Carb vacuum
 

I posted in another thread about finding fuel in front cylinder intake & it was hydrolocked.
I've read about the vacuum from the rear carb working the 'tap' on the tank.
Engine stopped or running, there is fuel-flow.
Can someone clarify to me that without the engine-created vacuum, it should not?


Apart from dirty inside (black 'bits') it was in decent condition. Diaphragm still sound, spring working, - but the o ring looks worn. Likely problem?

Re: Carb vacuum
 

When the engine is stopped the Vacuum operated Petcock should be OFF so no fuel should flow.

Re: Carb vacuum
 

Yep, thought so, the o ring in the valve had become cone shape. It's been replaced, & seems better, but with a full tank of fuel there's a leak......suspect it's the carbs. But if the vacuum's working, why are they getting fuel.....?

Re: Carb vacuum
 

I don't know if they are getting fuel but there is a level of fuel in the Float bowls which is controlled by the Carb Floats.

If you take the pipe off the Petcock does fuel flow.

This diagram is worth downloading as a reference,

https://i.imgur.com/CdvKng8.jpg

Re: Carb vacuum
 

Thanks for the diagram. Can you elaborate on "If you take the pipe off the Petcock does fuel flow.?"

Can you explain why when main pipe from tank is disconnected, the tank only empties at a drip? 2 litres taken hours.....




It so happens I have the same problem with Yam TRX850.....bought a jig to stand the carbs on to check the float heights. :smt010

Re: Carb vacuum
 

If you pull the fuel pipe,not the vacuum pipe, off the Fuel pump does fuel flow ? There should be none when the engine is stopped as there's no partial vacuum to open the Valve in the Petcock.You may have a faulty Petcock if there's been fuel leaking through.

You can buy service kits but....

I fitted a new cheap Petcock,

https://www.ebay.co.uk/itm/334067266...YAAOSw5HlhyYZh

However they some with a wrong larger spigot pipe so I use some larger diameter fuel tube and an in-line fuel filter to compensate,



https://i.imgur.com/OpqCM7n.jpg

Re: Carb vacuum
 

https://i.ebayimg.com/images/g/xegAA...-v/s-l1600.jpg


We got this. Not a great success....it leaks. I've nipped-up the four screws today & it's still seeping.
I mentioned above, fuel hose from tank disconnected....it only dribbles. ??

Re: Carb vacuum
 

Even if your fuel tap is leaking the float needles in your carbs should stop the carbs flooding. The rubber tips tend to go hard with age, especially if the bike has been unused for long periods of time. If you fix this issue I think the weeping fuel tap, whilst not ideal, would not be a great issue. When all fuel taps were manual, if you forgot to turn it to off, you only had a problem if the float needles were failing to make a good seal.

Re: Carb vacuum
 

As you posted this I was finishing reassembling the tank 'tap'. It was seeping with the new vacuum bits in it.



After taking the airbox off again last night, the front carb was flooded. What you've said above, could this be why the front floods but the rear isn't?
I don't know which part you're meaning though. Float needles....the long one visible below the plastic slide?

Re: Carb vacuum
 

This is what your Float Needle Valve/s and seat assembly look like,

https://cdn.shopify.com/s/files/1/00...g?v=1504835003

They come is a service kit like this,

https://www.ebay.co.uk/itm/403521485...IAAOSwR5diJLnL

The FNV sits in the middle of the floats attached by a small spring and metal pin.

You should still address the leaking Petcock though as it's not good to let it leak as it is. Where is it exactly leaking from ?

Also check the engine oil for fuel contamination.