will not stay running or idle

[notbydesign]

I just bought a 1980 honda Cb 750k. The bike revs all the way up to 10000rpms but it will not idle or stay running. I've cleaned the carbs three times, bought new spark plugs, and changed the choke cable. Any advice that's anyone can give me would be greatly appreciated.

[sgtslag]

Start with the simple, and work towards the complex.

Have you verified the choke is working?  Without choke, a cold engine won't idle.  My '79 K was rather cold-blooded, requiring choke for a few minutes.

We know you have a working electrical system (it starts, it rev's), we know you have fuel reaching the cylinders (it starts, it rev's).

Known good gasoline in the tank?  Not old, like from last riding season, or even older?

Is there a working, known good fuel filter in place?  Crud from the tank could make its way down to the floats, or the carb jets, so a filter is always a good idea.  Many of the OEM wire mesh filters ended up being removed from the petcock, leaving no filter in the system.

Can you give more details on what it does do?  Will it remain running as long as you hold the throttle open a bit?  Careful not to rev too high/too long without a load on the engine; keep it below 4,000 RPM to be safe.

Do you have proper weight oil in the engine?  Oil of 10W-40 weight is good, but a 50 weight will make it more difficult to turn over due to the high viscosity -- very doubtful this is the real problem, but it's always worth checking the oil (amount, quality, and filter's condition).

Let us know more details, and we will be happy to offer suggestions.  By the way, since you are new here, be sure to download the FSM (Factory Service Manual) from the home page.  Cheers!

[TOOLS1]

Could be the pilot jets are still plugged, or a vacuum leak.
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[notbydesign]

It will stay running with the throttle open. I think the filter was removed from it. The choke does work but I have noticed that the 2 choke floppy things on the left are a little tighter than the ones on the right. The gas and the oil are both fine. Like your bike it does require the choke to start. Does the bike need the air box on to work?

[TOOLS1]

No it does not need the airbox to run. When you cleaned the carbs, did you remove the pilot jets? You must remove them to clean them. There is no way to do it without removing them. Are your rubber boots between the carb, and head in good shape, or are they hard, and maybe cracked? That would cause a vacuum leak that will not let the bike run at idle.
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[notbydesign]

I haven't taken them out yet so I'll do that for sure. The rubber has been there since 86 and it feels hard. What would I call them off I went to a shop?

[TOOLS1]

Carb holders, or intake boots. Here is a link to vintage CB 750. http://www.vintagecb750.com/products/6/fuel-system/54/carb-holders-clamps I would suggest new clamps also if you want to replace them. However you might be able to recondition your old ones by boiling them in a solution of 1qt water to 10oz wintergreen oil.
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[Re-run]

If you hold the throttle at, say, 2000 rpm, does it stay running? Or does it die?
How low can you go before it dies?
Or does it only die if you totally let go of the throttle?