At wits end with dirt in the carb...help


40 replies to this topic
  • yam 3

Posted September 20, 2006 - 12:23 PM

#21

I too notice a little film of dust in the boot. Not enough grit to feel but you can see it when you wipe it with a finger. Always use lots of Bel-ray filter oil. I was worried also and milled out the inside of the cage to get 100% seal. One ride all was OK next ride really dusty and film was back. Like I said not gritty but there none the less. I was using a UNI filter.

  • Goosedog

Posted October 09, 2006 - 01:24 PM

#22

Tried all this, now we'll see on the next ride.

RTV siliconed the petcock to the tank and used red threadlock on the bolts. (There was a trace of dirt in that area.)

RTV siliconed the intake boot to the airbox.

Replaced neoprene filter ring in airbox.

New clamps on intake side of carb. (Trace of dirt there too.)

Snipped the vent hoses.

And probably most importantly....went back to oil-based filter oil (Maxima FFT) and no more No-Toil.


Three ride now since this post and the problem still persists. Any more suggestions?

  • Blue4Ever

Posted October 09, 2006 - 06:17 PM

#23

How is your fuel cap vent set up? It is possable for dust to enter this area as the fuel flows into the carb air is pulled in thru this vent to displace the fuel. Do you have the end of the vent hose routed into the steering stem?

  • Kent Rathgeber

Posted October 09, 2006 - 06:23 PM

#24

Could your airboot be cracked under the clamps? Or could there be a very small crack in the boot itself?

  • Goosedog

Posted October 09, 2006 - 06:34 PM

#25

I've replaced both the cap vent hose and one-way valve, also added an in-line fuel filter betweeb pet-cock and carb. I've had the airboot off and inspected and then I siliconed it to the airbox opening. I've got new clamps and there not pinching or cracking the intake boot or manifold side of the carb either. :devil:

  • Budlite

Posted October 09, 2006 - 08:12 PM

#26

What kind of shape is the fuel line itself in ? Is it coming apart on the inside ?

  • Goosedog

Posted October 10, 2006 - 03:49 AM

#27

What kind of shape is the fuel line itself in ? Is it coming apart on the inside ?



I've had it off the bike a lot recently and it appears OK. If there were tears in it then I'd have a leaking issue too.

  • YZ426F Rider

Posted October 10, 2006 - 10:04 AM

#28

...but it is a demonstration of the Bernoulli Principal.

Awesome. :devil:

  • Goosedog

Posted October 13, 2006 - 03:33 AM

#29

Now through the process of elimination, thrown tools and lots of cussing I've narrowed it down to one last possibility.

Could it be that dirt would find it's way to the bowl through the top of the AC pump rod hole behind the throttle wheel cover and travel directly to the pump area, into the bowl and thus start clogging my jets?

  • av

Posted October 13, 2006 - 07:11 AM

#30

As others have suggested but I don't think you have tried yet... I believe dust might be coming in through your carb's vent hoses. Try putting filters at the end of those lines. There should be 2 lines coming out of the carb then they may split via a 'T' fitting into 2 lines making a total of 4 lines exposed to dust, you'll need to filter them all out (or only use 2 lines, read further).

Those vent lines are used to equalize your carb so that it can flow fuel - i.e. if they are clogged with mud, your bike will starve without fuel and die. Since they do suck air to equalize the carb's pressure for flowing the fuel through, it makes perfect sense that if you ride in dusty conditions that the dust will also get suck up into the carb via these lines. Since those lines split via the 'T' fitting, you can actually seal one of the line from each side and just put the filters on the remaining 2 exposed lines. They are split so that if one of the line gets clogged with mud, the other line can still breathe. If both lines are clogged on one side of the carb then the equalization process for the carb is broken for that side of the carb (I hope that makes sense). Anyhow, if you plan to use filters at the end of these lines, you can save yourself some $$ by just running one line from each side of the carb (i.e. get rid of the 'T' split) into the air box (i.e. under the seat) where they will stay away from mud, then put the filters on those 2 lines (that are now under the seat) to filter out the dust that would have gone straight to the carb/bowl.

I hope that makes sense.

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  • grayracer513

Posted October 13, 2006 - 07:58 AM

#31

A couple of things to note about the vent hoses: First, their purpose is, as you have nearly said, to provide a balance between the atmospheric pressure outside the carb, and the air pressure inside the carb. If the air pressure inside doesn't match the outside, the metering by the jets will be inconsistent. Changes in fuel level, temperature, or outside air pressure are immediately balanced by the vents so the fuel metering is the same as intended.

