fuel out vent tube in prime

[Steve]

All suggestions are good, so far, but I have a question of my own:

Fuel streams out the vent tube between carb one and two while petcock in "prime" and bike not running.

Why do you have the petcock in PRIme and the bike not running? :-k

Yeah, I see where it happens when in the RUN position, too, but at least, when the bike is not running, the petcock will shut off the flow of fuel.

As mentioned (repeatedly), you have an issue with the float valves not sealing. You said the needles looked okay, but the needles are not the only sealing surface. The seats also need to be immaculately clean and it is so hard to see in there to check them. I just got new valves and seats from Parts-n-More for $8 each, so it's not all that expensive to change them. (Note that Parts-n-More has a $35 minimum, so you will need to find something else to buy, too. I got a new front brake switch for $6. )

One other item to check: are you measuring your float height at the correct location? You do not measure from the top of the float, you measure to the bottom of the step. This picture is not quite right, but you should have a better idea where to measure:

An actual float being measured:

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[moose200]

what about the screw on the bottom? that happened to me, it was not tight enough and fuel started comeing out of the vent

 

[Steve]

what about the screw on the bottom? that happened to me, it was not tight enough and fuel started comeing out of the vent

Which "screw at the bottom"?

I have to ask, because depending on what bike you have, there is only one screw or two screws.

On the '80 and newer bikes with CV carbs, the only screw on the bottom is the drain screw for the float bowl. If that is not tight, yes, it will leak.
If you have a '79 or older bike with VM carbs, there is also a pilot fuel screw under the carb, but it should NOT be tight, it should be out 5/8 to 3/4 turn.

However, the "tightness" of either of those screws will NOT affect whether gas comes out the vents.

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[Matchless]

Here is a little summary I made for someone a while back:
http://www.mediafire.com/file/0yucmnmwnjj/Needle and seat in BS34 CV and other carburettors.pdf

Maybe it will help you.

 

[GS1000S]

This may not be your problem but I had the inner rubber disc in my petcock, that controls fuel flow direction, start deteriorating after 30 years of use. It would shave a hardened piece of the disc about the size of a fleck of black pepper occasionally when switched to prime or between on and reserve and it would flow down and stick a needle valve open. Because it was on the downstream side of the tank standpipe filter it bypassed it. I tore my carbs apart 3 times before I finally found the small piece in the needle. When I disassembled the petcock I could see the area around one of the holes in the disc had been coming apart for some time.

 

[Mekanix]

I've had this problem for the past 3 weeks and finally fixed it.

Maybe this will help.

How can this happen?

1. If its the petcock or tank:

- Petcock diaphragm busted and fuel leaking into one cylinder ( the one your vacuum line is connected to)
- Tank is not venting and is pressurizing and forcing the gas past the needle and seats regardless of how well they seal.


2. Needle seats and needle/ Seat O-rings.

- Hard to tell if they are worn by sight unless its really obvious.
- O-rings will make them leak around the seats

- Setting the float height too high makes them top out and not close the needle valve
- Setting the float height too low makes them not have enough force to close the needle.
- worn needles and seats makes it harder to adjust the float level.


I fixed mine by looking at what it was actually doing with the float bowls off.

Took the float bowl off and set the petcock on prime, gas would flow until I pushed the floats high enough. Took that measurement and it was higher than the book specs so I set the float height again and again and still it would bypass the needle. but they looked great to the naked eye.

So replaced all needle, seats, and O-rings.

Problem solved regardless of what I set the float level too now.


P.S Avoid sandblasting a gas tank that isn't sealed perfectly

On mine, sand made it past the in-tank filter and the inline filter and the filter on the needle seat and ruined the needle seat.