I have a 3W75IUS, mounted inverted. When not in full power, level flight, the engine gets richer. I tried to lean the low needle but won't make anything as leaning it will caus the engine not to run anymore. I heard that it is because airflow problem which I strongly suspect it isn't the problem as there's no change in air flow when the airplane is inverted, same speed, level flight, etc.
- Is it possible that the excess of fuel spitting out the carb would richen it in this condition, as the carb inlet is now upwards ? If so how to correct it ?
- I heard about the weight of the diaprhagm. How could it be corrected ?

What are your experiences with this ?

Regards

Juan

The weight of the diaphragm is irrelevant (or so slightly relevant as to be irrelevant).

There could be one of a couple things happening. The most probable one, if you've not already addressed it, is related to the carb diaphragm. It's possible that the airflow past the carb is interfering with the metering diaphragm's operation. One of the carb diaphragm cover plates has a small breather hole in it. This is to provide static atmospheric pressure on the metering diaphragm. Airflow can change the pressure it sees, which will change the mixture. Many people solder a bit of brass tubing onto this hole, and route a fuel line into the fuse to allow it to sense in a stable air mass.

Could also be that the cowl and airflow are causing a ram air effect on the carb intake, this can cause mixture problems. This is likely if the problem goes away when flying without the cowl.... It can then be rectified with clever baffling and venting...

Good luck,

J

Several possible issues. One is ram air against the diaphragm opening, which does change with aircraft attitude. You don't want impact air, just static pressure. Another is fuel making puddles in the reed block. Both are common and require experimentation to correct. If the carb is spitting fuel you may have a reed that's bent or not seating correctly. Check and correct that first. A slim possibility is that you have an air leak at the carb isolator or reed block mount.

The diaphragm is the easiest to work with but may require several attempts to correct. One method is to run a tube from the diaphragm opening into a "dead air" space either inside the fuselage or inside a small box built out of balsa somewhere. It needs a pin hole to maintain correct static pressur. Another is to run a tube from the diaphragm opening to the front of the carb intake.

Good luck.

I wonder if anyone ever tried a pitot tube construction for carb reference pressure?
From information I have gathered so far, if the connecting line to the carb membrane plate is too long, it may set up pulsating vibrations and thus upset good carb function as well. In case of a long line, the reference line should be provided with some form of choke nearest to the carb connection. (i.e. piece of foam stuffed in)

You did not mention if you have leaned or adjusted the top end. Just some thing else to add.

Hi all:

Perhaps I didn't make my self clear with my first post: My problem happens only when the airplane goes from normal flight to inverted, or passes thorugh inverted position when rolling or loop for example
I understand all your inputs about air flow, but one thing that throws me off is why is there a change in airflow when inverted under the same flying condotion ? Eg level flight to inverted still level. This is why is surmissed about the fact that the carb in normal level flight spits the excess of fuel out, and when inverted the fuel is soaked in enrichening the mix.
However, I do have ( I know it is wrong ) ram air impacting the carb's inlet which in turn could potentially cause turbulence, but the bottom line (I guess) is the extra fuel being spit out that is being soaked back in. I already leaned the low and high with no results.
I will do a proper baffling, as well as run a static pressure line to the diaphragm, and will let you guys know results.

Once more thanks for your valuable input.

Juan

Fuel spitting out of the carb could be a reed seating problem. As for the diaphragm issue, you would be about number 3,000 to have experienced the problem if it is ocurring. Fuel puddling inside the reed block is still one of the potential issues, which is one of the reasons 3w came out with a new aluminum reed block sometime back. Warpage of the plastic version was another

Juan:

You didn’t say how much run-time you had on the engine; however, I too have a 3W75US mounted inverted inside a rather tight cowl. Initially, I also had nearly the identical issues as you describe, so I tried several different locations for the static pressure vent tube and various needle settings—nothing worked. Finally, I resorted to doing away with the remote vent pickup—just the short nipple coming out of the carb. I figured I would just have to live with this crazy behavior.

