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Roger Lee

Carb bowl obstruction - Don't be stranded

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Hi Guys,

 

Your flying along and the engine looses power to about 4000 rpm and runs rough. It could be a couple of things, but first suspect a carb bowl main jet obstruction. Here is a simple tip that may keep you flying and get your rpm back and if that fails how to do a self rescue and not be stranded and all you need is a screwdriver. You can do this fix and be back in the air in 30 minutes. I have done this and know it works.

 

The debris may never be detected during a run up on the ground. There usually isn't enough fuel flow for a good main jet suction and no sloshing or turbulence of the fuel in the carb bowl. In cruise flight you now have a good fuel flow, turbulence within the bowl and plenty of upward suction to draw that piece of debris right up against the main jet.

 

Here is an in air fix you can try, but there is no guarantee. While you are doing this next maneuver you should be looking for an appropriate landing spot. Either an airfield or a decent road. Stay away from most fields as you are very likely going to damage your front end.

 

Your first instinct is to go full throttle to get more rpm.That only holds the debris harder against the main jet, but it is human nature to try for more rpm. Once that fails and providing you have a little altitude pull the throttle back to idle and glide for about 15 seconds with a little gentle side to side "S" turns. When the throttle is back at idle the main jet is not in play and the suction is gone allowing the debris to hopefully fall away and settle to a lower point. The idle jet is separate from the main jet. After your 15 second glide slowly advance the throttle back to cruise rpm and head for an airport.

 

If this in air idle rpm fix doesn't work then continue to a safe landing.

Once on the ground then take off the upper cowl. Pop the carb retention spring off, then loosen the carb flange screw and push the carb back out of the rubber flange socket. It's snug so push hard. If you have the older stainless steel fuel supply lines you can now lift the carb upward enough to clear the drip tray to take the carb bowl off. If you have the red Teflon fuel lines all the better because you can lift the carbs and rotate them out towards you to have a little better access to the bowl and bale.

Now push or pry with your screwdriver the carb bowl bale back towards the firewall. Drop the carb bowl and look for your debris and toss the bad fuel out. Put the carb bowl back in place making sure it fits into its mounting outside edge grove and pull the bale back in place.Be mindful of the bowl gasket. Push the carb back into the rubber flange until you hear and feel it kind of seat in place. You will feel it snap in place and the carb will look like it is in far enough that it can't go any further. Tighten the rubber flange retention ring screw and do the same procedure to the other side. Once both are done put the cowl on and test run at full throttle for about 5 minutes.

 

If you have a CTLS with the stock Rotax airbox then you will need to loosen the airbox side rubber flange and slide it out of the way to push the carb back out of the rubber flange. So both sides of the carb gets the rubber flange loosened.

 

You should now be ready to take off and head for home.

 

It works.

 

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While you are doing this next maneuver you should be looking for an appropriate landing spot. Either an airfield or a decent road. Stay away from most fields as you are very likely going to damage your front end.

 

 

If anyone want to debate this, we can go to a different thread so as not to divert this one, but when I was sending a lot of students through primary instruction, the examiner would have failed them with this answer. The part of the road being preferable to the field would have been unacceptable.

 

I am not really interested in debating the issue, simply making an observation that someone going for a checkride may want to verify what the DPE wants to hear before volunteering to land on a road before a field.

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Having made an emergency landing on a road only to be ask later if I went under or over the powerlines and having to look to see where they were. I agree that landing on a road is not a good idea. Roger, I know roads are different here than they are there.

If you have a true emergency you shouldn't be thinking about damage to the airplane. You should be thinking about your safety. One more thing if the engine is still running and you can make it safely to an airport that is what you should do.

 

The information on clearing the debris is good stuff.

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Great troubleshooting advice. As to the landing, If I can still make 4000 or even 3800 rpm without the engine shaking itself to death, I'll be flying to an airport for a landing.

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In the high deserts roads and fields can be good choices. Lake beds can work well. Pumice flats are plentiful around here and the soft pumice will shorten your roll-out to about 10 feet then make the take off roll impossible.

 

It used to be that snow stakes were removed for 1/2 of the year but in the new economy they are left up and present a hazard to your wings.

