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Water Crossing - Carburator Tubes


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There are many tubes coming from my KTM Carburetor. On my old Kawasaki, I put T's in on a few of the key tubes and ran the tubes I added at the T joints to the air box. The reason I did that was to stop the bogging and stalling when I cross through water where the tubes that hang below the frame get into water and affect the vacuum. I know there are a few of them that affect the vacuum in the carb if the get into water on my KTM also as I've experienced the bogging and almost stalled.

Can anyone tell me which ones on the KTM need to have a T joint to prevent bogging in water? This carburetor is different that my old bike which had a CV Mikuni Carb.

I know the bowl overflow is not one of them.

Thanks for any helps.

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On the Keihin carb all five lines are vents. The tube out of the bottom of the bowl doubles as a drain line. I don't think it really matters which ones you route to the air box. Obviously you don't want to do that with the drain line so I route the two top tubes, one from each side of the carb, into the air box. Some people put tees in the line but I just run the tubes direct.

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Here's my piece on that, with the pics embedded.................

Carb Vent Rerouting

The stock carb float bowl vent routing from KTM is not only slightly overkill, but can cause problems during deep water crossings. The stock carb has a lower bowl vent, and two upper bowl vents. The lower vent goes to the bottom of the swing arm. The two upper bowl vents are on the right and left side of the carb. Each side is split in two. The lower of the two goes to the bottom of the swing arm. The upper of the two goes up and over the top of the carb to the oposite side, and then goes to the bottom of the swing arm (see photo below).

CarbVentStock.jpg

All of these 5 vent tubes are bundled together at the bottom of the swing arm. This allows all 5 tubes to be submerged together, no longer venting the carb to the atmosphere. This stops the flow of gas into the float bowl, starving the engine and causing it to die (usually at just about the worst possible time, literally mid-stream!).

Here's my solution.

OverflowRe-route1.jpg

I leave the lower vent tube on each side routed in the stock location, leading to the bottom of the swing arm. I re-route the two upper vents to gether with a "T" fitting, and take one extended vent up into the air box. This allows the two upper vents to stay above the water level, allowing the carb to vent to the atmosphere, allowing fuel to flow even if the lower vents are submerged (see photo below).

OverflowRe-route2.jpg

Additionally, you can pick up a little "extra fuel mileage" at the same time. The bottom bowl vent has a dual function. it will drain the bottom of the float bowl if you open the Allen-headed drain bolt there (if you think you may have some water gathered by the main jet), and it also vents the top of the float bowl internally by an extended brass tube. Over rough ground when the fuel sloshes around in the float bowl, quite a bit of it leaks out. By making a simple loop with the bottom vent you can prevent this fuel loss. If the bike is layed over, all the vents still work properly, helping to keep the bike from flooding still. And if you want to check for water and use the bottom drain it's a simple matter to temporarily pull the hose out of the routing clip and use it normally.

If you look closely at the photo below, you can see the fuel level in the tube closely matches that of the fuel in the float bowl.

OverflowRe-route3.jpg

Hope this helps some.

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Here's my piece on that, with the pics embedded..................

I like your set up, I had to read your bowl vent piece twice to see its function- that's a plausable solution... never thought of that before. Never worried about it enough to think about it too much I guess. Something to think about. I have heard of guys using that as a rule of thumb for checking the float height. But not affixed like that. hmmm, (thinking out loud) guess long as the only purpose is to drain fuel in a crash and and it has no venting purposes there's no negative effect...

I basicall do about the same thing with the vent tubes, I route the top ones to the airbox and leave the bottom two running through the swingarm. So I didn't use "Ts".

One small fyi tip to add:

or concider is to always ensure you have no loops or high-low-high points in your 4 carb vent hoses that could trap gas after a layover... can/will effect- venting.... (Not related to the bowl vent loop that supertrunk mentioned though...)

👍

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  • 1 month later...

I swtiched to Dave Hopkins' way (per the link DRZnXJnWI posted) after recent issues on a muddy day.

I was going to do something like SUPERTRUNK, but didn't feel like going to get a T. I was really feeling lazy.

The way Dave had them route is to connect the to top ones together as SUPERTRUNK, but with no T. The left line then goes down as normal and the right one goes up, over, into and down to the bottom of the airbox.

The way I understand it the top lines are there to stop the syphon effect should the bike be dumped and fuel starts out the side of the T. Doing it this way accomplishes the same thing with less parts. I've had no issues since doing it.

