Carb overflow

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Gacataki

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• Dec 19, 2022

• #1

Hi all, please help. I took delivery today of a 1980 CB750 Custom. The bike looks good, but I have one big problem. I open the fuel valve on the tank and everything is fine. As soon as I start cranking, the fuel literally pours out the back of all four carbs. I managed to remove all the bowls (on the bike), and if I hold all the floats up, I cannot blow into the fuel pipe. I put it all back together and it still does this. But only when I start cranking. The bike stood for 3 months. Please, I will struggle to believe that all 4 needle and seats packed up at exactly the same time.

 

BAHNST

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• Dec 19, 2022

• #2

Long shot.. drain screws on carbs loose??
Perished o-rings on the drains?

 

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Gacataki

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• Dec 20, 2022

• #3

Thanks I'll check. But could that cause fuel to run out the back of the carbs. Surely it would just drain straight through? Thanks for the help.

 

BAHNST

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• Dec 20, 2022

• #4

I misinterpreted it....

Yes floats are the primary suspect

 

Schooney

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• Friday at 7:01 PM

• #5

Polish the inside of the needle seats. Didn't help me, but it might work in your case.

 

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brettp

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• Friday at 8:06 PM

• #6

When you push the float up, your forcing the seal. Gasoline lifting them won't have that same force. Your float valves might need to be replaced.

 

loxley007

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• Saturday at 3:02 PM

• #7

I had that. Turned out the previous owner had put it eBay type floats (fully plastic, not adjustable). I replaced them with adjustable (plastic and metal) OEM substitutes from David Silver spares and that sorted it. That was on a 79.

 

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brettp

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• Saturday at 5:06 PM

• #8

I had good luck with floats from 4 into 1.

 

Schooney

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• Yesterday at 5:47 AM

• #9

loxley007 said:

I had that. Turned out the previous owner had put it eBay type floats (fully plastic, not adjustable). I replaced them with adjustable (plastic and metal) OEM substitutes from David Silver spares and that sorted it. That was on a 79.

Click to expand...

Adjustable floats seem more reliable. But, if the seat is bad, it wont matter...seems that a float just floats, should control the flow of fuel.

 

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brettp

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• Yesterday at 6:38 AM

• #10

Floats do just float. But if the tab holding the valve is off a bit, it can cause problems with not putting equal pressure on the valve into the seat.

 

mrtwowheel

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• Yesterday at 8:14 AM

• #11

Also we have the inherent fuel flow of these bikes. I think it's less than a 2 inch drop from petcock to fuel inlet on the carbs, that's not much fuel pressure pushing up on those floats. I always thought these bikes would benefit from an electric fuel pump, I'm soon gonna find out.