Need info. re: carb vent hose running thru the airbox

[Runtoeat]

I've been wondering about my carb vent hose which runs thru my airbox. This is the 1/4" plastic line which connects the vents on the carbs. My vent has four strange looking holes in it. instead of neat drilled holes, these look like someone took a pair of diag. cutters and cut out 1/2" long X 1/8" wide slits on the side of the hose. They are spaced in a manner which positions the outer holes close to the airbox walls. It is necessary to always make sure the hose is positioned properly to prevent the hose from shifting and allowing one of the end holes to get outside the airbox and causing engine vibrations. I've given the hose a wrap of tape and chinched a tie wrap where the hose enters the airbox on each side. This keeps it positioned. If someone knows the proper number of holes, holes size and spacing of these on a factory installation, it would be good info for me. Talking to Roger Lee and doing some research, I see that some installations on Rotax the vent hose does not connect the carbs together. Instead, a short vent hose from each carb is routed down from the vent nipple and tucked under the carb bowl bail wire on each carb and ends at the bottom of the float bowl with the ends of each hose exposed to the general engine environment. As long as the pressure differential is the same at each end of the hose, I guess this works. It would be much simpler than running the hose thru the air box.

[Tom Baker]

Dick, All I have seen had the scalloped type cuts as well, and yes they should all be inside the airbox. It has always been my understanding that the vent should be matched to the same pressure as the air going into the carb. If you have filters mounted dirrectly to the carbs then the vent lines can be right there. Our airplane have an airbox, so that is where the pressure should come from.

[Roger Lee]

What? You didn't believe me yesterday. :lol:

 

They just need the same pressure on each tube and should not be placed in an air stream like out of the cowl. One one gets a different pressure than the other the engine runs a tad rough. This is the first thing to check if your engine has a slight roughness. Takes a minute or two.

[mocfly]

Trust but verify......lol

[FredG]

Dick, consistent with what Tom said, 912 installations that run individual air filters mounted right on the carb intake housing have a hole cut in their rubber flange to allow insertion of the float chamber vent tube into the filter body. From this I infer that the vent tube should be at intake pressure.

 

The August 2012 912 installation manual says this (Section 2.1.1):

 

Float chamber venting lines (1): The float chamber venting lines (1) have to be routed into a ram-air and vacuum free zone or into the airbox, according to the requirements and release of BRP-Powertrain. See chapter “air intake system“. These lines must not be routed into the slipstream or down the firewall. Pressure differences between intake pressure and pressure in the carburetor chambers may lead to engine malfunction due to incorrect fuel supply.

 

Fred

[Runtoeat]

Good info from all. Isn't this forum great? Makes sense that the vent line should be exposed to the same air pressure that enters the carb, now that this has been pointed out. I imagine that the airbox gets a fairly good pressure while flying. Wonder what the pressure is at the normal 115+kts cruise?

 

Roger, the main reason for this post was to verify my funky looking vent hose was correct and Tom has verified it is. I was going to call you back and ask you about this but figured I'd already worn out my welcome with my other calls to you the last few days. :blush:

[mocfly]

Dick,

The pressure inside the cowl varies by location.

[Runtoeat]

Chris, this thread convinces me that there was a good reason for FD venting the carbs into the airbox. If I were to change one thing though, which I can't due to LSA rules, is to provide nipples on each side of the box, similar to the CTLS, which would accept each vent line directly to instead of running the vent line thru the box.

[mocfly]

Dick,

I have found that the structure of the air box could have engineered in area much better. After the actuating arm for the carb heat flapper assembly made the thru hole oblong, I had my A&P bond in a bronze bushing for the shaft to run thru. No more worries about unfiltered air getting into the carbs...

I agree with that sentiment...everytime I see something on the plane that doesn't make sense to me from an engineering standpoint I have to remind myself that this isn't a Cirrus......

 

[FredG]

Dick, one change I would have made would be to not cut the holes in the vent tube so close to the sides of the airbox. That way, there would be more tolerance for movement of the tube side to side, without unporting one of the vent tube holes to air outside of the airbox.

 

Chris, I agree with you about the carb heat flapper assembly. Having no bearing surface installed where the shaft passes through the airbox is a near guarantee for sloppy holes and entry of unfiltered air.

[Roger Lee]

You guys can always replace the vent tube and put the holes where you want them. There is nothing to prevent that. Don't just make one hole. It will need either two larger ones or the three, but closer together.

 

Dick's idea would also work with nipples out each side, but whatever you do here make absolutely sure nothing will come loose or break off because it will go right into the cylinder. Then you are just going to cry.

[mocfly]

I noticed a difference when I changed the position of the holes and the tube assembly itself....Because of the way the carburettor uses the sense ports and balance tube to regulate the mixture it is sensitive to the way the intake air moves, and to the conditions of the intake system.

In a perfectly balanced air induction system, the air will pass thru the filter and move thru the connecting device with no turbulence or pressure drop. The other key is that the clean side of the intake(the area on the carb side of the filter) should not be pressurized. Some intake systems have a ram air bleed to insure that this doesn't occur.

Someday I will setup a desiccator with a bing 64 and see what actually occurs inside the carb as the altitude increases...just need to find the time.

 

[FastEddieB]

...everytime I see something on the plane that doesn't make sense to me from an engineering standpoint I have to remind myself that this isn't a Cirrus......

 

Not everything is rosy in the Cirrus world.

