CB500 carbs leaking fuel from breather

[Kermitsbog]

Hello again,

2000 Honda CB500 - rebuilt the carbs with new float needles etc. Original floats, they are plastic but float in water fine. They are also pre-set so their height can't be adjusted.

Installed onto the bike. Wanted to test it runs ok before tank goes back on etc so rigged up a fuel can on the seat, got a syphon going and then connected the pipe to the fuel filter/line.

Shortly after fuel began leaking out of the carb breather so I clamped off the pipe.

Engine starts and runs ok but as soon as I undo the clamp it slowly begins to leak from the breather.

I've read it might be the floats and/or float valves.

However I know the floats are delicate little things so wondered if how I've rigged this up us creating too much head pressure, pushing fuel past the float valves? Is that even possible or have I just made that up.

I'm trying to avoid taking the carbs off again if I can help it since they are a bastard to get on and off

[stinkwheel]

Could be stuck or leaking needle valves.

Try tapping the carbs with a bit of wood, that can settle stuck ones down.

They could be leaking though. They could have been leaking before you stripped it but the vacuum fuel tap prevents them overflowing when the engine is off. It's a common issue on Japanese bikes.

If they keep leaking, I'd have a close look at the needle valve seats with a magnifying glass.

Incidentally, a CB500 is one of the easiest sets of carbs I've ever worked on. You do know the whole airbox pivots back off them complete with rubbers? As I recall you need to remove the battery and undo one bolt, loosen the airbox rubber clamps and pivot the whole airbox backwards off the carbs. Hold it there with a bungee.

You don't need to remove them entirely to remove the float/needle valves, dismount them from the rubbers and turn them upside down and you'll be able to remove the float bowl without disturbing the cables.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Kermitsbog]

Ah thanks that's interesting, I'll give that a go.

I can't say I looked in the needle valve holes, just blasted them with carb cleaner, maybe they are a little dirty still.

It's not the getting them out that's the pain so much, it's pushing them into the rubber inlet seals after. The clamps were loosened right off but I had to wiggle them and use a lot of force to get them back on. Maybe I should warm them up with some hot water or something?

[-]

When you replaced the float needles, where did you get them from?

I bought some from a well known parts place, fitted them and couldn't stop the carbs from pissing fuel. I took them out and compared them to the existing ones. There was a couple of mm difference in size, enough to stop them from sealing. I photo'd them and they just refunded me.

In the end I bought 2 sets of matched new float needle jets and cups from Allen's Performance. Not cheap, but the old float cups were worn too. You could see a ring around the top of the float needle and a matching one on the cup.

Do you have an inline fuel filter? I' d fit one between tank and carbs to stop any rust or dirt from the carb blocking the float needles in the future.

Hot water on the rubber makes a huge difference in putting back together. The only way my carb rubbers and airbox rubbers were going back on when I rebuilt my YZF750 a couple of weeks ago, was by soaking them in hot water first.
____________________
TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[Kermitsbog]

I bought a float bowl gasket set off eBay for the carbs that came with 2 float needles, £18 each so not the cheapest.

The bikes only done 16k miles so I would hope they are not too worn after that.

Sounds like I need to have a good look at them, good job I kept the original ones. I've got a micrometer so I'll compare them.

[-]

BTW, the float height is normally adjusted by slightly bending the metal bracket they're fixed to. Turn the carbs upside down and measure the height they sit at above the cab wall when upside down. Check with the manual, but probably somewhere around 15mm.

If they're not punctured and sinking though, this is rarely necessary.
____________________
TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[stinkwheel]

I heat up carb rubbers with a hairdrier. A smear of red rubber grease round the lip of the rubber makes life much easier too.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Kermitsbog]