SUPERTRUNK....I didn't think the float bowl line was a vent, but an overflow should the float run high. If the bike gets dumped and fuel flow out the overflow wouldn't it settle at the bottom of the loop negating the venting effect if it has any?

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Chicken Boy said

"I didn't think the float bowl line was a vent, but an overflow should the float run high."

That hose has two functions;

When opporating it is an overflow, taking fuel from above the float level to minimize flooding, and

When you open the drain screw it is a drain, taking fuel from the bottom. This is how the float level is tested at the Kehein factory.

I like the way Super did his hoses, comes out equal to my route except you have to buy a "T" and have a little less hose.

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  • 4 months later...

I have a question about these vent tubes..

i picked up my new to me 05 450 exc, ran it across town today to the DMV to get it registered...

when i got back, I noticed a pool of oil/gas under the bike.....one of the vent tuves was pukiing some gas...

it was about 92 today and I ran a bunch of stop and go traffic about 5-10 mins before I noticed this...

normal?

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I ran all my lines up to the airbox, except for the bowl overflow. That one I left hanging down. Turns out all my lines were long enough from the factory and they reached the airbox just like that. I snuck them up in the same slot under the tank edge where the cables route under the seat, then ziptied them together there. I was in a water hole this sunday that was almost up to the seat, and no stalling. BTW, I have the bottom of my airbox sealed up, and use black electrical tape on the seams where the side panels meet. I left a very small section of the flap unsealed so the airbox can drain itself after.

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I ran all my lines up to the airbox, except for the bowl overflow. That one I left hanging down. Turns out all my lines were long enough from the factory and they reached the airbox just like that. I snuck them up in the same slot under the tank edge where the cables route under the seat, then ziptied them together there. I was in a water hole this sunday that was almost up to the seat, and no stalling. BTW, I have the bottom of my airbox sealed up, and use black electrical tape on the seams where the side panels meet. I left a very small section of the flap unsealed so the airbox can drain itself after.

The problem with not putting a Tee in them and letting them run down also is if you dump your bike and fuel gets into the lines as it is design to do, you will retain fuel in the lowest point of the line and causae the same problem you are trying to avoid with water. It wil not run. The idea is to have the lines always open whether the bottom ones are in the water, or if you dump the bike.

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  • 2 months later...

Odok

No its not normal, that is not actually the vents (there are 4 in the factory setup) it is the overflow. However we do not fix this with the hoses, it is an internal issue, there is an Oring where the brass seat is pressed into the carb body and it likly got cut when installed at the Kehein factory. I fix them regularly

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if you are moving your vent lines into the airbox run t fittings on each line so they can vent from airbox and at the bottom.

if you directly move them to your airbox you will have problems.

say you dump your bike.......

you get fuel coming out your vent lines

then its trapped in the line because they are higher than the carb

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hmmmmm.....

By looping the overflow hose for the carb bowl ... up and over .... aren't you essentially blocking that overflow tube?

I understand how it would help gas mileage (which we all need!) - but it seems like you might cause other jetting issues by the overall gas level being higher in the bowl (after hard whoops or banging around) due to being trapped in the overflow line?

Just thinkin' out loud, eh... E-Ticket

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if you are moving your vent lines into the airbox run t fittings on each line so they can vent from airbox and at the bottom.

if you directly move them to your airbox you will have problems.

say you dump your bike.......

you get fuel coming out your vent lines

then its trapped in the line because they are higher than the carb

It doesnt really matter. If you dump your bike in the water, the best thing to do is drain your carb bowl.

I did not run my overflow line up to the airbox, it is still down where it is supposed to be, by the swingarm.

Lastly, I have a 7 series battery. I can crank it till the cows come home. By this time those lines will eventually drain back into the carb bowl that has drained itself thru the overflow.

I have proven this over and over as I drop my bike all the time and I have never had the issue you mention above. Remember the bowl has 2 vent lines, one each side. They will never BOTH overflow at the same time. If I lay my bike on the side, the gas in the bowl is now draining out from the overflow and out from the lowest vent line. The other one stays clear and pulls in fresh air. Remember Physics class guys? We have gravity on mother earth, gas will go down to the lowest possible point. The bike would have to be upside down for a couple of minutes to have both vent lines filled. Even if they do, they will most likely empty themselves into the airbox.

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as stated above there are 5 carb hoses to deal with. Separate the hoses and simply run the 2 hoses that go up into the top of the air box, and the other 3 go down behind the engine in front of the swing arm. When you cut off the excess hose use a angle cut to prevent clogging in the end.

DBM

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