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Author Topic: 78 CB750K Carb float weight  (Read 2758 times)

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Offline blparker

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78 CB750K Carb float weight
« on: August 18, 2015, 06:24:13 AM »
Hey all,
Does anyone have the weight of a brand new float for the carbs on this bike? Trying to determine if mine are to heavy and causing float level issues.

Thanks,
Ben
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Offline flybox1

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Re: 78 CB750K Carb float weight
« Reply #1 on: August 18, 2015, 07:09:33 AM »
It doesnt matter what they weigh.
If they are the black plastic, they cannot absorb fuel.
If you've put in aftermarket float valves and want to verify you have the correct float/ fuel level height, use the clear tube method off the bowl drain pipe.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline blparker

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Re: 78 CB750K Carb float weight
« Reply #2 on: August 18, 2015, 08:03:43 AM »
Quote from: flybox1 on August 18, 2015, 07:09:33 AM
It doesnt matter what they weigh.
If they are the black plastic, they cannot absorb fuel.
If you've put in aftermarket float valves and want to verify you have the correct float/ fuel level height, use the clear tube method off the bowl drain pipe.


That spec will work ok for the 78 k? Thanks for the illustration. I am getting sooty plugs on all four cylinders and nice black smoke. I set my float height at 14.5mm with the float barely touching the spring loaded needle. I am reading elsewhere to go 12.5mm which in my head would make the mixture even more rich. Another thing I am looking into is spark quality. I measured the caps resistance last night and found one around 10k ohms and another that wouldn't read at all. New caps are coming just as a piece of mind. Fuel screws are one turn out and I am running stock airbox with stock filter.
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Offline blparker

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Re: 78 CB750K Carb float weight
« Reply #3 on: August 18, 2015, 08:04:14 AM »
Also, I am not sure if it will matter, but I am running 78k carbs on a 76f motor.
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Offline flybox1

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Re: 78 CB750K Carb float weight
« Reply #4 on: August 18, 2015, 08:26:27 AM »
Yes, this applies.
im running a 78K carbs on a 78F engine.  does not matter.
get the fuel level at 3-4mm below the top of the bowl as per the diagram.
you should have #35 pilot jets and #110 main jets.
« Last Edit: August 18, 2015, 08:28:47 AM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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"Knowledge without mileage equals bullsh!t" - Henry Rollins

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Offline MCRider

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Re: 78 CB750K Carb float weight
« Reply #5 on: August 18, 2015, 08:45:31 AM »
"I am reading elsewhere to go 12.5mm which in my head would make the mixture even more rich."

The float level does not make it rich or lean, the jets do that. You want enough fuel in the bwl that it doesn't starve or flood.

OCICBW
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Offline flybox1

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Re: 78 CB750K Carb float weight
« Reply #6 on: August 18, 2015, 09:04:04 AM »
Yeah, id have to disagree, MC.
If got nothing but plug color and a butt dyno to support this though  ;D
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline MCRider

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Re: 78 CB750K Carb float weight
« Reply #7 on: August 18, 2015, 09:19:45 AM »
Quote from: flybox1 on August 18, 2015, 09:04:04 AM
Yeah, id have to disagree, MC.
If got nothing but plug color and a butt dyno to support this though  ;D

You are probably right, per this explanation (last paragraph esp.).

Too high, it runs rich. Too low, lean. But not in a controlled way as though the jets were too big or too small.

http://www.fix.com/blog/motorcycle-carburetor-float-height-adjustment/
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Offline 754

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Re: 78 CB750K Carb float weight
« Reply #8 on: August 18, 2015, 09:46:35 AM »
If it doesn't. Stop supply fuel when it should it starts pouring or sucking in to the venturi..mine is doing that.
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Offline blparker

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Re: 78 CB750K Carb float weight
« Reply #9 on: August 18, 2015, 11:52:29 AM »
That was my assumption. I understand pretty well the inner workings of these carbs. I am concerned that someone has been into the pilot jets and drilled them out for some damn pod filters since I bought the carbs off of ebay. I am going to assume I cant get replacement press in pilot jets either? I wonder how I could ensure this isn't the case? I am a Sales Engineer for a non-contact laser measurement company, if I had a hole diameter spec, I could scan them to see what they are. I will do the clear tube method tonight to see where I am. I have been exhausting the search feature and have been reading a lot of advice. Thanks for all of the replies.
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Offline flybox1

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Re: 78 CB750K Carb float weight
« Reply #10 on: August 18, 2015, 12:05:27 PM »
« Last Edit: August 18, 2015, 12:08:08 PM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline flybox1

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Re: 78 CB750K Carb float weight
« Reply #11 on: August 18, 2015, 12:06:24 PM »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline blparker

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Re: 78 CB750K Carb float weight
« Reply #12 on: August 18, 2015, 12:26:03 PM »
Quote from: flybox1 on August 18, 2015, 12:06:24 PM


I have been told over the years that jet size #'s are not the same hole size for each manufacture. Example being a #42 pilot from Mikuni may be a different ID than a #42 Keihn. Does this table apply to our keyhole carbs? Thanks for the table by the way!
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Offline flybox1

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Re: 78 CB750K Carb float weight
« Reply #13 on: August 18, 2015, 12:30:17 PM »
Just buy OEM/Keihin, and leave the worries behind.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline blparker

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Re: 78 CB750K Carb float weight
« Reply #14 on: August 18, 2015, 12:55:08 PM »
Quote from: flybox1 on August 18, 2015, 12:30:17 PM
Just buy OEM/Keihin, and leave the worries behind.

I completely agree. I just wanted to make sure the numbers on the chart were the corresponding sizes for our jets before I measure them.
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Offline flybox1

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Re: 78 CB750K Carb float weight
« Reply #15 on: August 18, 2015, 01:03:09 PM »
a #110 jet should measure 1.10mm.  #120 - 1.20mm, etc.
those numbers are not the drill size used.  they are the hole dia.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline blparker

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Re: 78 CB750K Carb float weight
« Reply #16 on: August 18, 2015, 01:15:31 PM »
Quote from: flybox1 on August 18, 2015, 01:03:09 PM
a #110 jet should measure 1.10mm.  #120 - 1.20mm, etc.
those numbers are not the drill size used.  they are the hole dia.

Thanks.
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