Fuel flow issue cb400 nc31

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Brussel
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Fuel flow issue cb400 nc31

Post by Brussel » Tue Jan 05, 2021 12:37 pm

Hi All,

I have a '92 NC31 with a fuel starvation issue.

It's definitely not the tank or the petcock as far as I can see. the tank vent is good and fuel flow through the petcock is fine with a small vacuum on the petcock. If I run the tank close to empty the fuel starvation issue is much worse. I need to keep 3 or 4l in the tank or I run into issues at speed on the freeway with a misfire eventually coming to stop and then eventually can get the bike started again. If I keep on the gas I run into issue a few miles later.

The issue also seems to be the right hand two carbs but there is no issue that I can find with the fuel lines running to the carbs and I've opened the carbs and checked flow using water and all seems good. There is nothing obvious, but I'm new to motorbikes and carbs and I'm needing some advice on what else I can check.

ideas?

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CMSMJ1
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Re: Fuel flow issue cb400 nc31

Post by CMSMJ1 » Tue Jan 05, 2021 4:06 pm

If when on low tank - that leads you to shit in the tank or filters
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cbr400rrn
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Re: Fuel flow issue cb400 nc31

Post by cbr400rrn » Tue Jan 05, 2021 9:37 pm

Brussel wrote:
Tue Jan 05, 2021 12:37 pm
Hi All,

I have a '92 NC31 with a fuel starvation issue.

It's definitely not the tank or the petcock as far as I can see. the tank vent is good and fuel flow through the petcock is fine with a small vacuum on the petcock. If I run the tank close to empty the fuel starvation issue is much worse. I need to keep 3 or 4l in the tank or I run into issues at speed on the freeway with a misfire eventually coming to stop and then eventually can get the bike started again. If I keep on the gas I run into issue a few miles later.
Are you sure the tank is venting? When the bike dies next time open the fuel cap (obviously with the ignition off) and see if you get a whoosh of air entering the tank.

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Re: Fuel flow issue cb400 nc31

Post by Brussel » Wed Jan 06, 2021 5:59 am

CMSMJ1 wrote:
Tue Jan 05, 2021 4:06 pm
If when on low tank - that leads you to shit in the tank or filters
The bike never had a filter. I put one in for several months. issue remains and there is no buildup in the filter and the tank is all good too.

Brussel
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Re: Fuel flow issue cb400 nc31

Post by Brussel » Wed Jan 06, 2021 6:01 am

cbr400rrn wrote:
Tue Jan 05, 2021 9:37 pm
Are you sure the tank is venting? When the bike dies next time open the fuel cap (obviously with the ignition off) and see if you get a whoosh of air entering the tank.
Yip I can blow air freely through the vent. And when the bike has died I have opened the tank and no woosh as you describe indicating a vacuum on the tank.

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Re: Fuel flow issue cb400 nc31

Post by CMSMJ1 » Wed Jan 06, 2021 10:24 am

OK :)

So, how's about the vacuum hose for the pump being split - and once the bike is hot and the fuel draw is too much then the tap cant flow enough as the vacuum hose is not 100% working?

(caveat being - I have no idea if these bikes have a vacuum tap or whether they ave a pump..)
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Re: Fuel flow issue cb400 nc31

Post by Superjack44 » Wed Jan 06, 2021 8:33 pm

Have you ever had the petrol cock off the tank ,there is a filter over the petrol pipe, the reserve is a small hole at the base of the main feed pipe on the petrol cock .
There are two rubber pipes fitted to the petrol tank 1 large 1 small the large (this is not the breather pipe) one is to drain water /spilled fuel from the filler cap recess, the smaller is the vent pipe this is fitted to underneath the petrol cap accessed by removing three allen bolts from the petrol cap located 12oclock 8 oclock and 4 oclock ,l cleared mine by pushing a length of bowden cable down and spinning it with a hand drill pushing at the same time.


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