Small amount of dirt in the botom of the carb.

Everytime I take out the bolt at the bottom of the carb to change main jets on my 07 YZ450F I am noticing a more than normal amount of dirt in the bottom. Is this normal? I have checked all boots and seals, cleaned the carb and everything was and is perfect. I can only assume I am getting it from my gas or the can.

Is there not some sort of microfilter that can either be part of a funnel that I pour the gas into or part of my gascan to filter out this stuff?

HMMRDWN.

Last time I checked it before my ride and cleaned it. I only rode for 20 minutes and checked it again. Once again there was a small amount in the bottom. I don't seem to rememeber this problem on my 250F.

Dirt is either in your tank, or your fuel can. Presumably, the gas pump your fuel originally comes from should be filtered, but it may be the source of the problem, too. Actually, it's for sure that it's in your tank at this point.

One thing you should not do is use a separate hand held filter sitting in the fuel neck unless it is made for the purpose. Pouring gas across certain kinds of materials can generate enough static electricity to create spark under the right conditions.

we had this happen on my yz125 a few times.. we put a fuel filter on the fule line which helped.. i asked all my mates what they do some do filters some nothing. i suppose the biggest rule be careful

Everytime I take out the bolt at the bottom of the carb to change main jets on my 07 YZ450F I am noticing a more than normal amount of dirt in the bottom. Is this normal? I have checked all boots and seals, cleaned the carb and everything was and is perfect. I can only assume I am getting it from my gas or the can.

Is there not some sort of microfilter that can either be part of a funnel that I pour the gas into or part of my gascan to filter out this stuff?

HMMRDWN.

Last time I checked it before my ride and cleaned it. I only rode for 20 minutes and checked it again. Once again there was a small amount in the bottom. I don't seem to rememeber this problem on my 250F.

Clean the vent lines from your carburetor. You will probably notice dust inside them. When you pull the CHOKE knob on your bike as you are cold starting it, the choke circuit draws air into the float bowl to operate. The only passages for air to enter the float bowl are from the vent lines down by your swingarm. When the air is drawn inwards it can suck dirt into the float bowl at the same time.

(A poor design.... ? )

James

Try adding fuel in you tank by pouring it through a coffee filter(paper style) inside a funnel. This should help catch some of the fine debris and it may help narrow down the source of your problems.

Try adding fuel in you tank by pouring it through a coffee filter(paper style) inside a funnel. This should help catch some of the fine debris and it may help narrow down the source of your problems.

Pretty much exactly what I said not to do:

One thing you should not do is use a separate hand held filter sitting in the fuel neck unless it is made for the purpose. Pouring gas across certain kinds of materials can generate enough static electricity to create spark under the right conditions.

Clean the vent lines from your carburetor. You will probably notice dust inside them. When you pull the CHOKE knob on your bike as you are cold starting it, the choke circuit draws air into the float bowl to operate. The only passages for air to enter the float bowl are from the vent lines down by your swingarm. When the air is drawn inwards it can suck dirt into the float bowl at the same time.

(A poor design.... ? )

James

LIGHTBULB! Thanks JD. I have seen a lot of threads and posts trying to figure out the cause. The Bournoulli effect from having one vent hose longer than the other and your contribution of dirt being drawb into the carb when the choke is on seem like the best candidates with your choke circuit being the leading candidate. In the future I will warm up in the parking lot / pits and push the choke off before entering the track just to be safe.

What do you think GrayRacer?

I actually just went to look at the carb hoses and noticed the one at the very bottom of the carb was actually off. I also noticed that the other end was plugged solid with mud. Did this get blown off from the pressure of the other end being plugged or was that just coincedence? Would that hose at the very bottom of the carb being off maybe be the reason the dirt was in the bottom of the carb?

