Fuel ‘disappearing’ from carbs.

[sharpeyie]

Hi all.

Really strange one here. Trying to setup the carbs on Z1B. They were setup on the bench as per service manual. The float levels were checked and tweaked until they were as they should be. Trying to balance them whilst on the bike this week with two funnels feeding fuel, noticed that cylinders 3 & 4 fuel level was dropping in in the funnel whilst the engine wasn’t running, however nothing coming from carb overflows. None of those present (some much nore experienced than I) could figure WTF.....

Where is this fuel going and how? Answers on a postcard

[zed1015]

It doesn't just disappear so its got to be going somewhere.
If it isn't overflowing out it's got to be going in the cylinders.
Unless that carbs 3 and 4 were taking longer to fill due to a partially stuck float needle etc which gave the impression of taking more fuel than they actually were.
Not really any other options than those.

[moizeau]

Check your oil level and also the airbox for a petrol smell, hopefully it was just a float valve being slow as Rob said.

[sharpeyie]

It doesn't just disappear so its got to be going somewhere.
If it isn't overflowing out it's got to be going in the cylinders.
Unless that carbs 3 and 4 were taking longer to fill due to a partially stuck float needle etc which gave the impression of taking more fuel than they actually were.
Not really any other options than those.

Yeah, it has to be a float issue. Thought I had missed something else.... thanks.

[sharpeyie]

Check your oil level and also the airbox for a petrol smell, hopefully it was just a float valve being slow as Rob said.

What are your thoughts regarding oil level?

[zed1015]

Check your oil level and also the airbox for a petrol smell, hopefully it was just a float valve being slow as Rob said.

What are your thoughts regarding oil level?

In severe cases overflowing carbs will pour fuel into the cylinders , past the rings and into the engine which increases the level in the sump..
It's a common occurence on bikes with vacuum taps that fail and drain the fuel tank when stood overnight.
A mates tap on his 600 SRAD went kaput at a rally with a full tank.
The next day the tank was empty , the engine was full and petrol was pouring out of the exhaust end can.

[sharpeyie]

Check your oil level and also the airbox for a petrol smell, hopefully it was just a float valve being slow as Rob said.

What are your thoughts regarding oil level?

In severe cases overflowing carbs will pour fuel into the cylinders , past the rings and into the engine which increases the level in the sump..
It's a common occurence on bikes with vacuum taps that fail and drain the fuel tank when stood overnight.
A mates tap on his 600 SRAD went kaput at a rally with a full tank.
The next day the tank was empty , the engine was full and petrol was pouring out of the exhaust end can.

Oh FFS. Right. Mmm that’s intersting ‘cos when I looked at it today the fuel had drained and it wasn’t sittingin the cylinder so may well have gone past the rings. I’ll keep an eye on that. Would the rings stop it if they were 100% - is this an indicator that rings probably past their sell-by date?

[zed1015]

Would the rings stop it if they were 100% - is this an indicator that rings probably past their sell-by date?

No! Fuel will get past even on a new engine.
It mainly seeps through the expansion gap in the rings.

[rpeters]

Someone may correct me, but its also possible that the fuel can drain back into the airbox and back to the gearbox through the little breather pipe from the airbox to crankcase breather. Sure i had that before on an ELR?

[zed1015]

Someone may correct me, but its also possible that the fuel can drain back into the airbox and back to the gearbox through the little breather pipe from the airbox to crankcase breather. Sure i had that before on an ELR?

Yes! That too.