# Bernoulli and carb T-vents

Discussion in 'Old's Cool' started by HellSickle, Sep 26, 2005.

1. ### HellSickleScone Rider

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Bernoulli was on to something. Way back in the 1700âs he built the framework for the fluid dynamics behind heavier than air flight and the carburetor. Bernoulliâs Law states that for horizontal fluid flow, an increase in the velocity of flow will result in a decrease in the static pressure.

Hereâs the most basic principle to understand about your carburetor. As air flows into a carburetor, the area available for flow narrows. For a given mass flow rate, this means that the speed of the air increases as the area decreases. Because of the decreased pressure in the throat, the atmospheric pressure can push fuel through the jets into the air stream. This is all assuming that the fuel in the bowl is still at atmospheric pressure. Thatâs what your carb vent hoses are for.

If the vent hose to your carb bowl gets plugged up with, say, water, then you will no longer have the pressure differential needed to push fuel from the bowl through the jets. If this happens in a water crossing, this could be you:

Many bikes are prone to this problem unless you make a simple modification commonly known at the âT-ventâ. The basic principle is to insert a T-fitting into your carb vent line as close to the carb as possible, and route an additional vent to an higher location on the bike. This will allow you to do deeper water crossings without plugging the vent hose, which is usually routed to the lowest location on the frame.

After nearly having my bike die in a deep water crossing on our last ride, I pulled the tank and seat to install a T-vent. I could have sworn I had one in the stock plumbing. What I discovered is that while it was T-vented, the T was down near the swing arm pivot, thus reducing my enabled water depth by several inches. The following pictures show how I installed a new T-vent at a better location.

Here is the before picture:

You can find T-vents such as these at your local hardware store:

As it turns out, I chose to use the Y-fitting that the Honda engineers has chosen to put at too low of a location on the bike (sorry about the focus).

When I was done inserting the Y-fitting, this is what it looked like:

If you look closely, you can see where the new upper vent is routed upward, but loops back downward at the end. This helps prevent water and dirt from falling into the hose.

As a final modification, you should make some changes to the very bottom of your vent hoses. By cutting the end of the hose at an angle, and slitting an inch or two up the hose, you make it less likely to plug with mud.

If this can prevent even one set of pickled feet, it will have been worth it.

-Jeff-
2. ### cRAsHBanned

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Excellent!

Thanks Jeff, I'm on it.

No more pickled feet for me if I can help it...
3. ### SantaFocused on the Future

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Excellent piece Mr Hellsickle.
Santa
4. ### arroyosharkNeeding some space

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Any particular reason you couldn't just run one longer piece of tubing up above airbox, under the seat, and then turn it down? Esteban thought this would work comparably to the t-mod.

I have a DRZ.
5. ### GregInBoulderJerry rules. Get over it.

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My thoughts exactly.
6. ### cRAsHBanned

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We could start a club...
7. ### HellSickleScone Rider

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Are you suggesting that the open end be left up high?

The biggest drawback is that when (not if) the bike is tipped over, the gas will drain into this higher location. Also, in the event of a stuck float valve, your bike will flood horribly, rather than the gas simply running out the overflow vent.

Still, this idea has some merit. In preparation for a water crossing, I can see a lot of advantages to temporarily shutting off the lower vent hose. This would prevent water from entering the carb if it should reach that high. Of course, at that point the water is probably coming in the air box.

I have another theory about the T-vent. Sometimes, while riding rough terrain, gas will slosh up into the bowl vent hose. In a normal vent, the gas will try to run downhill. If there is enough, it could plug the hose as a vacuum in the bowl trys to pull it back up. I believe that this is responisble for the spluttering that sometimes happens in rock fields. With a T-vent, the hose will quickly drain, since it is now vented much like the plumbing in your house has to be vented. Building codes are very strict about vents needing to be within a certain distance of the drain. Without the vents, your toilets wouldn't flush very well at all.

-Jeff-
8. ### DoctorItvrooom!

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Until you go supersonic of course.

Good post though, science is fun.

Every once in a while I have to remind myself that I do know something about flows other than the creeping kind.
9. ### SteveAZLong timer

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I remember nearly a decade ago working in the chemical industry. I was working on large distillation equipment that were controlled by monitoring nothing but pressure differentials all over the place. Often heating was by steam and cooling by water. Many things learned from this and some very non-intuitive issues came up trying to accurately measure (and control) the pressure differentials of vapors, liquids, mists and gasses. But one very big "rule" was that you always needed high point vents and low point drains and no horizontal (less than 10:1) runs. Every high point would need a vent and every low point needs a drain. Heck even household plumbing basically follows these rules.

Around this time there were a lot of folks having problems with there motos stalling in water crossings and in heavy rain. That got me thinking about these very same things and how they applied to or carbureted motos. Sure helped out a lot of folks that were having problems back then.

But to do it right, just likes been posted, you really should have a high point vent *and* a low point drain on the carb vent. No hose runs that are closer to horizontal than 10:1 too...
10. ### StephenLong timerSupporter

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Excellent.

Just did this to my 950 yestiddy.

Except now I'll have to re-do it with the upper loop and the bologna-slash bottom.

There has been much discussion over in OC about where the vent lines should end up. Apparently, leave the vents in just the right spot and good ol' Mssr. Bernoulli steps in when the wind is right and has some fun with the pressure in yer float bowls, yielding some interesting surging effects. What a prankster, that guy! I'm thinking (wishfully?) that the T will defeat Mssr. B when the ill winds blow. Any of you pro fluid guys care to comment?
11. ### Yellow PigAllergic to asphalt!Supporter

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Any special notes for a DRZ400S where there is a vacum line that goes from the carb to the fuel petcock?
12. ### HellSickleScone Rider

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Some of the newer 4-strokes, most notably the DR & WR, have way too much plumbing on the carb. If you're unsure which hose is which, consult a good service manual. Perhaps a quick way to check would be to lay the bike over & see which hose gas dribbles out of.

You certainly don't want to do anything with any of the vacuum lines.

As we discovered on Crash's bike, the DR lacks an easy way to drain the carb. You have to pull the plug on the bottom of the bowl, rather than simply turning a drain screw.

-Jeff-
13. ### HellSickleScone Rider

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I hadn't thought of this before, but I can see this being a problem. If the sole vent hose is in an area of variable pressure, that's what the carb bowl would see. Yes, as long as you put the upper part of the T-vent in a sheltered area, then it would stop this from being a problem. I sometimes run the upper end of my T-vent hose into a hole drilled in the side or top of the air box.

-Jeff-
14. ### Disquisitive DaveNot so wise fool

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So last week I did my first proper steam crossing.

The engine died hard when the water got about 1/2 way up the case. Pickled feet for me! So I need a T-vent eh?

I've got 3 vent hoses hanging off the carb. Looking at the exploded diagrams doesn't seem to help me figure out which hoses need vents. Do em all, eh? OK. First an experiment. I start the engine and stick the ends of the hoses into a bucket of water one by one. Engine keeps on putting away. Even with all three vent hoses underwater. Even at 5000 rpm. There must be more to this story...

Ok so there were some non-trivial sand bars in the stream bed, but I don't think the stall was caused by bogging in them. Could sudden cooling be an issue?

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