Topic: Having trouble getting my '77 CB750k to run.

[banderson221]

Hello everyone,
About a year ago I bought a 1977 CB750k from a friend. The bike looks pretty good but wasn't quite running and I bought it in hopes of learning a bit more about motorcycles and mechanics in general (I'm a complete noob). I have lurked around on this site for a year now and figured I might as well actually make a post to address my specific problems. So I apologize if my issues have been addressed elsewhere on the site.

My main problem is: when I start the bike it will idle (not very well) but as soon as I twist the throttle I hear a hissing sound like air being sucked and the engine immediately dies. I have taken apart the carbs many times and they are clean, I replaced all the carb gaskets, o-rings, tubing, and accelerator pump. One issue I have noticed is that when I twist the throttle I do not see any gas squirting out of the accelerator pump post. I replaced the accelerator pump but cannot seem to figure out where the problem is. Could this be why the engine is dying?

My second, unrelated problem is: one time when I was trying to start the bike, I noticed smoke coming from under the seat. All the wiring from my rectifier had melted and part of the main wiring harness had too. If I replace the rectifier and the wiring harness would this fix my issue?

Sorry for the long winded post and my general lack of knowledge. I am very excited to be learning this stuff though! Any advice would be appreciated. Thanks!

[flybox1]

Hello everyone,
About a year ago I bought a 1977 CB750k from a friend. The bike looks pretty good but wasn't quite running and I bought it in hopes of learning a bit more about motorcycles and mechanics in general (I'm a complete noob). I have lurked around on this site for a year now and figured I might as well actually make a post to address my specific problems. So I apologize if my issues have been addressed elsewhere on the site.

My main problem is: when I start the bike it will idle (not very well) but as soon as I twist the throttle I hear a hissing sound like air being sucked and the engine immediately dies. I have taken apart the carbs many times and they are clean, I replaced all the carb gaskets, o-rings, tubing, and accelerator pump. One issue I have noticed is that when I twist the throttle I do not see any gas squirting out of the accelerator pump post. I replaced the accelerator pump but cannot seem to figure out where the problem is. Could this be why the engine is dying?  Clean the accel pump circuit.  Typically, the check valves, one in the accel pump bowl, and another at the top of the carb bowl itself, are clogged with debris.  Spray carb cleaner helps.  Clean the accel pump circuit tubes between the carbs.  Clean the individual accel pump squirters, one in each carb throat.

My second, unrelated problem is: one time when I was trying to start the bike, I noticed smoke coming from under the seat. All the wiring from my rectifier had melted and part of the main wiring harness had too. If I replace the rectifier and the wiring harness would this fix my issue?   Check that your fuse panel is not fried.  pull it out and inspect it carefully.  only use the appropriate fuses.

Sorry for the long winded post and my general lack of knowledge. I am very excited to be learning this stuff though! Any advice would be appreciated. Thanks!

Hondaman's book, and the shop manual are great resources.  The shop manual can be downloaded free in the FAQ section.
Hondamans book is available to purchase here
http://www.lulu.com/shop/mark-paris/my-cb750-book-hardcover-edition/hardcover/product-22322967.html

[kerryb]

Hondaman's book is a must-have if you really want to understand your bike!  While going through issues similar to what you describe I discovered/read about using single strand of copper wire from lamp cord or the smallest wire bristle from a brush to clean the accelerator pump nozzle (the tube sticking into the carb throat)  I learned to use the handle from an xacto knife as an extension handle to reach in and poke the wire into the clogged nozzle.  This has worked for me on more than one occasion.  On another note, I have almost quit using carb cleaner except when all the rubber bits have been removed.  Spray carb cleaner has cost me several expensive diaphrams and O-rings.

just for fun...kb

[flybox1]

+1 Kerryb
spray carb cleaner will melt those accel pump diaphragms.
X-acto knife pen w wire....nice accel pump cleaner hack!
I always used a needle bent w a 'U' at the end.

[Phinn]

The hissing sounds like a vacuum leak. How pliable are your insulators and carb intake boots?  Air filter?  Air box?  Using pods?  If there's a leak, it loses the negative atmospheric pressure, which is what causes the entire carb system to work. Without a partial vacuum, fuel won't draw up from the bowls, and air will not flow past the venturi properly.

As for the accelerator pump, I don't think that it's something that causes dying, not as dramatically as you describe.  I ran my 78k for a few months before fixing the pump.  It'll accelerate slowly without a functioning pump, but wont cause it to just die, not in my experience.

The key to the pump is in the base.  The check ball in the bottom gets gunky because it sits in old gas, and it's virtually impossible to access.  I had to soak mine in solvent for days and days before it finally started working. 

Also, mine was assembled incorrectly.  It had the spring on the wrong side.  It was never going to work. 

[banderson221]

thanks for the input guys, I will look into that book once I get a few paycheck coming in. I'll see what I can do with unclogging the accelerator pump nozzle (awesome idea with the Xacto knife!). I am using the stock airbox and probably will continue to. The intake boots from the airbox are not very pliable, actually quite hard to get on so maybe theres a chance that they don't have a good seal.
Thanks again, I'll let you know how things go.

[madmtnmotors]

The intake boots from the airbox are not very pliable, actually quite hard to get on so maybe theres a chance that they don't have a good seal.

The boots from the airbox won't have as much impact as the boots between the carbs and the engine. I would suspect "hissing" noises to be coming from these.