Topic: Why does this tube under my bike spill gas every once and a while???

[mathbishop]

This little tube under my bike spills out a good amount of gas every once and a blue moon. Im sure its something im doing wrong or just something im doing that im not aware of, any info would be great!

[BeSeeingYou]

If your bike has the recessed gas cap it may be the overflow tube that drains the recess around the gas cap.  Should only leak if you spill a bit when filling up or maybe if you overfill the tank and some leaks out the cap vent.

[Greggo]

Looks like a float bowl overflow drain.  Is it connected to the bottom of one of the carburetors?  If so, you have a leaky float valve, or a cracked bowl tube.

[farinhite]

+1 on the float valve, or could be a stuck float. Give the bowls a little bang with a screw driver, or take the carbs off and check float heights.

[ekpent]

Do you turn off your petcock when not riding ?. These are not vacuum operated.

[mathbishop]

i usually do not turn it off, the last time it happened I had left it sitting for a week or so without riding with petcock on.

[flybox1]

Look at what it attaches to, and you'll find your answer.
Looks a bit big to be a carb bowl drain tube, but who knows what the PO put on there.... how about condensate from your valve cover rebreather?

[MCRider]

i usually do not turn it off, the last time it happened I had left it sitting for a week or so without riding with petcock on.

ITs SOP to turn the petcock off when setting for any time. I'll get out of bed and go to the garage to turn it off when i forget. I like to turn it off when entering the driveway. That way it can suck the bowls levels down a tad for extra safety.

You have (probably) a float valve that is sticking open occasionally, fed by the open fuel cock. Run some SeaFoam or equivalent thru a tank of gas. You should still turn the pc off though.

[malcolmgb]

You don't say what bike it is, always a good idea. Looking at your picture I would guess a 400 in which case you should have 4 overflow pipes, 1 from each carb.
Always turn tap off when engine not running, it may save you losing $ of fuel or the bike/building through fire.

[Prospect]

i usually do not turn it off, the last time it happened I had left it sitting for a week or so without riding with petcock on.

ITs SOP to turn the petcock off when setting for any time. I'll get out of bed and go to the garage to turn it off when i forget. I like to turn it off when entering the driveway. That way it can suck the bowls levels down a tad for extra safety.

You have (probably) a float valve that is sticking open occasionally, fed by the open fuel cock. Run some SeaFoam or equivalent thru a tank of gas. You should still turn the pc off though.

I also turn off the fuel as I'm riding towards my house.  I turn it off about 1/4 mile from my house and it just starts to loose power (from fuel exhaustion) as I enter my driveway and open give it moderate throttle. 

Also, often the petcock can leak gas through it as the rubber inside gets old and brittle causing fuel to constantly flow into the carbs.  So turning it off will not do anything. 

[Mooshie]

+1 on the float valve, or could be a stuck float. Give the bowls a little bang with a screw driver, or take the carbs off and check float heights.

Mine does this intermittently also.  Always turn off the petcock too.  Screwdriver takes care of it but there is probably a granule of gunk stuck so it is not sealing properly-shaking it up seems to work well until I can fix it

[Mainerider]

It looks too large to be a carb drain line; I believe it is your air box re circulation drain tube.  Your engine doesn't burn all of the fuel in the first pass-through and the lighter, unburned particles get re-circulated back into the combustion chambers to be re-burned, the heavier particles (unburned fuel and a tiny bit of oil with it, well hopefully a tiny bit...) collect at the bottom of the air box. There should be a plug at the end of that hose so you can pull it out and drain the tube occasionally.  Completely normal.  If you are missing the hose end plug or you have a tear in the hose it will drain out on its own.

Are you seeing clean fuel drain out or is it more of a sludgy mix?  You should pull out your air filter and look at the bottom of the air box; is it relatively clean and dry or damp and/or sludgy looking?

[TwoTired]

If your gas tank is as rusty as your drive chain in the pic, I'd expect the float valve(s) to be blocked open by rust bits.  Follow the tube to its source.  If the tube is from an air plenum or air box drain the carb have overflowed into the carb throats and to the air box.

I'd check your oil for gas contamination as well.  Liquid gas in the carb throat can go toward the engine cylinders, too, and pollute/dilute the oil supply.

Can't tell any more detail without knowing WHAT BIKE we are looking at.

You sure it's gas coming out and not battery acid?

[mathbishop]

Its a CB400f. its definitely gas. And it does seem to be a little on the sludgy side. That makes sense.

[trueblue]

Your engine doesn't burn all of the fuel in the first pass-through and the lighter, unburned particles get re-circulated back into the combustion chambers to be re-burned.

