oddy

Hello Everyone.

I attached a tuned pipe and a long header to my Mitsubishi 33cc conversion with very good results. (7,800 RPM´s at 2,400 meters osl with a 18x6 prop)

Anyhow I´m having troubles, I set the needles and get very good RPM´s on the floor and when taking off.

But at the first minute of flying the engine began acting as if it was flooded, even closing the high needle

The problem comes even worst If I Invert the plane it lowers down a lot the RPM´s getting totally flooded and getting back to normal when getting the airplane back to a normal flying position.

The engine is mounted with the piston up

Could this be a problem with the membrane of the carb being old and not flexible enought which is allowing to much gas to the carb when inverted?

In advance thank you very much for your help.

av8tor1977

My Feedback: (6)

When an engine runs good on the ground and then goes rich in flight, it is usually caused by the carburetor regulator sensing ram air from flying, which makes the carburetor go richer. What you need to do is solder a 1/8" tube into the regulator cover on the carb, and then run a hose from there into the fuselage where it can sense static, and not ram, air. The regulator is the cover with four screws and an open hole in it. When you make the existing hole larger and solder a tube into it, make sure that you grind everything flush and smooth on the diaphragm side of the cover so that the diaphragm does not get damaged by any rough or high spots on the cover.

Also note that when running an engine with the cylinder oriented upwards, if the engine is running a bit rich in general, fuel can accumulate in the crankcase, especially if you are flying around at mid or low throttle settings. Then when you invert the plane, the accumulated fuel drops into the airflow inside the engine making it suddenly go very rich.

AV8TOR

AA5BY

Good advice from avi8tor.

I'll add that among my several gas planes, I've one that goes richer inverted. I suspect the cause is gravity influence on the atmospheric diaphragm. It is a rear carb 35cc and the diaphragm is on the bottom of the carb when upright. I suspect that the little spring is slightly weak and that when inverted, the weight of the diaphragm opens the float needle slightly more. If/when it gets rebuilt, I'll substitute a slightly stronger spring and see what happens.

pe reivers

+1 for AV8tor

av8tor1977

My Feedback: (6)

Thanks guys.

AV8TOR

oddy

Thanks everyone, I will be doing this modification today and testing it on Sunday.


Thank you very much.

rcairboater

Care to share a pic of this? I would be interested in this.

av8tor1977

My Feedback: (6)

Sorry, I don't have any pics to document it, but I have seen/encountered it many times. Even a glow engine will do it. When it is idling very rich, you will notice the engine slowing down more and more. Then when you give it the throttle, it will blubber and sputter way rich until it has used up that accumulated fuel in the crankcase. As a matter of fact, that is why the idle bar glow plug was invented. The rush of fuel from a semi flooded crankcase would douse a normal plug after low speed rich running, followed by acceleration which made the mixture go way rich. The idle bar prevented that raw fuel from hitting and cooling/dousing the glow plug.

The same thing can happen with an auto engine fitted with a carburetor on a very cold start. The fuel falls out of suspension and puddles in the intake manifold. That makes the idle/slow speed mixture lean and the car stumbles lean and has trouble running. Then, when you accelerate, the rush of air vaporizes some of that puddled fuel, and then all of a sudden the engine is too rich. Baffling to trouble shoot for mechanics that don't understand what is happening. It is especially problematic on hot rod cars that have had the manifold heat blocked off, and are running an aggressive camshaft. The problem is not very often encountered now with modern fuel injection.

So, back to gasoline model airplane engines. Just try it sometime. Set your idle mixture about 1/2 turn or so too rich, and then let the engine idle for a while. You will notice it slowly slowing down. Then, accelerate rapidly. The engine will go very rich, and stumble and four stroke and burble until it has burned up that accumulated fuel in the crankcase. The rush of air and turbulence from higher rpms "scoops up" that fuel and puts it back into suspension, causing a momentary very rich mixture. (This experiment will only work with the cylinder oriented upwards.)

I have a flying buddy that has an airplane with an upright cylinder mounted engine and a combination of a too rich midrange/idle, and a too small prop on the 25cc engine in his plane. You can hear it going into an on and off four stroke mode when he is putting around at half throttle. Then, when he inverts the airplane, it puts out a big puff of smoke, four strokes for a bit, and then cleans out. It is that accumulated fuel in the crankcase getting suddenly mixed into the moving fuel/air stream in the engine. It kind of makes me crazy, but he is happy with how it runs and doesn't want to fine tune it, so who am I to worry about it?

Anyway, it is just a fact of operation of two stroke engines, and sometimes with four strokes too. That is why polished intake ports on four stroke engines are no longer the rage. Now it is understood that some turbulence in a four stroke intake tract helps keep fuel from falling out of suspension, causing carburetion problems.

Now after having gone through all that, you'll probably tell me you were referring to the ram air problem! However, that has been well documented here in the forum many times.

Edit: Well, I tried to post a couple of pics of a carb modified for sensing pressure from inside the fuselage in case that was indeed your question, but the photo posting function for RCU is on the fritz.... AGAIN!! [:@]

AV8TOR

av8tor1977

My Feedback: (6)

SERVER ERROR - 500 INTERNAL SERVER ERROR.

