Topic: what is the breather/vent from airbox for?

[mattd]

hi all no silly questions from me for a while cos ive been out riding it and havent broke it yet. however yesterday i did a couple of hours then parked it up for a few more. when i came to start it up and ride away as i warmed it through i noticed fluid venting under the bike. this stopped immediatly and i rode home. as i arrived home it sprayed another patch as it sat idling but only breifly. im sure this came from the large breather which come from the airbox as it meets the airbox to carb rubbers. does anyone know why this happened? the bike is a 550 f2 and does have a split in the left hand airbox to carb rubber ( furthest carb left when sat on the bike). any advice greatly appreciated cos im worried now!!!

[sick_seven_fifty]

Are you sure it's not coming from the carb vents?  Could be gas overflowing from a leaky float needle.  That would also explain why it's only doing it while at idle (not enough gas being consumed to stop it from overflowing).  Did you smell the fluid?

[TwoTired]

The air plenum drain, the filter box drain, and all four of the carb overflow tubes are all routed behind the engine and in front of the swing arm pivot.

Only reason for them to get active is if the carbs float valves aren't closing as they should.  All these tubes need to be cleared of insect nests so they can do their job. if the air box is filling the tubes are probably plugged, and there is a reasonable change that the carb intake runners filled with gas, too.  This would seep past valves and rings and dilute the oil supply.  So check the oil for too full and reeking of gas.  Finding this, change the oil.  Then track the carb float valve cause and correct.

Rust in the tank and a corrupted fuel filter can block the carb's float valves open, as could other sediment.

You never told us, but we have to assume your bike is all stock, since you didn't inform us of any changes to the make and model you mentioned.

Cheers,

[mattd]

thanks guys. yes 750 i smelled then fluid and it was defo petrol. the bikes completely stock 2tired. if its a bit of crap holding the needle up whats the chances of it clearing on it,s own? should i drain the float bowls and see if that helps?

[TwoTired]

thanks guys. yes 750 i smelled then fluid and it was defo petrol. the bikes completely stock 2tired. if its a bit of crap holding the needle up whats the chances of it clearing on it,s own? should i drain the float bowls and see if that helps?

It is possible to clear on its own.  It could also return when you are far from home.
I would drain the carb bowls into a clean pan and see examine what comes out with the gas.  (Sediment, rust, flakes, gum, sightless insects?) I've even flushed them with aerosol carb cleaner through that drain hole.  The next step depends on what comes out with the drainage.  Removing one of the outer bowls isn't too hard and allows you to see the reservoir deposits.  Then make a decision on what next steps are required.

Cheers,

[mattd]

just been out checked the oil its still at the level on the dip stick and doesnt smell of petrol so i think i got away with that. soon as i can ill remove all 4 tubes and check their not blocked and drain all 4 float bowls. then give bike a good blast and see what happens. 2tired can i take it that provided my drain tubes are unblocked that any unwanted petrol would just be dumped on the ground and not end up in the oil?

[TwoTired]

When the fuel level reaches the carb throat level  (or, air jet level), it can travel two ways and is guided by gravity.  If the carbs are tilted toward the engine, it flows that way.  If the carbs are tilted toward the rear it fills the air plenum.
The carb bowl overflow tubes are supposed to prevent the fuel level reaching the carb throats, and dump errant fuel on the ground.

Cheers,

[WarwickE36]

Definitely drain them..

If the rust (or whatever it is) is allowed to stay in the bowl it will ultimately end up plugging your idle jet...

Its possible for it to end up going right through your main jet but I wouldn't bank on it.

Also shut your petcock off and pull the sediment bowl, check clean it out and check the screen for holes.  My old petcock screen had a massive hole in it and was allowing the tank rust to run into the carbs...  It only took 1 time on reserve to cause 2 float needles to stick open.  I ended up with the same problem you had, and until i replaced the petcock, drained the carbs, and cleaned out the idle jets it was running rough at idle and leaking out the overflows.

[mattd]

now in terms of stupid questions then you'll love this warwick! how the hell do you drain them? after my last post on this subject with twotired i went staright out with the intentiuon of draining em but i dont know how!! each carb has a large screw in the side of the float bowl and when i cracked it open a little on the first carb fuel came out but i didnt want to go any further and do damage in ignorance (i thought the fuel would come out the drain tube not out of the screw) so does it actually come out the screw? im assuming then you just balance a pan on top of the gearbox to catch the petrol and drain away. wish someone would tell me cos i cant beleive i can be getting this so wrong!!!! cheers in advance

[TwoTired]

The CB550 F carbs have a large screw head near the bottom of the bowls. It is simply a drain plug.  If clean, it will dribble/flow when loosened from a bleed hole in it.  But, it can clog up. In which case, the whole brass plug will have to be removed and it will all drain out in gush.

Cheers,

 

[WarwickE36]

In your case just like TT said, pull the large flathead scre/bolt out ALL THE WAY.  use something to catch the fuel so you can see what was swimming around in it.  Then you can play show and tell with us.  Bring a camera

[mattd]

sound advice boys and very much appreciated! however you might wait a while for a photo just posting questions and replies on here is my limit at the mo but im going to get the wife to show me how to post pics - i could do with one to go next to my name

[Duanob]

I got in the habit of leaving my drain hoses off the carbs until i was sure everything was working properly. Like when taking out from winter storage or after a carb cleaning. It's a lot easier to see which is the offending carb. After that put all the drains back on for the rest of the summer.

And don't forget to shut the petcock before cracking the cabr drains. I only speak from experience.