However, the vents being blocked will not stop the carb from functioning. It seems that there is an assumption here that fuel drawn out of the bowl through the jets will create a vacuum if not replaced by air, when in fact it is replace by more fuel from the tank. Additionally, the vents and overflow are not the only route air has to enter the carb. There is an array of air jets at the mouth of the carb through which air can get in, and would if the bowl were unable to vent properly. And it is true that all of this would upset the proper operation of the carb, but it's very doubtful it would stop the engine.

Secondly, even though there are 5 hoses leading from the body of the carb, they all go into the same air space in the float bowl. That means that the bowl will vent if any one of them is open. One hose, the one that comes from the float bowl itself, is the overflow line, which limits the level fuel can rise to, and won't vent very well if there's fuel coming out of it. The other 4 come from two ports on the carb body, and lead to the top of the air space in the float bowl. These two ports are, as you said, each teed into two lines each for the sake of redundancy, since there is the very real possibility of the lines getting plugged by mud sticking to the ends of them. But again, the bowl only needs one of these to be open in order to work.

Last, because all they do is balance to the outside air pressure, there is not at any time any significant flow of air in the hoses in either direction, nor any significant vacuum formed in them, except under the odd circumstance that Bazooka Joe outlined. In any such hose through which dust had traveled, it would be visible on the inside of the hose when looked at from outside, just as the dirt that gathers on a fuel leak is.

One very common practice that will prevent mud clogging the entire set is to take the pair of lines that loop over the top of the carb from each vent port and run them up and back into the air box instead. Cover the open ends with a bit of an old air filter, and keep it clean and oiled.

  • av

Posted October 13, 2006 - 09:54 AM

#32

However, the vents being blocked will not stop the carb from functioning. It seems that there is an assumption here that fuel drawn out of the bowl through the jets will create a vacuum if not replaced by air, when in fact it is replace by more fuel from the tank. Additionally, the vents and overflow are not the only route air has to enter the carb. There is an array of air jets at the mouth of the carb through which air can get in, and would if the bowl were unable to vent properly. And it is true that all of this would upset the proper operation of the carb, but it's very doubtful it would stop the engine.


This has happened to a few of my buddies who have ridden through mud and have had their hoses clogged up - their bikes would NOT start. They had to call it a day until we found out it was just the clogged vent lines - cleaned off the mud and bike started right up. Try clogging up those lines with clamps, then try to start your bike - I bet it won't start and if it did, it wouldn't run long or run very well.

Secondly, even though there are 5 hoses leading from the body of the carb, they all go into the same air space in the float bowl. That means that the bowl will vent if any one of them is open.


This is a different statement than what you stated above (i.e. "vents being blocked will not stop the carb from functioning", as in ALL vents?). If one of the 4 vent hose is open and can equalize the carb appropriately, then yes, maybe the bike will keep running - I won't claim that it will work since I don't know for sure...

One hose, the one that comes from the float bowl itself, is the overflow line, which limits the level fuel can rise to, and won't vent very well if there's fuel coming out of it.


I don't believe this is correct. There are 5 hoses but the hose coming from the bottom of the bowl is for draining the bowl and is typically closed - it only opens up if you undo the Allen screw to drain the bowl via this hose. See link below:

http://av-motorcycle...ls/IMGP3473.JPG

The other 4 come from two ports on the carb body, and lead to the top of the air space in the float bowl. These two ports are, as you said, each teed into two lines each for the sake of redundancy, since there is the very real possibility of the lines getting plugged by mud sticking to the ends of them. But again, the bowl only needs one of these to be open in order to work.


I'm not sure how accurate you are as far as the other 4 hoses leading to the space in the bowl... the other 4 hoses are connected to 2 outlets, one on each side of the carb near the top of the carb's throat/venturi - they may lead to the space at the top of the bowl, but they must also do something else otherwise why would they design for those hoses to connect way up there instead of directly to the bowl? See link below:

http://av-motorcycle...or/IMGP3795.JPG

Those 2 vent ports on the side of the carb's throat must lead to some of the air jets, etc. to equalize the carb and the flow of fuel - IMHO, this is the area for dust to enter since there are NO filters on these hoses. Most of us don't bother with filtering these hoses, but this person obviously rides in pretty dusty conditions and is trying to solve the mystery and I believe this may be the port of entry for the dust that he sees in the bowl.

Last, because all they do is balance to the outside air pressure, there is not at any time any significant flow of air in the hoses in either direction, nor any significant vacuum formed in them, except under the odd circumstance that Bazooka Joe outlined. In any such hose through which dust had traveled, it would be visible on the inside of the hose when looked at from outside, just as the dirt that gathers on a fuel leak is.


From experience, I would disagree with that, I believe there is an intake air flow going through those hoses. If this is the source for dust to enter his carb, I bet if he looks closely, he will see dust inside of those hoses - if people ride in dusty conditions, their hoses have dust in them, just look closely.