LIGHTBULB! Thanks JD. I have seen a lot of threads and posts trying to figure out the cause. The Bournoulli effect from having one vent hose longer than the other and your contribution of dirt being drawb into the carb when the choke is on seem like the best candidates with your choke circuit being the leading candidate. In the future I will warm up in the parking lot / pits and push the choke off before entering the track just to be safe.

What do you think GrayRacer?

Next, you do understand who JD is, right? You want me to evaluate his opinion? Very well. I am and have been skeptical of the contention that a significant amount of air is ever moved through these vents, but I have not taken enough time to fool around with it to see that one way or other. I will say that none of the vent tubes on any of my own bikes have any dirt in them whatsoever. So why do I doubt it?

First, there are 5 vent hoses on an FCR that plug into 3 ports, one on each side above the bowl, and one on the bottom of the bowl. The top two split into two for redundancy, and the bottom one serves as an float level limiting overflow tube, a drain, and an additional vent. All of these lead to the float bowl in the air volume above the fuel.

The level of fuel in the bowl is controlled by a simple (crude?) float and needle arrangement that I suppose I shouldn't criticize with being able to offer something better. When fuel leaves the bowl, the level drops, the float drops, the needle valve opens, the level rises again, and closes the valve. If we could have this arrangement operate in a totally vibration and shock free environment at a constant temperature, we could run the carb without a bowl vent, but real life ain't like that. The float gets bounced all over the place and there are times when a little too much gas is let in at once, and the needle valve then will stay closed until the engine uses fuel down to the correct level again. But if we expect the fuel to flow out of the bowl consistently with all this going on, we have to vent the float bowl to allow the small amount of air that must be moved in or out to do so freely, but this process should never cause a rapid movement of a large volume of air in either direction through the vent hoses, especially for a long enough time, or with enough force to raise dirt into the carb.

Carburetors do require air to flow through the different circuits to pre-atomize fuel and to help transport it through the passageways, but they virtually always get this air from a "clean" source on the filtered side of the intake stream, usually at the intake bell. The air for both the main and idle circuit on an FCR come from there, and not from the bowl. Likewise, the clean air feed for the hot start comes from the chamber above the slide, which breathes only through the air inlet bell. I was certain, up to now, that I remember the passage that feeds air to the "starter" circuit (choke circuit) coming from the mouth of the carb, too, but I don't have a carb off to look at, and I doubt I'll pull one to examine it for this reason. If Mr. Dean or anyone else can tell me for certain that the choke circuit air feed comes from the float bowl (or simply doesn't exist, drawing fuel straight out of the bowl without additional air), I'll have to take their word for it.

Even then, I wonder about how strong the vacuum in the tubes would be, since the strongest flow would be up the center tube on the bowl, and that would be no more than a third of the vacuum actually generated by the circuit.

So, to summarize, I doubt it, and that's why I doubt it, but I'm not in a particularly good position to be able to say yes or no. I would need more information.

But on the Bernoulli Effect thing, no.

little tiny bit of dirt = no good at all

I bet the dirt in the carb is very very fine particles.....

remove your tank and flush it out with clean gas

Check you airbox seal inside and out for possible leak - if you have removed your carb boot check this and the metal gasket to make sure both are on right.

I have not had any dirt sux'd into carb through ventlines - but in my case it was one of the 2 reasons above. Also when you put gas in at track....dont do it in a dust storm.

A separate gas filter can help but keeping your gas clean from debree and dirt out of the tank can help elliviate this altogether. Do it before the float gets stuck.....?

More than likely you are drawing dirt in through the vent lines of the carb. Look here and scroll down a little ways. Really nice carb vent filters. https://www.thumpertalk.com/forum/showpost.php?p=5182425&postcount=30

Thumpers pull a huge amount of vacuum through these lines. Its also a good idea to run your crankcase breather tube into the airbox or put some sort of filter on it. It's doing the same thing everytime you let off the gas.

