1980 Honda CB750k

[FourtyOne]

I have a Honda 750k 1980 that i recently purchased. I had the carbs rebuilt and checked but the bike still does not hold an idle. you can ride it and get where you need to go, but if at any time you stop giving it gas it will die instantly. all the fel systems have been looked at and seem fine. the firing was good and cmpression was a bit low. there is no engine noise and it sounds good. I am stumped over this issue and would like to fix it so it will idle on tis own soon. Thank you

[TOOLS1]

Welcome to the forum. Try adjusting the idle screw up on the carbs. If, you are not sure about where it is at, or how to do it, you can download the manual from our manual section.
TOOLS

[FourtyOne]

yea, i tried that also

[TOOLS1]

In that case I would take it back to the person that rebuilt the carbs. Your idle circuit has a problem.
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[FourtyOne]

is that something he should have changed out when rebuilding them?

[Re-run]

Well it is hard to say. It could be as simple as your jets being plugged up some. There is also and air diaphragm in the carbs and if this develops cracks, the a/f mix can be off and cause poor running.
Are you using the stock airbox or do you have pods on?

[TOOLS1]

If you paid to have it rebuilt, it should work properly.
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[FourtyOne]

its the stock airbox. He said he thoroughly went through the carbs and even replaced a diaphram cause it had a pinhole in it. he hasnt answered me back yet about the idler circuit

[FourtyOne]

He said when he brought the bike back that he went through it again to make sure it wasnt nothing to do with the carbs or the fuel system but didnt know why it was still dieing if you wasnt giving it gas. it has me stumped. I know how to do most but have never really messed with carbs, always outsourced that work. I keep going back to the carbs also but all I have is his word that he did all he said he did. Im still waiting on an answer about the idler circuit.

[Re-run]

A common mistake shops make is to only dip or spray carb cleaner though the carbs. If the idle jets get blockage, then the best way to clean them is to use a strand of copper wire and poke through the holes to clean the blockage.
From there, you should pull off the airbox and look at your slides and make sure your idle screw is raising the slides as it should.

[Re-run]

Now I am not sure about this part but on my 78K I have a fast idle cam that is actuated by the choke mechanism. It raises the rpm while the choke is applied.

[Rodgil]

I believe the US market bikes have an accelerator pump, needed to give a bit of extra gas when you open the throttle, needed because they run a leaner mixture for that market. (Not fitted in other countries) If that is not working properly, the engine will stall when you open the throttle.

[FourtyOne]

its the exact opposite rod. it only runs when i open the throttle. it wont idle

[FourtyOne]

Can the issue be anything outside of the carbs? He said he rebuilt them and double checked them when it still wasnt holding an idle. He said it has to be something outside of the carbs

[FourtyOne]

I just noticed the wires were meted going to the voltage rectifier were extremely melted.. could this be the issue? or is that an issue itself?

[Re-run]

Well melted wires are certainly not good. If wires got burned up, it could be possible that the ignition is not getting the power required.

At what rpm will the bike stay running?

[FourtyOne]

Well, since it wont stay running while idle, id have to guess the rpms to keep it running would be around 2500+. id be able to answer you better if i could get it to turn over. i was trying to figure out why it stopped turning over when i found the wires all melted.

[LukeM]

Quick question regarding the wires: are they 3 yellow wires? If so, this is a common problem with older Honda charging systems.  The 3 yellow wires are the coils coming off the alternator going to the rectifier and regulator.  There is a connector between the two which will degrade with time, increase resistance, and eventually fail.  The connector can be bypassed by soldering the wires together, with some shrink wrap to protect them from shorting or the elements.

If not, please disregard.  I agree: burnt wires are a bad thing, usually a sign of excess resistance.

Luke M

[FourtyOne]

Luke - yes those are the wires. could those wires be the problem with it not being able to stay running when idle? i plan to bypass it all that way. I just was wondering if it could be the root cause of my issue. thanks :)

[Lucky 1]

If every thing is right about the carbs and electrical then the one thing that can cause it to not idle is a tight valve. A valve that is not adjusted right, a tight valve will make it not be able to idle.

But 9 out of 10 times it is a clogged idle jet .
Does it hesitate or pop or backfire when trying to rev up the engine?