'80 cb750k idles at 5-6000, getting 28mpg, air cleaner case leaking oil

[Steve '80 cb750k]

Hey everyone,
Let me start by saying I'm new to motorcycles and motorcycle mechanics. I just bought a 1980 CB 750 k and it idles between 5000-6000rpm. Is this normal? If not how do I adjust it? I removed the carb bank and cleaned the carbs but did not remove individual carbs from the bank. They were very clean, didn't really need cleaning but I figured I'd give it a shot.

Also, I'm getting 28 mpg on the highway. This seems really low to me, can anyone confirm? Could this be related to the fast idle? I replaced the plugs and checked the new plugs after 200 miles and they don't seem particularly fouled to me (rules out overly rich mixture?) but I could be off, not much experience in this area.

Finally, there is oil in my air cleaner case. I "think" it's coming from the emissions control system which runs through the top of the case? Has anyone had experience with this? Is it a problem? Should I just disconnect the input hoses from the top of the engine to the air cleaner case?

Thanks for any advice,

Steve

PS I'm in the Lansing MI area if there is anyone local looking to talk shop.

[swiffer]

I have the same bike.Idle should be 1000 rpms hot standing still.If all the carbs are all right and all you need to do is set the idle look under the the carbs and you should see a black knob.Just turn it until it idles down to 1000+-100th.Oil in the air cleaner case is not good .The EPA make honda reuse the gas emissions on these bikes so maybe might be coming from your crankcase breather hose.As always you need to get a repair manual first .I like the CLYMER .It will serve you well for many years.Also check out these two sites.http://www.dansmc.com/MC_repaircourse.htm
http://polymer52.webs.com/motorcyclediysinfo.htm
Great for getting started

[Steve '80 cb750k]

The oil is from the crankcase breather hose, but what do I do about this? Why is there oil coming through this hose?

[swiffer]

Old Hondas leak after a while.Most of the time it is no problem you just keep a eye on things.That being said it may be time for a re-build.New gaskets all around.That is what i think may be happening.Dont want you to think i am a mack though.I am not ,just seen some things.You really dont need the breather hose,but if you take it off it will effect the running a little because it is part of the air-gas combo ratio and you may need to do some adjusting .Also if you take it off you need to get another filter to go over the breather hose tube coming out of the crankcase{DO NOT BLOCK IT OFF}because it still has to breath.You can get a resperator filter that is just right {1/2"} at a auto store for $10.

[sjackson]

Did you ever figure out what was going on with the oil in your crankcase?

[Polymer-2]

Like Swiffer said the idle is likely adjusted too high.
I have an 80 750F and the last time I checked I got 39.2 mpg.

[OTTODOCTOR]

  Steve , you never said how many miles are on the bike , and like the other post said , idle should be around 1000 rpm . Oil in breather box is from blow-by , which is probably caused by compression leaking past the piston rings and overpressurizing the crankcase . This may lessen after you get the idle down to where it should be , but it sounds like a rebuild is in the near future . -- As far as gas mileage goes , my 81 custom gets 45 -47 m.p.g. -- Have seen 50 a couple of times too . -- Otto

[Polymer-2]

OTTODOCTOR wrote

 As far as gas mileage goes , my 81 custom gets 45 -47 m.p.g. -- Have seen 50 a couple of times too . -- Otto

If you hit 50mpg you drive too slow.
Sorry couldn't help myself.

[OTTODOCTOR]

     Hey , I can take it . And besides , I'm gettin old . Seen my share of road rash , and don't want anymore . However , we all know that it's not a question of if you fall , it's when , because if you ride , you fall , sooner or later .