Dirtbike carburetor question

[cordex]

I'm working on an old Honda dirtbike (a 1979 XL250S) and recently had a friend help me rebuild the carburetor.  The bike runs great if you manually fill up the float bowl with gasoline ... until the bowl is empty.  The float seems to release gas when the bowl is off, but when the bowl is on no gas flows.

Does anyone have any experience with getting a carb to dump gas into the float bowl?  I know I can adjust the float tang, but is there anything for it besides trial and error?

[Brad Johnson]

The fuel petcock is turned on, right?   

Brad

[JonnyB]

Somewhere, there ought to be a technical manual that lists the float setting - usually in fractions of an inch. In this case, it may list a metric measurement.

Google may be your friend.

Is the float resting on the bottom of the bowl when assembled? Needle valve bad? Perhaps the seat is partially unscrewed.

jb

[cordex]

Brad,
Yeah.  We checked that.  Like I said, we get plenty of flow when the float bowl is removed.

Somewhere, there ought to be a technical manual that lists the float setting - usually in fractions of an inch. In this case, it may list a metric measurement.

I've had several people suggest I find that setting for my bike, but I have no idea what I'd be measuring to or from.  I can measure a couple of millimeters, but without knowing what I'm measuring ...

Is the float resting on the bottom of the bowl when assembled? Needle valve bad? Perhaps the seat is partially unscrewed.

It's kind of hard to tell what the position of the float is when the bowl is on.  Maybe I should try putting some black grease on the float itself and see where (and if) it contacts the bowl.

[coppertales]

Does your float have a leak/hole in it preventing it from floating?  If the hole is in the bottom, the gas might drain out before you can see it when you remove the bowl...chris3

[cordex]

Does your float have a leak/hole in it preventing it from floating?  If the hole is in the bottom, the gas might drain out before you can see it when you remove the bowl...chris3

The float itself is plastic.  If it wasn't floating, wouldn't the gas keep coming instead of not flowing at all?
There is a drain tube coming off the bottom of the float bowl, but we can tell when gas is coming out of there.  And as I said, we can fill the bowl manually and bike runs just fine, so I don't think the problem is that gas is getting into the bowl and then leaking.

I guess I'll keep trying to tweak the tang.

[JonnyB]

To measure the float setting, you generally measure from the top of the float to the top of the float chamber. The tech manual should give both the measurement and the reference points. (Often, the carb lays on its back for this.

If the float is a sinker, rather than a floater, yes, you'll have fuel running out all over. There may be a link that connects the needle valve to the float; perhaps this got fubar'ed and the needle won't open with the carb assembled. Will fuel come out when the bowl is removed? If so, can you lift the float to stop it? Be gentle! These things are sorta fragile.

Have you removed the needle and seat assembly? Perhaps there's an obstruction. Also, gas with ethanol may have caused the rubber parts to swell, cutting off the supply.

jb

[cordex]

To measure the float setting, you generally measure from the top of the float to the top of the float chamber. The tech manual should give both the measurement and the reference points. (Often, the carb lays on its back for this.

Okay, that explains how to measure.  I think I've found the correct measurements elsewhere.  It'll be worth a shot, anyway.

If the float is a sinker, rather than a floater, yes, you'll have fuel running out all over. There may be a link that connects the needle valve to the float; perhaps this got fubar'ed and the needle won't open with the carb assembled. Will fuel come out when the bowl is removed? If so, can you lift the float to stop it? Be gentle! These things are sorta fragile.

Fuel does come out (quite quickly) when the bowl is removed.  When the float is lifted, the fuel stops.

Have you removed the needle and seat assembly? Perhaps there's an obstruction. Also, gas with ethanol may have caused the rubber parts to swell, cutting off the supply.

I haven't taken the seat out, but when I had a friend (who sadly doesn't live nearby) rebuild the carb they replaced all the parts.  The needle comes out when I pull the float.

If I get a chance, I'll take some pictures.

Thanks for your help on this!

[coppertales]

Pardon me, I got the problem backwards........measure the depth the float opens, then the depth of the bowl.  Adjust the float so it opens sooner.  Maybe when you are installing the bowl, it is pressing the float closed.....chris3

[Marnoot]

Having the opposite problem with the Rebel I've been riding. Left the petcock open overnight and the float must be stuck open as fuel was leaking out the overflow tube all night onto the garage floor.  Going to have to pull and clean the carburetor. I'll be installing an inline fuel filter to try to prevent it in the future.

[cordex]

Having the opposite problem with the Rebel I've been riding. Left the petcock open overnight and the float must be stuck open as fuel was leaking out the overflow tube all night onto the garage floor.  Going to have to pull and clean the carburetor. I'll be installing an inline fuel filter to try to prevent it in the future.

Our bikes need to rub off on each other a bit.

In trying to solve my problem, I found a suggestion to help diagnose the root issue in your situation.  Replace the drain tube with some clear fuel line, open the float bowl drain and lift the end of the clear line up above the level of the carb.  Turn on the fuel at the petcock and watch the fuel level.  If it rises past the level of the carb and starts to spill out, it's probably a stuck float or needle.  If it stops about where the float should be to close off the fuel, then the problem is either a bad or open valve on the float bowl drain or a maladjusted float tang.

[Leatherneck]

I bet the bowl is backwards, or bent causing binding on the float in some way. At least you know the tubes are clean if it runs when you put fuel in the bowl. Press on, thinking "mechanical interference with the float."

TC

PS: Have a beer.

[cordex]

I bet the bowl is backwards, or bent causing binding on the float in some way. At least you know the tubes are clean if it runs when you put fuel in the bowl. Press on, thinking "mechanical interference with the float."

Only one way for the bowl to go on as it has some pegs keyed to the carb itself.  Good idea, with the mechanical interference idea, though.  I think I'm going to try to use something like a grease to determine if and where the float is making contact with the bowl.

PS: Have a beer.

Would that help?  Should it be applied directly to the float? 

[griz]

I'll second Leatherneck's diagnosis.  Mechanical interference.

Was the float replaced with a different one?  Might be the wrong one.  Also, on some, the part that actually seals when the float rises can be installed so that it sits above where it should and stops flow way to early.  Some floats can also be installed upside down.  Pics would help if the problem persists.

[cordex]

Thanks to all that helped.  Especially to a friend and his dad who figured it out before I had a chance to look at it again. 

Leatherneck and griz were the closest - the problem was that the float was mounted upside-down.  It operated correctly without the bowl, but was too thick when mounted that way to work, as the bowl would contact it.  Flipping it upside-down solved the problem.

Now I've got a dozen other things to work on with the bike, but it at least will run.

Thanks again!