Author Topic: CB750K Carb Problem  (Read 1940 times)

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cubbie85

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CB750K Carb Problem
« on: June 04, 2009, 01:10:13 PM »
Hello everyone!

I have been going through previous posts and can not find a specific answer to my current problem. I have a 1977 CB750K which i have just put the carbs back on after cleaning. The problem I am having is the the bike will only run on full choke and dies immediately after I take the choke off. I also have fuel coming out of the over flow tubes under the bike. Same problem I was having before I removed the carbs. I heard it could be a float needle problem, but wanted to see what everyone else thought.

Offline Inkscars

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Re: CB750K Carb Problem
« Reply #1 on: June 04, 2009, 01:12:30 PM »
I have the same bike as you
Slow idle jets are sticking.
as for the leak. Our floats should be set between 12mm and 14mm or else they will leak.I set mine at 14mm.

I should note that I do not KNOW that these are the causes of your problems, only that I had the exact same problems and that is what fixed mine.  ;D
« Last Edit: June 04, 2009, 01:37:57 PM by Inkscars »
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MrHellpop

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Re: CB750K Carb Problem
« Reply #2 on: June 04, 2009, 02:57:35 PM »
I too have the same problem of the bike dying when the choke was not fully opened. I just took the carbs off and went through the cleaning process, or at least what I could figure out how to do since the K7/8's carbs are very different than the K0-K6s. The floats are way more than likely the cause of the fuel overflow as the previous poster said. My #1 float sticks occasionally even though I have cleaned it and it looks and acts exactly like the others. When that happens I tap the float bowl lightly with a wrench a few times and it seems to come unstuck. Since I have yet to take my bike for a ride since I got it, I have no idea if this problem will come up when riding around town. I hope not!!!!

I also opened the airbox and found a partially disintegrated foam filter which I am sure was contributing to the problem. Have you checked your air filter?


Offline JS550

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Re: CB750K Carb Problem
« Reply #3 on: June 04, 2009, 03:00:15 PM »
Tear carbs back off, pull the jets & clean them out real good. My 75 750 did the same thing.
If you're worried about how many calories are in beer, is losing weight really your biggest problem?

MrHellpop

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Re: CB750K Carb Problem
« Reply #4 on: June 04, 2009, 03:02:26 PM »
I have the same bike as you
Slow idle jets are sticking.
as for the leak. Our floats should be set between 12mm and 14mm or else they will leak.I set mine at 14mm.

I should note that I do not KNOW that these are the causes of your problems, only that I had the exact same problems and that is what fixed mine.  ;D

Inkscars, how do you KNOW the slow idle jets are sticking? Maybe that's what's happening to me! I am having all sorts of issues figuring out which parts are what, since I can't find too many pics that have labels describing what these things actually are. Then again, I'm pretty stupid. Are those one of the two little cylanders in each carb throat?

Offline Inkscars

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Re: CB750K Carb Problem
« Reply #5 on: June 04, 2009, 03:06:38 PM »
Hellpop
Twotired is one of the few complete gurus on here.
He more or less walked me through my issue.
Let me find my picture again... lmao
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Offline my78k

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Re: CB750K Carb Problem
« Reply #6 on: June 04, 2009, 03:10:13 PM »
Running on full choke tends to be indicative of plugged idle jets. Yes they are pressed in and yes they will pull out with vise grips and a gentle twist and pull. They must come out to get them clean. Dont' just spray some carb cleaner in there....soak the jets first and then use some copper wire and make sure it will pass through and use compressed air to get all the little holes clear.

As for the over flowing....could be a couple of things. Improperly set floats, crud in the tank that is blocking the floats from closing or potentially cracked over flow tubes *the least likely but still potentially the problem...oh also double check the drain screws are tight (sounds dumb but it can happen).

Dennis
« Last Edit: June 04, 2009, 03:11:46 PM by my78k »

Offline Inkscars

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Re: CB750K Carb Problem
« Reply #7 on: June 04, 2009, 03:16:18 PM »
Ok. On ours and 78'S
I BELIEVE
we have a slow idle jet/circut that it pressed into the body of the carb.
If you boys were anything like me, you look right past it when pulling jets and such to clean.
it looks as if it is a piece of the body. It is in BETWEEN the main jet and the air mix screw. This is the best picture I can find.