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Most fields are not suitable for the CT landing gear. Most who have landed there have collapsed the front nose gear. It takes very little in bumps, crevices, rocks or small shrubs to collapse the nose wheel and you cant see any of these obsticles from the air until its too late. I have landed on many a road (with and without power lines and barbed wire fences), school yards and parking lots. Roads or other better surface will at least let you take off again or not fold the plane up. Land in most fields and you are there to stay. I set a helicopter down on a Hwy too. I want to not only save me, but the plane too and fields no matter how much wider than a road tend not to be a good place unless you have a plane setup for that type terrain and the CT isn't one of them. Nothing wrong with nice asphalt roads, they are a little narrower and you need to pay attention for obstructions, but there are tons of them out there. Power lines and fences aren't the problem, just lack of attention and planning on the pilot's part.

 

The whole idea here is for self rescue and being able to fly back out.

 

After hearing about just such an incident I took my CT with a passenger and could not maintain altitude at 4000 rpm. I had a great extended glide, but I was loosing altitude. It might be doable with a single person.

 

 

No matter what any of us use as a landing strip in an emergency I hope the above info about a quick and simple way to clear the carbs will help someone some day. I know two that I have used this method on and it worked like a charm and we were off and flying again in 30 min.

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Roger, thanks for taking the time and effort to provide the carb clearing video. The picture quality was really good. I've seen some other videos you've done on the Rotax Owners website and these too are well done.

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The system Roger demonstrated is really a great and easy way to inspect and clean your carb bowl without the need for rebalancing. As he said, all you need is a screwdriver.

 

I recently had an experience with debris in both carbs and could just barely hold approx. 3300 rpm. At that RPM I could maintain altitude; but elected to land at an airport. FWIW I’m a featherweight, had no baggage and approx.. 20gals fuel. Situation might have been different for a size lg. guy with more fuel.

 

As luck would have it Roger was on same flight, came back and did the above procedure and we were on our way .For me it was a great learning experience.

 

I agree with Roger regarding preferring the road over field for the CT. I’ve landed the 152 on fields & roads but the nose gear on the 152 is considerably more robust than the CT.

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I have not been closely following the carb bowl debris threads. We do check bowls at each condition inspection, and have never found anything notable (approx. 1000 hrs. TTIS on one engine, and 600 hrs. TTIS on the other). It seems by the posts here, that power loss is more common than it should be in my opinion. Hence an off airport "self rescue" procedure. Shouldn't this topic roll into the question of why debis is allowed to find its way into the fuel metering system in the first place? And what is the mfg doing to insure the problem is corrected?

 

Also, I'm not a pilot in the same sense as most on this forum, but in my meager 1000 hrs. PIC (no off airport landings), I would be hard pressed to take off from an an off airport location after power loss, in an aircraft that could be trailered out, unless I could absolutely find and correct the problem. To me, "stranded" in an airplane has always been synonomous with AOG on the ramp, not in a field or on a road.

 

Doug Hereford

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On a thread about carb debris, it would make sense one could easily get the wrong impression about carb debris being a big problem...

 

Any carb is subject to getting something in it, and I'm sure there are a bunch of piles of melted aluminum at wreck sites that could tell a story. It's just that, probably because of the glut of 5 year hose replacements, Rotax-powered aircraft have had more than their share over the last year. As the threads here exposed, extreme care is needed when messing with the fuel system. And the barbed things didn't help things. Checking the bowl at condition inspections is smart. I know that the discussion has caused several folks to look... and they found "stuff".

Lots of planes have had off-field landings, fixed their problem, and took off from the same place. Sometimes it's as easy as adding fuel.

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Bad news and good news.

 

Last October I was on a non stop leg direct from VGT to RYN. We took off at the calculated weight of 1320lbs, two on board. I had not flown into RYN before. Weather CAVU.

 

Bad news, at about 30 miles out from RYN engine started to run rough, temps were cold on one side and about 4400 RPM max with much vibration. We found carb heat, and a 100 RPM reduction to 4300 was tolerable vibrations. We were a few thousand feet above RYN elevationn. We checked nearest and started down on course, but could have maintained altitude that day, under those temps, etc. Landing uneventful.

