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Author Topic: Cleaning Keihin carbs as a bank  (Read 2174 times)

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Offline cb750fbomb

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Cleaning Keihin carbs as a bank
« on: February 21, 2009, 12:15:24 AM »
Any of you '77-78 F owners, or any year/model for that matter, ever clean these as a bank and not completely apart?
Also, any suggestions from F owners (77-78?) for jetting for pods? I don't think when the pods were installed, by the PO, that it was jetted accordingly.


I know that this is something that's tackled on a bike-by-bike basis and that my '77 F could act diff. from the next, but a ballpark would be great.

Stock is 105. Go up to 110 in a jet and raise the needle clip up 1 notch maybe?

Year Model Casting # Type Float   Main   Slow JN Screw
77   CB750F PD41B       2    14.5    105    35    2     1
« Last Edit: February 21, 2009, 02:46:48 AM by cb750fbomb »
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Re: Cleaning Keihin carbs as a bank
« Reply #1 on: February 21, 2009, 04:05:37 AM »
I never unstack the rack unless I find a broken/nonrepairable body.  I have had to unstack maybe twice in 160+ carb cleanings.  Your jetting estimate seems reasonable.  Remember, jets control idle and full throttle.  The midrange and transitions are all on the needle.  When tuned correctly, those F models are a blast to ride.
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Offline Spanner 1

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Re: Cleaning Keihin carbs as a bank
« Reply #2 on: February 21, 2009, 04:40:57 AM »
Look at the big carb. brain on you, Bomb....jets,needles.. how much of this did you know  1 month ago ??
Ain't it great ?  Ya gotta love this forum !
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Offline cb750fbomb

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Re: Cleaning Keihin carbs as a bank
« Reply #3 on: February 21, 2009, 04:50:15 AM »
I didn't know a thing and I still don't!!  :) I'm mildly terrified of getting this back together and it being worse. It sounds like it should go something like this:
Remove carbs, adjust, install carbs, remove carbs, adjust, install carbs, test, remove carbs, adjust, install carbs, test, remove carbs, adjust, install carbs, test.............
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Offline cb750fbomb

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Re: Cleaning Keihin carbs as a bank
« Reply #4 on: February 21, 2009, 05:23:51 AM »
Any other changes? Do I need to change the slow jet size or anything else when the main jet and the needle position is changed?
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Offline KB02

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Re: Cleaning Keihin carbs as a bank
« Reply #5 on: February 21, 2009, 06:03:25 AM »
Quote from: cb750fbomb on February 21, 2009, 04:50:15 AM
It sounds like it should go something like this:
Remove carbs, adjust, install carbs, remove carbs, adjust, install carbs, test, remove carbs, adjust, install carbs, test, remove carbs, adjust, install carbs, test.............

That sounds about right.  ;D ;D ;D ;D ;D
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Offline TheHun

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Re: Cleaning Keihin carbs as a bank
« Reply #6 on: February 21, 2009, 07:09:45 AM »
how do you raise the needle?
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Offline cb750fbomb

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Re: Cleaning Keihin carbs as a bank
« Reply #7 on: February 21, 2009, 07:28:18 AM »
Faintly circled in red, tried to do better with that part, are what look like venturis but only one has an air passage. The one on the left looks melted and the one on the right looks fine. How did this thing melt, what are these and how do I replace them?
« Last Edit: February 21, 2009, 07:49:28 AM by cb750fbomb »
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Offline cb750fbomb

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Re: Cleaning Keihin carbs as a bank
« Reply #8 on: February 21, 2009, 07:45:32 AM »
BTW, the main jets are already 120's. Hopefully she'll be running well here soon with clean carbs, properly oiled pods, new plugs, oil change....... I also had to get new push and pull throttle cables. The old ones were stretched out about 5 inches a piece.
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Offline scondon

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Re: Cleaning Keihin carbs as a bank
« Reply #9 on: February 21, 2009, 09:27:41 AM »
The piece circled in red is a stand pipe for the accelerator pump. When the throttle is twisted, the accel pump squirts gas through these pipes. I have no idea what could cause one to end up in such a condition.

    I would imagine they are press-in, much like the pilot jets. I'm not sure how well they would stand up to a pair of pliers though. Maybe test to see if it is functioning first. Fill bowls with gas and operate the throttle, gas should squirt out of all four.
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Offline fishhead

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Re: Cleaning Keihin carbs as a bank
« Reply #10 on: February 21, 2009, 09:28:23 AM »
Quote from: cb750fbomb on February 21, 2009, 07:28:18 AM
Faintly circled in red, tried to do better with that part, are what look like venturis but only one has an air passage. The one on the left looks melted and the one on the right looks fine. How did this thing melt, what are these and how do I replace them?

 Thats the accelerator pump nozzle. The one on the left looks broken. Thats where the fuel is squirted in to the carb bore.
  I ALWAYS dismantle the carb bank for cleaning. Especially accel pump models. They have very tiny holes in them and get clogged very easily. Dismantling them completely is the only way to "get it right" and to "get it right the first time".
   If crud is stuck in the fuel line connections (from the tank, accel pump lines,etc) and not removed with a thourough cleaning, it will end up in your tiny passages of the carbs and you will be removing them again for another "not to thourough" cleaning. Any crud that is stuck anywhere in the system will find its way to be an inconvienence.
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Offline Gordon

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Re: Cleaning Keihin carbs as a bank
« Reply #11 on: February 21, 2009, 09:50:51 AM »
Quote from: cb750fbomb on February 21, 2009, 04:50:15 AM
It sounds like it should go something like this:
Remove carbs, adjust, install carbs, remove carbs, adjust, install carbs, test, remove carbs, adjust, install carbs, test, remove carbs, adjust, install carbs, test.............

With most others it's like pulling teeth trying to get them to accept the fact that they need to pull their carbs and work on them, but you catch on quick!! ;D

You'll be much better off for it in the future.
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Offline Jim F

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Re: Cleaning Keihin carbs as a bank
« Reply #12 on: February 21, 2009, 12:47:10 PM »
When I installed pods and a header system using stock carbs.

I went 1 size larger on the pilot
raised the needle (lowered the clip) 1 place
and went 2 sizes up on the main and that seemed to be a good point to start checking
your plugs if you don't have access to a dyno and most people don't

good luck

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