I have a 3W 100 and am having some difficulties when inverted. If at part throttle, it will sputter, act like its loading up. At full throttle, I have no problems. I was having general issues with throttle transition and leaned out the bottom end. It cleared up, but it remains to have issues when inverted. Any ideas??

It sounds like the low end is FAT. Try leaning the low end until the engine will not transition well and then back up slightly.

Do you have a fuel pump diaphragm vent tube adapted, a shroud or the 3W intake adapter with the vent tube?

I have a vent tube, no shroud.

All what to do is to modify the carburator.
This will finished all inverted flying problems with any gas-engine.
To see at www.zdz-modelmotor.cz under " news and FAQ´s.
The Bing and the Walbro carburator have the same construction at this detail.

I am also having inverted problems with a 3W 150. I have tried everything including a spring change in the needle. I am wondering if there is a mfg. condition that some of them fall out of tolerance and the spring needs more tension. I will try it and repost.

Chris

That's a nice photo series on the carb. What are they actually changing. I'm at work and can't spend too much time on it. Thanks_bob

They're strenghtening the spring that regulates what's called pop off pressure...does it work ? Maybe...The Walbro engineer says it doesn't make much difference, there is a factory setting for each carb, depending on the end use...

Are they pulling it apart and then micing it?

Interesting. Looks like it is being pulled apart a total of 2.47 mm and re-installed. I'll give it a try.

They are lengthing the spring. It will put more preload on the popoff or the needle valve that lets gas into the metering side of the carb. Gokarters regularly change this setting and have special pressure guages that check the point that the valve releases. You can actually get springs of different strengths to set this valve.

In any event I will also try it tomorrow. My 150 four strokes/goes rich when inverted and I have changed everything. I have flown with the cowel off and it made no difference. The fuel tank is just above the carb in level flight and below it when inverted. I am now assuming that the pump works harder inverted and overcomes the popoff pressure some how.

I am beginning to think there is something else wrong with the engine but I sure don't know what. I even tried turning the ignition upside down.

CB

I have had the same problem with my 3W100 and have tried venting the regulator reference point into the fuselage (no help). Also bought the 90 degree intake manifold and connected the regulator to the intake manifold (no help). It is better when you run the low end needle valve very lean, but then it sometimes quits.
I thought perhaps that excess fuel was somehow pooling up in between the carburetor and the cylinders since the carb is below the engine in normal flight. Then when the engine is turned over, gravity causes this excess fuel to be dumped into the cyclinders.
I admit this doesn't seem possible, but something is clearly happening here. Can anyone explain why changing the pop-off pressure fixes this? I believe the tank is pretty much in line with the carburetor anyway. And if the pump side of the carburetor is pumping properly, it shouldn't really matter.
Anyway, I may go ahead and attempt this modification. Others please let us know if this is the fix.

My 3W-100 suffered with this problem going rich when inverted. I rebuilt the carb - new gaskets, diaphragms and inlet needle. That didn't do much. I put in new NGK CM6 spark plugs. That didn't do much. I installed the 90 degree carbon fiber intake horn and reset the needles on the carb. That fixed it.

If the engine is running rich when upright and level, the excess fuel will pool in the bottom of the reed cage. If you roll inverted, this fuel will get dumped onto the spinning crank and get whipped into the transfer ports, making the engine burble very rich. The intake horn helps to ensure that the mixture is correct over the entire throttle range so fuel won't puddle.

The gas pooling in the read block area is not happening, I really thought it was the problem. My problem is with a 3W-150 in a Carden 40% 330. I have flown the thing around at low throttal killed the engine, landed, and checked the gas content-none in the reed block. If you think about this for a while you realize the pump pulse holes are there and the carb wouldn't work if it was full of gas.

I have also put on the elbow and it had no effect, I have even turned it around facing forward, no change. To make this worse I have a 3W-100 in a Areoworks Edge and it works perfectly in every attitude. I just vent the senser to the fuze.

I am increasing the spring length tonight and trying it tomorrow morning, I will let you know.
CB

A good reason to learn to ADJUST your own carb, don't just buy it and fly it with whatever settings it comes with.....
There is no way ANYONE can predict your prop, rpm, altitude, mixture, etc....The gasser carbs are not very touchy, and will run the engine when adjusted WAY wrong...

Does your engine load up while taxiing out for a flight?
If you nail it to full throttle after extensive idling, does it take some time to clear out and reach max rpm?
Does the problem get worse, the longer the motor has been idling?
If so, these are the signs that you have fuel puddling at low throttle conditions.

So the adapter will correct this? Some have claimed to have installed the adapter with no success. I think I'll try the spring and keep fiddling with the mixture for a while.

Keep the info coming. This is just what we need to stay on top of our engines and their maladies. That being said, I understand that the 100 is no longer available and is replaced by the 106. Does anyone have anything to say about the newer designed engines? The obvious differences are the cylinders have an opposite stagger (RH Forward) and the carb blocks are of a different material. Have any of these changes made a difference in the running characteristics of the 106 class engines_bob

Lean or Rich is the question. After many, many adjustments, new carbs, intake elbows, new ignitions, flying without a cowl, and many other changes I still have an engine in a plane that everybody has, that won't run inverted and an engine that runs perfectly in another plane that everybody has and there is no obvious major difference between them.

I checked the spring tension between two carbs. If you lengthen one spring and compress them over a brass tube, both springs compress together at the same point. The only thing that I can see is the lengthened spring has more preload at the compressed length in the carb. This should put more pressure on the popoff and reduce fuel pressure on the low end, hopefully reducing fuel dribble or whatever is happening.

As always if anybody has any ideas please chip in.

Chris

well... The spring thing didnt work for me! Couldnt tell if the spittering stopped because the engine wont run now! Fires for a few minuets, sloppy transition, then dies. Guess I'll go back to new spring and adjust, adjust, adjust.....

Sometimes the little leve that lifts the inlet needle gets bent...It should be flush with the casting just next to it..Be sure you get the fork in the diaphragm hooked into the lever, if it has that type...Also the gasket goes against the carb, with the diaphragm on top......

The adjustment on the low end needle was the fix on my 3W85,did the same thing inverted until the low end was leaned out.
I have the new 106 bob-nj and it is a POWERHOUSE !!! It runs great in any attitude and pulls the Hangar 9 Sukhoi I have it in with no problem. No loss of rpm's or speed on the uplines and you definately have to throttle back on the downlines.It runs very well and very smooth,I can't wait until it breaks in and I get used to the plane,great combination !
I have personally talked to a couple of guys who have both the new 106 and the old 120 and have compared the 106 in power to the larger 120, I don't have an old 100 or 120 for that matter so I can't comment personally on that but the 106 I do have has alot of bang for the buck !