I carefully i emphasize CAREFULLY because people tend to clamp too hard. It's my understanding that this piece is brass and can be pinched shut all too easily.
I used a small pair or vice grips and patience. after taking the #1 float and needle out, I realized if you're careful enough, You can just depress the float to get a hold of it.
If you haven't already went through the carbs, take them out and clean everything.
I clamped, gave a 1/8 turn either direction and pulled upward. It comes out easily enough.
Clean, clean clean. There was 8 holes i believe in mine and out of all 32 on the slow jet circuts there was only 2 clogged. I cleaned and reassembled. Take this time to adjust your floats. According to manuals they should be between 12mm and 14.5 mm. I set mine at 14 and have experienced no drippage since then.
When you go to put the jets back in, they don't take alot of pressure and make sure to make it level, don't just jam it in there.
Hope this helps.
« Last Edit: June 04, 2009, 03:19:36 PM by Inkscars »
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MrHellpop

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Re: CB750K Carb Problem
« Reply #8 on: June 04, 2009, 03:24:32 PM »
Running on full choke tends to be indicative of plugged idle jets. Yes they are pressed in and yes they will pull out with vise grips and a gentle twist and pull. They must come out to get them clean. Dont' just spray some carb cleaner in there....soak the jets first and then use some copper wire and make sure it will pass through and use compressed air to get all the little holes clear.

As for the over flowing....could be a couple of things. Improperly set floats, crud in the tank that is blocking the floats from closing or potentially cracked over flow tubes *the least likely but still potentially the problem...oh also double check the drain screws are tight (sounds dumb but it can happen).

Dennis

Dennis! This is exactly what I am talking about! An illustration with NO labeling of the parts!!!! ha ha ha not to be a dick, but I have no idea which parts are which without being shown which parts are which!  ??? I'll figure it out eventually  ;)

upperlake04

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Re: CB750K Carb Problem
« Reply #9 on: June 04, 2009, 03:58:16 PM »


Offline my78k

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Re: CB750K Carb Problem
« Reply #10 on: June 04, 2009, 05:03:53 PM »
SHOW OFF!!!! Just kidding..nice pic UL

Dennis

Offline Inkscars

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Re: CB750K Carb Problem
« Reply #11 on: June 04, 2009, 07:41:43 PM »
Damn you and your awesome photo skillllz.
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Offline jx25

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Re: CB750K Carb Problem
« Reply #12 on: June 04, 2009, 10:16:05 PM »
those pics are perfect!! ;D
1978 CB750K (slightly modified)
1973 Suzuki GT250
1968 Ducati Monza 250(needs title)
1965 Bultaco 250(needs clutch work)

MrHellpop

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Re: CB750K Carb Problem
« Reply #13 on: June 05, 2009, 11:13:46 AM »
those pics are perfect!! ;D

Agreed! Somebody should add this to the carb faq!

Offline Colonel

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Re: CB750K Carb Problem
« Reply #14 on: June 05, 2009, 11:32:44 AM »

Offline Johnny5

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Re: CB750K Carb Problem
« Reply #15 on: June 05, 2009, 11:41:01 AM »
You didnt put your floats in upside down?
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cubbie85

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Re: CB750K Carb Problem
« Reply #16 on: June 05, 2009, 12:41:56 PM »
No, I know that the floats are not upside down. Sounds like I have some work ahead of me this weekend!

MrHellpop

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Re: CB750K Carb Problem
« Reply #17 on: June 05, 2009, 02:05:07 PM »
I managed to get the pressed in slow jets out pretty easily... They seemed pretty crusted in and wouldn't budge, so I put ONE DROP of CLR around the edge of where the jet meets the body. After about a 5 minute wait, they came out with very little effort! Might try that if you have a hard time getting those suckers out. By the way, 3 out of 4 of mine were clogged. Can't wait to get my new air filter in the mail and fire it up!!!

Offline Inkscars

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Re: CB750K Carb Problem
« Reply #18 on: June 05, 2009, 02:08:16 PM »
Congrats!
One down!  :D
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cubbie85

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Re: CB750K Carb Problem
« Reply #19 on: June 13, 2009, 05:55:54 PM »
I pulled all of the slow jets and let them sit for over an hour in carb cleaner. I have a steel guitar sting that I have been trying to poke through each jet when it is pulled out, but with no success. Any advice on cleaning these out?

Offline my78k

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Re: CB750K Carb Problem
« Reply #20 on: June 15, 2009, 09:52:49 AM »
Compressed air usually works. I don't do the guitar string method as I don't have a guitar but I do use some thin guage copper wire. Any chance the string is too thick?

Dennis