Thread: Both Carbs leaking!

Here is a strange one. On startup after an hour on the ramp on a $100 burger run, my normal instant startup took a few more cranks. Choke off, I cranked a couple of seconds, turned off the key and cranked again. Initial rough running but it smoothed out in 2-3 seconds. Runup, climb and cruise seemed normal. During cruise power descent we smelled strong fuel odors and had brief engine roughness at power reduction. Removing the cowl, BOTH carb drip trays were filled with (dirty looking) fuel. No hose leaks could be found. In retrospect the carb drip trays have appeared dirty prior to this.

One carb was replaced on this plane, leaking, three months and 30 hours ago. 235 hours total now. Normally I don't need any throttle at all to start with choke. Could the fuel pump somehow be hyperactive, sending too much pressure to the carbs?

Any ideas?
WF

The carb was probably flooded and this cause the engine to shake in a vicious cycle. It is possible you might need to replace you float bowl gaskets. When I replace float bowl gaskets I message them with a fine film of white grease. That grease works fine around fuel. This helps them seat in well when the bowl bale is tightened. If the carbs are out of balance the engine shakes and the bowls flood and don't regulate fuel well because they are shaking so bad. You probably had this and spilled some fuel in the drip trays.
I would do two things. First double check my carb balance and after that if you notice any more fuel in the drip trays then replace the bowl gaskets.

WF,
The engine roughness you experienced at power reduction sounds like something that happened to me... at least I thought that’s what was happening at first. After the problem got progressively worse and I did some careful testing (over an airport) I realized that the roughness was actually related to low or negative g loads. Pushing the stick forward or turbulence would cause the engine to miss.
The only thing I could think of was that there was something foreign floating around in the float bowls – maybe water which would normally set at the bottom coming up to the outlet port with negative g’s.
Fortunately, I had just completed a class where we took apart Bing carburetors (thanks Dean). I dropped the bowls, dumped them out, and the engine hasn’t missed a beat since.
You may wonder what was in the bowls before I dumped them. Unfortunately, I didn’t get a good look (I was working outside on a rainy night a couple thousand miles from home) but it seemed like the fluid in one looked different than the other and one had some black residue on the bottom of the bowl which I wiped out before reinstalling it.
To my discredit, I hadn’t been checking the gasolator sump for water regularly, the fittings the fuel caps fit into leak a little where they join the wing, and the plane had been out in the rain.
I don’t know if you have the same problem. Before touching the carbs you probably should determine whether the roughness you experienced was an isolated event or if it is consistent and related to negative g loads. Also, water in the float bowls would not explain either the leaking or smell you experienced, and probably doesn’t explain the starting difficulty either (I didn’t have that).
I wouldn’t recommend removing the bowls yourself anyway, unless you have some familiarity with this operation. You have to rotate the carbs to get the bowls off. That means removing a hard line with a fitting that’s hard to get at. Furthermore the float mounts look delicate and if you have the old-style clamps you can damage the rubber boots that hold the carbs on if you over-tighten them.
Mike Koerner

My mechanic has been on this problem twice now, no joy. The fuel looks to be clean and I have always used a very clean process to refuel. But we saw small dirt specks in the carbs on two occassions, which he thinks could cause the rough running and fuel overflow from the carbs. Our best guess now is that dirt is coming in from the air filtration system. There is a gap where the carb heat flap does not seal perfectly. The hoses from the air filter box to the carbs may be deteriorating (225 hours total time), so these get replaced next along with the air filter element.

Has anyone with a CTSW just disconnected the carb air intake hoses, put the small conical KV air filters on the carb throats instead? It looks like they would fit and be cleaner, although you would have no carb heat. It might also change the air flow patterns in the cowl, depending on how and whether you plugged things up or not. I infer that the air intake system is not the greatest design, since they redesigned it completely on the LS.

WF

Hi Wf

All the UK one's have the cone shaped K&N air filters on them i have the water based carbheat kit on mine very simple just fit it and forget it , it stay's on all year round

Cheers Mike
http://www.skydrive.co.uk/proddetail.as ... %2D912%2D3 carb heat kit


http://www.skydrive.co.uk/proddetail.as ... 551&cat=38 air filters

The latest round on this problem, after three trys from my mechanic, is that it was float bowls overflowing from dirt that partially obstructed the jets. The adjustment of the float levels can change over time, which might be related to experiencing a relation to g forces. But my mechanic's guess is that dirt from the air filtration system can get into the carbs, even if the fuel is clean. (Can that happen?)

He suspects that the hoses between the air filter box and the carbs may have deteriorated and caused the problem. We replaced the hoses, which were the black versions, with the orange versions, after cleaning the carbs out. I only have a couple of hours on it since the "fix," but so far, so good.

This experience makes me more interested in converting to two conical oiled air filters, but I wonder about two things. (1) without the ram air from the NACA scoop on top of the cowl into the air filter and carbs, and with the conical filter drawing hotter air from behind the engine, would it hurt performance? (2) Should the NACA scoop on top of the cowl be plugged, or allowed to push air into the space behind the engine?

WF

WF,

The hot air from under the cowl will impede you performance slightly, just like giving it carb heat all the time. Cool dry air is the best. Then you would need FD's approval. This mod may or may not fly. The Allegro 2000 uses air filters just as you want. Depending on the time of year the opening might help cool in the summer (an unknown), but be a big cooling issue in the colder months. The best suggestion is to leave it alone.
It is possible for the fuel or the air to contaminate the carbs. Unless you can identify the culprit I wouldn't just shoot in the dark and make serious changes.
Bottom line it would work, but the gain may be little to none and cause other issues you have to deal with.

At a recent safety seminar the guy talking about the rotax 912 had picture of black stuff in the float bowl that caused a problem. The black stuff came from the floats. He said it was from vibration and time. Look the floats over real good around where the shaft goes through.

This is why it is recommended at the 200 hr. inspection and every 200 hrs. there after you are technically supposed to rebuild your carbs. Internal parts do wear from movement and vibration. Yes some people don't do anything to their carbs for many hundreds of hours and don't seem to have any issues, but you can and the moving parts and "O" rings in the carb do wear and the tolerances become bigger. Black stuff from float wear,,,, maybe, but I would think hoses first. If the floats are wearing that bad from vibration then the prop and carb balance needs better service. I takes nothing to drop a float bowl once every 50-100 hrs. and just take a peak at the bottom and look for debris. Do I look every 50 hrs, no, but I have never found any debris in my bowls. If something like that starts up you need to follow up.

WF,
I personnally wouldn't recommend changing the air filter set-up either. The Induction system on the CTs is designed so that the air is drawn from a central point outside the warm turbulent air that is inside of the cowling. This does three things, first it allows relativly cooler air to enter the carbs which in turn will lead to a slight increase in horse power. Second because your drawing the air from a central source, it is insuring that the air pressure entering both carbs is identicial this helps with carb balancing and over all smoothness of your engine, think about it this way, your Air scoop balances the inlet air, The carbs and comp. tube balance the manifold pressures and the exhaust system balances the back pressure in the muffler. These are all key points that FD addressed when designing the systems on the engine, and to say the least alot of manufactures never seem to take them into consideration. Oh and before I forget the Third thing that the Central induction system does is if you notice the clear plastic tube going from your carbs to just below your air filter, this is put in place to ensure that the float bowl pressures are identical to, which also plays a role in the smooth running of the engine. (might want to check that out if your having over flowing problems on one carb) If that tube isn't positioned properly, one carb is going to be using outside ram air as a reference and the other carbs will be using warm air from inside the cowling. Just one more thing to check...