CB500 fuel problem

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Got a new to me 1994 CB500.
Riding to work the other week and fuel ran out.
Fair enough, I wasn't sure how much in before bought.
Put on reserve and went to petrol station.

Filled up, about £10 worth so say 8 litres give or take.
Reset odometer to check mileage when happened again.

Today it did, after about 90 miles
That's just wrong.

Filled up with 7 litres, say 1.5 imperial gallons.
A combination of Wikipedia and Haynes tells me tank capacity 18 litres and tank range about 196 miles.

So I'm thinking fuel tap blocked?
Just wanted to run it past others for a sanity check.

I guess internal tank could be damaged restricting capacity.
Other though was carb icing, but that does not explain starting on reserve straight away.

Any other input appreciated

P.S.
Working out consumption for 18 litre tank = 3.96 gallons for tank range 196 miles is about 49.5 mpg
So my 7 litres is about 1.5 gallons for 85 miles is about 56mpg. So that's right. So does this point to an issue with tank and fuel tap?
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[YBR Ric]

Fuel consuption seems OK for a 1994 model https://www.fuelly.com/motorcycle/honda/cb500

"So I'm thinking fuel tap blocked?
Just wanted to run it past others for a sanity check."

Yup, most likely the reserve outlet on the fuel tap is blocked.

[stinkwheel]

With the exception of a couple of other left of field suggestions I suspect you're right.

Might be that the vacuum has become detached or damaged so it's not opening the tap fully in which case it's cutting out when the head of perssure in the fuel isn't enough to force its way past.

Also consider the possability of a blocked tank breather.

Maybe have a look inside with a torch first to check the tank isn't totally mangled and 50% filler.

In any case, whipping the tank and fuel tap off and having a look should tell you what you need to know.

I usually take the whole tank off and lay it in an old tyre, tipped over far enough so the fuel has sloshed away from the tap. Quicker and easier than draining the tank and removing the tap in-situ and allows you to inspect the hoses properly too.Seems to work well with anything less than 2/3 full. Any fuller and you get fuel leaking out of the cap.
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[davebike]

If you fill a CB500 on the side stand it will only get to about 3/4 full
If filling on the main stand and to proper full you may have lots of muck /water in the tank or major filled dents but but that would be unusual

Dave
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[pdg]

Could be a blockage, could also be someone fitted a tap from a different model with a longer 'reserve tube' affair. Does the tank look half full when you need to switch to reserve?

My (completely different to yours) bike's fuel light comes on anywhere between 85 and 130 miles depending on what angle I park it at and what mood it's in - judging by the amount of fuel it takes to fill from that point it's about half full....

[Robby]

I recall my old CB500 having quite a big reserve, I was switching onto it after 120 miles.

An aftermarket fuel tap with the main intake a few mil higher than standard would explain the problem. Have a look- if it has a standard manual fuel tap instead of a vacuum affair, this could be your problem. A standard aftermarket fuel tap costs about the same as a repair kit for a vacuum tap, so I always go for a normal tap.

If you are checking the tank breather for blockages, don't test it right after you've had the tank upside down to check the tap - you'll have fuel in the breather, which will make it seem blocked until it evaporates or drains off.

I seem to remember the fuel tank being a bit of an odd shape, so to really brim it requires filling the last couple of litres quite slowly.

[Vincent]

Does the tap have a plastic pencil like plastic filter inside the tank? A bit of a shot in the dark, as I'd have thought they all work the same but could someone have replaced it with the wrong one....thus changing where reserve starts?
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