WTF, if it isn't burned on the first pass through the engine it is poked straight out the exhaust, there is no EGR set up on these bikes.  Even if there is, an EGR's sole purpose is to reduce NOx emissions, it has nothing to do with HC emissions.  If your engine is in a proper state of tune then the amount of unburned fuel should only measurable as PPM, and definitely nowhere near combustible.  The hose that goes into the airbox is a crank case breather, it's purpose it to burn off the vapour that comes off the hot oil in the crankcase, rather than venting to atmosphere.  If your rings are shot then you will have excessive blow-by which, if bad enough, can push oil into your airbox. 

Back to the OP's question, like has been mentioned here a few times, it is likely caused by leaving the fuel tap on.  If the carbs are overflowing into the airbox, the fuel will pick up some of the oil scum that is normal in the airbox and run out the drain tube, giving the fuel a sludgy look.

[Mainerider]

The blow-by consists of carbon monoxide, hydrogen sulfide, nitrogen dioxide, selenium oxide, and hydrocarbons.

To quote from pages 108 and 109 of the CB550 Honda factory service manual,

" The gases consist of not only the remainder of burned mixture ( I.e. fuel ) and combustion products but also a leakage of compression past the cylinder wall or from the crankcase. The latter is known as blow-by gas , and accounts for 20-40% of the total amount of hydrocarbon to be emitted in the air. Since blow-by gases have not been completely burned and, then therefore, must be burned again by means of the blow-by gas scavenging device to minimize the amount of the gas to be emitted into the air."

What makes it into the cylinder head is a mix of the above, and that includes unburned fuel. By some estimates, up to 70% of what passes by the piston and into the crankcase consists of unburned fuel.; that number only increases as wear between the rings, piston, and cylinder wall occurs.

You should know that 100% burn rate never occurs, and there is always a small amount of unburned combustion products, even in a well-tuned engine.

[trueblue]

I know 100% burn never occurs, but the amount that doesn't get burnt should only make up about 200ppm of the exhaust gas.  I have never had an engine that I couldn't get below 400ppm of HC, and that includes some fire breathing monsters.  Don't forget that lubricating oil is also a hydrocarbon, and when it heats up and is splashed around like it is in a crankcase it too gives off hydrocarbon gas.  Which is the main reason for the crankcase breather.  Seeing as you seem to have all the answers, can you explain to me how the remainder of the fuel mixture can get back into the cylinder through another method other than blow-by, which is what you seem to be trying to say. 

[Mainerider]

God knows, I don't have all the answers and if I did I certainly wouldn't have time to hang around forums occasionally. Yes, you are correct, blow-by is the only way. I will also freely admit that my initial post wasn't particularly clear or well-written. I am also guilty of doing what I think every poster does at times, and that is Posting while Distracted (should be a chargeable offense).  Posting while you are supposed to be working, posting while watching the kids, etc.  I've even come across a few previous posts of mine, saw what I had written and thought, " Why did I write that? I know that's not correct. Where was my brain at the time?". We are all flawed beings; That's why they say on the forums at times, " Youse gets what's youse pays for..."

When you or I or anyone else sees info that is vague, ambiguous, partially incorrect or just flat out wrong, it is our obligation to point it out.  Politely pointing it out always earns one extra credit, though; Condescension, rudeness or profanity just leads to defensiveness and makes other parties less willing to entertain divergent viewpoints.

So, this is the beauty of the forums. An issue/question is posted and the OP receives many responses, some good some bad. Out of the ensuing debate, knowledge-swapping, petty bickering, etc. the OP eventually  finds his or her answer.  It is messy, contentious, drawn-out, and oftentimes just plain painful to watch, but democracy always is.

[Cundalini]

Seems to me to be the drain pipe from the Air filter intake box! Especially if the residue is oily and sludgy! You also need remove the Air Filter and clean and dry out the small piece of foam filter underneath on a regular basis!

[Cundalini]

Looks like a float bowl overflow drain.  Is it connected to the bottom of one of the carburetors?  If so, you have a leaky float valve, or a cracked bowl tube.

The float bowl over flow pipes on a 400 four are four small pipes which come together held together by a plastic ring.This seems to be the larger single drain pipe, to allow condensation from the Air filter box to drain away.

[HondanutRider]

You know how it is...occasionally you have this urge that needs relief.  Sometimes it's the middle of the night....sometimes it's while out on a long ride...whatever.  SOHC4's sometimes also have that same urge...you know - just to take a piss.