Couldn't upload pics last night.... still can't this morning. [:@]

AV8TOR

av8tor1977

My Feedback: (6)

Here are a couple of pics of a carb fitted with the remote sensing static air line. The yellow tubing will go into the fuselage in an area of undisturbed air. This allows the carb to sense actual ambient air pressure and not the higher ram air pressure or prop blast pressure that is often present inside a cowling. It doesn't even have to be in a cowling to be problematic sometimes. If the installation ends up with the regulator cover facing the oncoming air stream while flying, or is in the prop blast, it can throw off your carb settings. It doesn't always happen; Bill from CH Ignitions calls it "black magic." Sometimes a remote static line isn't necessary. But if you are having problems with an engine running great on the ground but then going rich in the air, this is the first thing to look at. I generally just go ahead and install them on all my setups just to eliminate a potential problem area.

AV8TOR

pe reivers

Why not connect that remote static to static ports and get REAL static pressure?????????????????????????????????????????? ??

av8tor1977

My Feedback: (6)

Yeah PE, you're probably right, but running the line into the fuse seems to work well enough, and saves the hassle of making a true static port.

AV8TOR

Nosedragger

You can't set needles with an expansion chamber on the ground and expect it to work in the air.When the prop unloads and rpm rises the mixture will be wrong.


That is dead wrong. The Walbro is an ALL ANGLE carburetor.Get Walbro document C-1022.

you probably answered your own question, replace the metering diaphragm and the pump diaphragm.

AA5BY

I'm all ears if you have a more likely explanation... otherwise, that is my theory until I hear a better one.

Nosedragger

Read C-1022

AA5BY

The manual crashed my Adobe plugin reader. No loss, I doubt that I need to read the service manual that likely says that a Walbro carb is an all position design as I'll stipulate that.

If you read my full post, it noted that I've many gas airplanes and add that most of them run Walbro or Walbro knockoff carbs and I've only one of the many that richens when inverted. What I know for a fact... is that it does that. I also know that there must be a cause.

One more time... until someone provides me with a better theory, my belief is that gravity plays an effect with the metering circuit on that carb. The atmospheric diaphragm has the task of providing a constant to wit the needles can be set. If the needles do not hold a mixture setting, then it likely is that the metering circuit is varying. As Av8tor outlines, varying ram pressures in the cowl can do that but simply inverting the aircraft doesn't change ram pressures. What does change is the gravity effect upon the components of the metering circuit and albeit they are designed not to be influenced by gravity and evidently most often are not as evidenced by my many planes that will invert with no issues, I've one plane that does have an inverted issue.

What would you have me to do... tell myself that the plane is not richening when inverted? How do I go about persuading other observers that they are also mistaken?

pe reivers

Please define Hassle?
You mean this is much work?

av8tor1977

My Feedback: (6)

Yeah!

As to the other comments, the Walbro is indeed an all position carb. However others have noticed as well that if the carb is mounted on the aircraft/engine so that the metering diaphragm is in a horizontal position, high "G" loads can cause a momentary change in mixture. The Walbro is an "all position" carb, but not necessarily an "all "G" loading" carb.

The only thing that will give you a perfect mixture in any and every possible condition is fuel injection. (Maybe.)

AV8TOR

oddy

Hello Everyone.....

Just to tell you that following the recomendation, I solder a tube into the pressure hole and run a tygon line from there into the fuse tank compartment.

All my problems were solved.....the engine gets easily tuned with just 2 turns of High and low needle,,,,,and never varies in the air giving me a great flying sunday with 7,800 Rpm on a 18x6 wood prop and very happy to finally being able to get my conversion engine completely ready, (and without noise given the tuned pipe and long header)

I´ll post some pics as soon as I get my camera.

Really thank you very much to all (happy to say that I convert this engine from scracth following all the recomendations given in this forum)

Best Regards
VÃ*ctor.

AA5BY

Victor... good to hear the report. Do keep an eye on cooling as a need to run an atmospheric line into the plane often means there is ram air pressure within the cowl and it can have another affect, that being inadequate cooling.

pe reivers

You are right, but only when the large WB carbs are concerned. WA and WT type carbs on smaller engines do not suffer from this, unless in high G manouvers where the membrane weight makes itself noticeable.

Nosedragger

what a waste of my time
two different guys with seperate problems

pe reivers

Nobody forces you to read all the crap we post.

Nosedragger

yeah I know, they make up for it by ignoring my efforts to help solve problems.

pe reivers

Like referring to C-1022 without posting a link or using quotes?
I know it's the Walbro service manual, but who else does as well? Come on, be fair!
Even then, The service manual does not provide a solution for the WB carbs going richer with the regulator on top (carb in downdraught position). Their carbs are not meant to fly and experience 20+ G-forces while doing so, but are built for hand-held appliances. We find out they are not perfect, so we are searching for solutions to the problems.

If that is like wasting your time, like I said, nobody forces you to read on.