 

A suggestion I made in a post to COPA for a new position they might consider - Obvious Drek Inspector:

 

Several recent threads on declining Cirrus values, what a used Cirrus is worth, Cirrus reliability, etc. have got me thinking.

 

What I think they need at Cirrus is an "Obvious Drek* Inspector".

 

Much of the Cirrus is well thought out, well engineered and substantial. Good job there, Cirrus.

 

But I have to think back to the first time I saw that the double-sided tape holding on the wing root fairings of my 2003 SR22 was coming loose. I mean double-sided tape! The kind you might use around the house, but never for a critical application.

 

Here's a possible end result of that choice (Rick McClanahan's plane here - I just meant to link the last photo):

 

(imagine a photo of a wing root fairing pulling away as the tape pulled away )

 

I mean, what were they thinking? Someone must have looked at options for attaching a needed fairing and decided tape was a good idea. Did they stop to consider its long term durability? The affect of cleaners, or weather, or just time on it?

 

Someone should have been in a position to look over their shoulders and say "NO - that's obviously drek!".

 

Similar items are critical navigational antennas held on with regular Velcro, and the quality of some of the electrical connectors chosen.

 

When my plane was grounded due to a broken ALT1 field wire, I had to look at the broken ring terminal and think "drek!". And this is drek the failure of which takes out a substantial amount of the plane's "all-electric" capability. I'm sure many of you looked at the original Emax connectors and thought the same thing.

 

I'm sure there are other examples.

 

If someone was looking at a Beechcraft and a Cirrus side-by-side, little "drek-y" things like this on the Cirrus would really stand out and might make one question the long term durability and short term reliability of the Cirrus. This may be part of the drop in value many of you are seeing on your planes.

 

Just a thought!

 

 

 

* a slang Yiddish word meaning "trash" or "dung"

[Doug G.]

A similar word (in Greek) shows up in the bible. I find it somewhat humorous that we don't say what it really means.

[Runtoeat]

Really can't complain a lot about my CTSW. It never ceases to amaze me that I ended up with such a high tech machine for the price i paid. There were other LSA aircraft available for me to buy but for me, it was a no brainer to purchase an airplane constructed of carbon fiber with a 15 year history with no problems. At one time, the only machines that offered a carbon fiber structure were the very exotic and very expensive Formula 1 race cars. One could only dream about ever riding in something like this, let alone owning something made of carbon fiber. It blows me away that our CT's not only have a carbon fiber structure, but that it appears to be so well done (in my non-expert opinion). My 2006 now has 690 hours and there isn't anything that shows signs of deterioration. People who haven't seen other Flight Designs come over to look at my airplane at fly-in's and don't believe me when I tell them it's not a new airplane. I've had a grommet glassed into the airbox for the flapper pivot rod as Chris mentions and as many others have also done. If there ever was an example for something being a "Drek", this would be it. Although I'm a happy camper, if I might "bitch" a little bit, there are two other areas that FD might have done better and it must be noted that they did rectify these areas when they came out with the CTLS. The first is the main gear axle bracket. The only way to get adjustment for both toe-in and camber is to go with the Matco system of axle, brakes and wheels (done on "later" CTLS). The other area that could use a better design (done in CTLS) IMHO, is to have a better designed engine mounting setup. The boxer (pancake) 4 cylinder engine produces some unique firing forces and I am not convinced that the CTSW mounts adequately isolate these from the airframe, especially when the rubbers get some hours on them.

[cdarza]

My old vent hose line did not have any holes (in the section from the carb to airbox)  - Although, the line inside the airbox has the holes as discussed in this thread which i understand.      The new vent hose line i purchased has multiple holes in the section from carb to airbox ...    this is ok and normal?

Pic - Holes not quite in focus. 

[Anticept]

That... huh? I'm pretty damn sure that is NOT normal, unless something changed recently. I think you have the wrong hose.

 

I don't have clear hoses anyways, mine are actually rubber?

[Roger Lee]

The original CTSW hoses were clear plastic for about 4" then coupled into white rigid plastic. This then ran through the airbox and coupled to the carb vent hose on the other side. This gave both carbs equal pressure. The only hole in the white plastic hose were three cut outs that were inside the airbox.

 

Now that was what FD had set up. In truth the carb vent hose can be placed many places so long as both carbs are getting the same pressure. You do not want them in a vacuum area or a pressure area. Many Rotax owners have them stuffed right along side each carb under the bale with that 4" clear plastic hose. It comes this way from Rotax if you do not have their airbox. Some have the vent tubes running into the air filter and like FD and Rotax they have them run into the airbox. You do not want a pressure differential between carbs because this vent line causes the fuel level in the carbs to change or be unequal. Both tubes must be in equal pressure areas so the air pressure inside each carb is equal.

 

The issue with a vent hose with holes on the outside of the airbox and then on the inside too is that it is getting two different pressures. You either need to change the hose so the tube only has holes in the vent tube inside the airbox or don't put it in the airbox at all and leave them on the outside of the carb.

 

 

I'm sure you can buy tubing locally. There is nothing special about it.

[cdarza]

OK thanks for that info.   i had ordered the vent hose from Lockwood (as per what was shown in the Rotax  Carburetor parts list)   Many months later, as i was ordering parts again, and had forgotten i already had the hose from Lockwood,  i placed and order from CPS - again, going on the parts listed for Rotax Carb.   Argh - Double mistake.    I guess this hose is ok when in a different set up to the CT - as mentioned by Roger.   

[Tom Baker]

Don't feel bad. I'm sure others have made the same mistake, I know I have. Though it was just once, and a long time ago.