 

Good new, Roger's home base. A small blue piece if material about 1/8 inch in diameter was found sitting on the float of left carb. It had smooth edges and was as thick as ten sheets of paper. Aircraft had more than 100 hoursTT, with no preivous indicaions of trouble. No one had worked on cabs since new, except Tom Baker balance at 30 hours. Ran perfect after material was removed. Next 45 hours uneventful.

 

Thanks ROGER, I have joined the list of those you have helpd. And thank for the new airplane performance after repitching the prop.

 

Farmer

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We had issues with bad fuel hose and the old floats flaking which both added debris to the carb bowls. After installing the right hose and new floats an inline filter was added between the fuel pump and carbs. I like this second line of defense. Here's link for the filter, http://www.golanproducts.com/filters.html - very nice billet design and no concern about heat issues. See "super mini reusable filters."

 

Our CT is registered as an ELSA.

 

Roger Kuhn

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My personal preference would be to *not* use something like the super mini filter. At 10 microns and with 4 square inches of filter surface, It would only take a pinch or two of crud to plug it. The crud could easily get past the relatively coarse filter in the gascolator.

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I was treated to carb bowl contamination, on the way to LSRM training, of all times.

I made a precautionary landing at a nearby airport.. Got a mechanic over ( I wasn't licensed yet.)

pulled gascolator, got a fine full power runup on the ground. took mechanic around the

pattern. Well, it got rough again when I resumed the flight to Corning. I had punky floats in the

carbs. CHips came off easily with a fingenail. I did a carb rebuild (it was instructional...)

Now, I would certainly do as Roger described first.

Oh, the carb float part no. had changed twice since my originals. No problem with the

new ones, through another 250 hrs.

Luckily I was within glide distance of airport(s), so didn't have the field vs road choice.

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I just opened up a set of carbs with 250 hrs. on a 2007 CTSW from the factory. They have never had any fuel hose work or even opened the fuel system. I found a glob of silicone in the gascolator and enough to cover a .50 cent piece and 2-3 black flakes in each carb bowl. Nothing had happened at this point and it was in for a hose change, but it was an issue waiting to happen. Have your mechanics pop the bowls off to check them during the 100 hr and annual inspection. Carb debris can happen at any time and any place. As the original post suggest a quick bowl check or even the self rescue scenario is worth the time and easy enough to do.

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Roger,

 

Are pushing the carbs back from the rubber flange now the preferred method to clear the drip trays? I believe in the past this was done by removing the four intake manifold screws on each side.

 

Roger Kuhn

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Hi Roger,

 

It kind of depends on what I'm doing and where I am. In the field use the one posted above. It works in the shop just as well. If I need to get further into the rear drip tray screw or do something else to the carb more complicated I take out the 4 manifold screws.

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Hi Dick,

 

For the CT there is nothing newer they have been using the same float for years. Once in a great while they can flake, but this is easy to see when looking at them. The float surface should be smooth and without any blemishes. When I say blemish I mean a spot that looks to be flaking or a pores looking spot. Once in a while you may see a seam, but that's okay. Do not scratch the surface or gouge it with your fingernail. If a float is bad it will sink below the fuel level that is equal on he pin sticking out of it. If itis flaking you can usually see a rough spot on the float and see the flakes in the bowl. It does happen, but isn't that common to sweat it.

All the CT Bing carb floats are good for ethanol.

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Hi Roger, thanks for the info. I sent both carbs to Leading Edge for a rebuild and recalibration during the rubber change. No problems noted but I'll have the mechanic remove the bowls and check things out at the next 100 hour.

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Rogerck, going by-the-book, if you remove the intake manifold you need to replace the o-rings. So I prefer to pull the

carbs out of the rubber mount flanges. Put it back in, and and torque the screw to spec. I used to disconnect the air

intake hose on my CTSW, but don't any more. There is enough play to move the carbs. I don't disconnect the

throttle or starting choke cables, and have found the carbs retain balance after the work.

On CTSW, make sure the carb vent tubing that runs thru the air box is centered afterward, the holes

must be inside the airbox.

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Roger and Bill, thanks for the input on carb removal. This method makes bowl inspection much easier than the intake removal method and may result in more owners doing inspections. Bill's comments on keeping control cables attached and not needing carb re-balance was good info.

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