One very common practice that will prevent mud clogging the entire set is to take the pair of lines that loop over the top of the carb from each vent port and run them up and back into the air box instead. Cover the open ends with a bit of an old air filter, and keep it clean and oiled.


Routing those hoses into the air box is pretty much what I said for him to do to avoid getting them clogged, adding the filters to those hoses is something that should be done but most of us don't. See link below... :devil:

http://av-motorcycle...fing/index.html

  • Goosedog

Posted October 13, 2006 - 11:11 AM

#33

Thanks for all this debate about vent lines but..............first of all the conditions I ride in are not all that dusty so it's gotta be something less complicated than equalized pressure inside the carb, and secondly what about the AC pump rod hole...it seems like a possibility to me. And is there supposed to be a seal at the top of it?

  • av

Posted October 13, 2006 - 11:40 AM

#34

BTW, those same hoses can let water get sucked into the carb so routing them higher up under the seat also avoids this problem, i.e. if you ride through deep water creeks/streams.

  • grayracer513

Posted October 13, 2006 - 02:24 PM

#35

Try clogging up those lines with clamps, then try to start your bike - I bet it won't start and if it did, it wouldn't run long or run very well.

This is a different statement than what you stated above (i.e. "vents being blocked will not stop the carb from functioning", as in ALL vents?). If one of the 4 vent hose is open and can equalize the carb appropriately, then yes, maybe the bike will keep running - I won't claim that it will work since I don't know for sure...

I have done that, to prove a point, and it works, although not very well. Keep in mind that there may be a good deal of difference between "functioning" and functioning properly.

I don't believe this is correct. There are 5 hoses but the hose coming from the bottom of the bowl is for draining the bowl and is typically closed - it only opens up if you undo the Allen screw to drain the bowl via this hose.

The hose that drains the float bowl does so because the allen bolt you mention opens a port that tees into the overflow stand pipe drillway, using one hose for two purposes. If you doubt this, blow a little air through it, or take the bowl off and look. The overflow standpipe is located near the center of the bowl, and stands about at the level of the gasket flange. Fuel reaching this level is intended to run down the pipe and out before it runs over the top of the main jet discharge nozzle and into the engine.

I'm not sure how accurate you are as far as the other 4 hoses leading to the space in the bowl... the other 4 hoses are connected to 2 outlets, one on each side of the carb near the top of the carb's throat/venturi - they may lead to the space at the top of the bowl, but they must also do something else otherwise why would they design for those hoses to connect way up there instead of directly to the bowl? Those 2 vent ports on the side of the carb's throat must lead to some of the air jets, etc. to equalize the carb and the flow of fuel

Again, all that is necessary is to run some carb spray through these passages when you have the carb off, and you can see right where they go or don't go. The two vent ports run from the point where the hoses connect straight down parallel to the slide bore, straddling, but not intersecting, the carb throat, and enter the top of the float chamber on either side. See for yourself.

All airways in the carb through which a significant amount of air actually flows, including the intakes for the choke and hot start, are placed in the intake bell, which means they receive filtered air from the air filter. There simply is no significant amount of air actually drawn through these hoses, and the engineers have made a point of ensuring that. All that goes on in the float bowl is that fuel is poured in when it is below a certain level, and is drawn out through the jets when the engine uses it, to be replaced by more, in a basically zero sum situation. Venting the bowl is only a MAJOR factor when the bowl is empty and needs to be filled, and in that case, air is forced out, not in. After that, things more or less balance themselves. Venting is, of course still necessary to prevent air from having to be stolen from the metering passageways when that air is supposed to be doing something else.

  • av

Posted October 13, 2006 - 02:53 PM

#36

The hose that drains the float bowl does so because the allen bolt you mention opens a port that tees into the overflow stand pipe drillway, using one hose for two purposes. If you doubt this, blow a little air through it, or take the bowl off and look. The overflow standpipe is located near the center of the bowl, and stands about at the level of the gasket flange. Fuel reaching this level is intended to run down the pipe and out before it runs over the top of the main jet discharge nozzle and into the engine.


I see the tube you speak of, thanks for the explanation.

Posted Image

Again, all that is necessary is to run some carb spray through these passages when you have the carb off, and you can see right where they go or don't go. The two vent ports run from the point where the hoses connect straight down parallel to the slide bore, straddling, but not intersecting, the carb throat, and enter the top of the float chamber on either side. See for yourself.