I was certain, up to now, that I remember the passage that feeds air to the "starter" circuit (choke circuit) coming from the mouth of the carb, too, but I don't have a carb off to look at, and I doubt I'll pull one to examine it for this reason. If Mr. Dean or anyone else can tell me for certain that the choke circuit air feed comes from the float bowl (or simply doesn't exist, drawing fuel straight out of the bowl without additional air), I'll have to take their word for it.

The passage feeding AIR to the choke circuit is in the float bowl cavity at the top, near the pillar supporting the choke and main jets.

If you want to confirm, place all 5 of your vent tubes into a pail of water with the bike running and pull the choke knob. You must place all of them in the fluid. It will pull the water up the tubes and kill the bike when it becomes starved for fuel by the vacuum.

See the hole below.

James

The passage feeding AIR to the choke circuit is in the float bowl cavity at the top, near the pillar supporting the choke and main jets.

If you want to confirm, place all 5 of your vent tubes into a pail of water with the bike running and pull the choke knob. You must place all of them in the fluid. It will pull the water up the tubes and kill the bike when it becomes starved for fuel by the vacuum.

See the hole below.

James

Maybe they figure you won't cold start your bike in a foot of water.

But, at least the water problem can be prevented by running the top two vent hoses up into the air box.

Thanks JD. ?

Well, you're right, that's not a good idea, and it goes against decades of common carb building practice because it creates an unfiltered air path into the internals of the unit. Dumb. How hard would it have been to run that to the intake bell?

Maybe they figure you won't cold start your bike in a foot of water.

But, at least the water problem can be prevented by running the top two vent hoses up into the air box.

Thanks JD. ?

Gray, I was checking out a funny looking little filter today at work that you plug all your carb vent lines into and mount it in the airbox? you ever seen this? I forget the name. Its rectangular and has about 12 nipples sticking out of it...6 on each side.

The hose coming from the bottom of the float bowl must run down or it can't serve its main purpose as a level limiter.

The hose coming from the bottom of the float bowl must run down or it can't serve its main purpose as a level limiter.

WELL............I didn't mean ALL...as in ALL! YOU know what i'm saying.

It does seem as though the dust would get pulled into the bowl using the choke as JD says as it would under normal circumstances as witnessed by motors dying when crossing streams.

Another source however must come from the petrol tank itself which fills with dusty air as fuel is being depleted...as well as the dust that gets through the air filter imo

I would think that vent hose length would also make a difference (as in longer hoses are closer to the ground) and if you ride the bike with the choke on (would make more dust). Just trying to help.

Next, you do understand who JD is, right? You want me to evaluate his opinion? Very well.

Gray, I know who JD is and hold both of you in high regard. I am glad to have provoked further discussion on this topic because I recently had the same problem, but could find no dirt or dust in my air boot, gas tank, or the top of the carb.

This excerpt from 7602racing is consistent with JD's carb vent vacuum theory.

http://www.7602racing.com/prod_t6cf_works.php

"During normal operation carburetors need to equalize the air pressure inside the float bowl with the air on the outside. The air pressure is equalized by way of the vent lines that exit the carburetor, and hang down under the frame. Raw unfiltered air is drawn into the carburetor through these lines (ever notice how dirty they are on the inside). Our T6 filter uses a foam filter element to clean this air before it is allowed to enter your carburetor. The second function of the T6 filter is to maintain a path to open air at all times. Should the vent lines become obstructed or submerged during a water crossing the carburetor will not be able to equalize the pressure inside the float bowl. Your motor will soon begin to bog down, and likely die altogether. You may have experienced this when crossing long streches of water. We have designed the T6 filter so that all the vent lines meet in one shared pocket of air within the filter. This helps to ensure that as long as at least one line remains open all remaining lines will be able to function properly. We recomend that two of the lines exiting the T6 filter be routed up under the seat. This configuration maintains a path to open air even during water crossings. "

I respect you Gray because you are always willing to stick your neck out in the interest of finding the right answer and are open to alternative, well thought-out, viewpoints.