'81 CB750 woes continue

[THansenite]

I am working on getting a buddy's bike ready for spring riding. He bought it last fall and could never get it running quite right. I've tried a bunch of things to no avail (original thread: http://www.dotheton.com/forum/index.php?topic=55477.msg630457#msg630457) and I'm hoping the experts on here can help me out.


The bike is an '81 CB750 that is stock to my knowledge. The bike is hard to get started when cold. The choke has to be completely pulled out and I need to play with the throttle a bit to get it to fire. It refuses to idle below around 3k. Below that, it will simply die. When riding it, the bike runs fantastic and smooth with the choke pulled, but when you pull in the clutch to slow down for a stop, if it drops below 3k, it will die.


I've also noticed when I am warming it up and have it idling around 3k with the choke pulled out, it will be running fine, then will stumble and die for no reason. The engine rattles a little bit when it does this, but I don't know if it is an engine issue, or just noise from a 30 year old motor.


Below is what I have checked:
- Carbs and all passages/jets/etc have been gone through and set 3 times
- New plugs (they look great when pulled after it dies, no indication of rich or lean)
- New fuel filter
- SOS valve removed (fuel line runs from petcock to filter to the carbs)
- Battery is putting out good voltage (multimeter shows 14+ plus volts up until it dies)
- New air filter


Is there anything else you guys can think of that might be causing this issue?

 

[HerrDeacon]

Go through the maintenance tasks outlined in the manual so then you can start eliminating things and ensure things are set to spec. This will include valve clearances, ignition timing, check battery, set float height in carbs, etc.

By your description it sounds like its running a bit lean on the idle, something is preventing the correct amount of fuel (clogged passages, incorrect float height, etc.). I'm not familiar with the 750's do they have a choke or enrichener?

 

[THansenite]

Go through the maintenance tasks outlined in the manual so then you can start eliminating things and ensure things are set to spec. This will include valve clearances, ignition timing, check battery, set float height in carbs, etc.

By your description it sounds like its running a bit lean on the idle, something is preventing the correct amount of fuel (clogged passages, incorrect float height, etc.). I'm not familiar with the 750's do they have a choke or enrichener?

I verified all passages are clear and made sure the floats were set correctly when I had the carbs apart before (I forgot to mention that above). I'm not sure which type of choke the CB uses. There is a pull-style choke up on the handlebars, but I'm not sure of the type of operation.


I'll try to track down a service manual to verify the specs on the other items you mentioned. Hopefully, something will pop up that will explain and remedy the issue.

 

[my-79-750-k]

Air leak? Check the carb boots, they are notorious for cracking, especially No's 2/3. They run hotter due to being on the inside with less airflow. Other than that, check the ignition. If the return spring for the advance isn't knocking your timing back to idle TDC, it would cause it to want to die. Idle doesn't like 30* BTDC, supposed to be somewhere in the 8-10* range at idle.

 

[HerrDeacon]

Just remembered that I had a similar issue on my KZ, next time you get it running check to see if all the cylinders are getting hot. Just spray a bit of water on each header to see if they are hot. In my case turned out my #2 cylinder carb had the enrichener pickup in the float bowl blocked.

 

[Hoosier Daddy]

How can you have gone through the carbs 3 times and not know what choke it uses?
They are plates...

Sure sounds like it is running lean and yes, the carb insulators are a prime suspect. Even if they aren't cracked if they are rock hard they may not be sealing. I go my replacements from www.vintagecb750.com and were a perfect fit at a good price. Much easier to tell when the carb rack was fully seated in place when the "POP"ed into the grooves.

When you removed the SOS did you cap off the vacuum port on #2 carb that opperated it? If not, that is a dead vacuum leak.