Then why do they put the port holes way up top? :devil: Even if the carb spray comes out in the bowl, are you so sure that there are no other holes connected to those passages internally? Seems like an odd design if all they do is to equalize the space in the bowl - and why would they need 2 such ports, both placed up high like that? :thumbsdn:

All airways in the carb through which a significant amount of air actually flows, including the intakes for the choke and hot start, are placed in the intake bell, which means they receive filtered air from the air filter. There simply is no significant amount of air actually drawn through these hoses, and the engineers have made a point of ensuring that. All that goes on in the float bowl is that fuel is poured in when it is below a certain level, and is drawn out through the jets when the engine uses it, to be replaced by more, in a basically zero sum situation. Venting the bowl is only a MAJOR factor when the bowl is empty and needs to be filled, and in that case, air is forced out, not in. After that, things more or less balance themselves. Venting is, of course still necessary to prevent air from having to be stolen from the metering passageways when that air is supposed to be doing something else.


I don't think that is totally correct. Take a straw from a water bottle and suck the liquid out, you'll need that vent hole to equalize the pressure and that hole will suck air not blow air, even if you have an unlimited supply of liquid coming into the bottle from another hole, the air equalizes the pressure within the chamber much faster than the incoming liquid can and that hole will still suck air not blow IMO - it's a physical phenomenon. A gas can works the same way, you need a pressure equalizer hole and it will suck air as it empties the fuel. If you look at a huge tank, with more gas than what's in a carb's bowl, hence you don't need another incoming feed line (i.e. gas tank), eventually you'll still need that hole to equalize the pressure and guess what, it will suck air - hence, if those hoses are clogged, the engine will starve of fuel and die.

  • grayracer513

Posted October 13, 2006 - 03:33 PM

#37

I see the tube you speak of, thanks for the explanation.

No problem.

...why do they put the port holes way up top? Even if the carb spray comes out in the bowl, are you so sure that there are no other holes connected to those passages internally? Seems like an odd design if all they do is to equalize the space in the bowl - and why would they need 2 such ports, both placed up high like that?

Yes, I am sure. The sense in running them up that high is simply that it makes it that much harder to spill fuel out over the top than if they were located lower, while it doesn't restrict the ventilation at all. Why two ports? Why two hoses on each one? Redundancy.

I don't think that is totally correct. Take a straw from a water bottle and suck the liquid out, you'll need that vent hole to equalize the pressure and that hole will suck air not blow air, even if you have an unlimited supply of liquid coming into the bottle from another hole, the air equalizes the pressure within the chamber much faster than the incoming liquid can and that hole will still suck air ...

Well, it will be easy enough for you to demonstrate that for yourself, but as long as you're analyzing it, picture a sealed 200cc container, half filled with fluid with one outlet and one inlet. The inlet is connected to a separate supply of fluid that has an independent vent source. The outlet is below the fluid level. If you pinch off the inlet, and remove 20cc of fluid through the outlet, you will expand the air space by 20cc and create a partial vacuum by doing so. If you open the inlet, this vacuum will be eliminated by 20cc of fluid flowing in to replace the fluid flowing out. Left on its own, this setup could flow an unlimited amount of fluid out of the outlet, and the fluid level in the container would remain a wonderfully constant 100cc with no vent at all but the inlet fluid source...until some variable came along, like a rise in the temperature of the air in the container, or a change in barometric pressure. And of course, using this setup to control the fluid level in a carb would be a particular PITA when you had to initially fill it, because getting fluid to go into the container through a single inlet without any coming out would require that we release some air, wouldn't it? So then we need a vent we can turn off, so that we can open it, let the container fill to the required level, and close it back off. Obviously, using something like this on a real-world carb would require fairly continuous periodic intervention to manage the fuel level. And the way that the early carb engineers decided to manage that problem was to put an air vent in the float bowl, and control the level with a float and needle valve.

You are kind of swerving into something with the concept of air being easier to move than fluid, and proper venting does contribute to the carb's ability to respond near instantly to sudden changes in fuel rate for that reason. But the change in the actual air volume even during that moment is very, very slight, and is reversed once the required fuel flows back in. Remember that any fuel removed results in the same amount of fuel being allowed into the bowl through the needle valve, so in that sense, the change in air volume is a net zero.

  • Goosedog

Posted October 13, 2006 - 03:47 PM

#38

Now my threads been totoally hijacked...I feel so used. :devil:

  • av

Posted October 13, 2006 - 04:05 PM

#39

Now my threads been totoally hijacked...I feel so used. :devil:


Sorry dude, but this is an interesting discussion - I still think you should try the filters for those hoses... :thumbsdn:

Gray - what you say makes sense but the fact still remains that on more than one occasion, when those hoses are clogged with mud, our bikes die and won't work until those vents are cleared again. This tells me that there is more to those hoses than net zero pressure changes. I've found fine silt in my fuel too, but not always and I still believe that those hoses do suck air and water (if submerged).

  • Blue4Ever

Posted October 13, 2006 - 06:09 PM

#40

I am not sure if you have been asked this yet, but are you running any kind of race fuel or additive in your fuel?





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