum leak, or even both.
TOOLS

[johnb]

I have an 98 F and I had to move the front mounting pegs and make a plate  for the rear mounting