The good news. As the run-time on the engine increased (about 10 gallons later) and after continued fine adjustments to the carburetor, the issue went away! I’ve now burned nearly 20 gallons and, with no other changes, the engine runs solid through all flight regimes without any noticeable changes in mixture—rolls, knife edge, snaps inverted, it just doesn’t care.

The bad news. I recently found that the engine would momentarily cut out completely during some maneuvers! Although these occurrences were only a second or less, they were very disconcerting. I won’t go into all the details of the time wasted trouble shooting this one—instead here is the bottom line. I utilize an electronic kill switch and, while I had correctly set “off� parameters to positively kill the engine, I accidentally had set the “on� parameters in the transmitter set so that the electronic kill was just barely “on.� All it took was just a little noise while in flight or a slight reduction in radio signal and the kill switch would do its thing—kill the engine!!

All is right with the world now! The 3W74US starts every single time on the second flip, never misses a beat and continues running like a Swiss watch. Hope this helps.

Ed

I have also had this same symptom with some new 3W engines. Although the other suggestions are all good ones for most any gas engine, I have found that this issue goes away completely with time on the engine and the leaning out that comes with it. After you get past break in and are on full synthetic at 50-1, and the needles are tweaked, all these problems seem to go away on mine. I had the same problem with the 3W 55US that I just got broke in last year. I also cut those looong tillison needles down to size (like Walbro) and re-slotted them, while I was at it.

I know it isn''t fun to go through the process, but my engines never quit altogether, just an annoying sag, till the came in. It is worth the wait for these power houses. Better have your 5-way on.

I find it easier to just adjust the needles on the first run as you would at any other time. Tweak them if need be when more fuel is run through.

Unfortunately, I have recently encountered the same problem with my 3W75. Having some wizbang engine mechs. on my side has done nothing to resolve the problem. I have a lengthy static presure vent tube off of the diaphram cover and this may be contributing to the difficulty. My carb intake is directly in the path of both airflow and propwash.
I called the 3W importer who is local in NJ and he afforded some insight (perhaps being aware of the trouble). He says the air over the venturi creates a vacuum in flight due to both the propwash and the added air speed. He wanted to know if the engine ran ok on the bench and I told him it did. He said that I didn't need a new carb. but needed to make a small modification.
He suggested a piece of metal mounted to the screws of the throat of the carb. to force air into the venturi as the plane moves forward. I have done this but haven't tested the results as yet.
I'll report back with the answer.

The two airflows: 1 from the prop wash, and the other from the plane moving through the air, causes too much air across the face of the intake of the carb.. It acts like a siphon (it sucks the air out of the intake instead of pushing air into it). The carb then goes rich and chokes on too much fuel. If the intake is exposed directly to airflow, you might have this problem, but if you can cover the intake with the cowl, it should stop. It is just too much air over the carb. If it runs ok on the ground with just the prop wash, then it is as I have suggested. Since I could not get the intake under the cowl, I had to buy a 3W velocity stack which has worked out very well for me. The 3W distributer here told me that I could make one from sheet metal that would direct air into the intake but I just bought their stack and have no more trouble.I hope this has been of some value.
I could not email to Christian so this is for him in Romania.

I also had the same problem with a 50i and a tillitson hd232 carb and and plenty of hours. I asked for help here and got nothing. My engine mixture would change even when turning / rotating model over on the ground and was worst at about 1/2 throttle. Ian nicols at IADMODEL DESIGNS in scotland put me on the right track. He said that it was a problem with the needle seating and the weight of the diaphram and needle going from say vertically done to vertically up. In the end I fitted a few walbros from chainsaws as a test and saw an immediate improvement.I finally fitted a carb from a DA, as it was all that I could get. The problem went away until ignition packed up. The tillitson has 4 holes in the venturi and makes for better and smoother midrange, the walbro gives more of a step type of throttle response. Having proved it with various carbs even smaller bores that they all ran and ran better even if they were off a smaller engine eg 30cc. So who knows. The 3w has been replaced by a mt 57 because reeds are now worn (£20) and ignition faulty (£119) excluding postage and new engine £250. Hope this helps because the 3w is a powerful motor ie lots of torque loves large props at about 6000 6200 rpm

Hi,

I have a similar problem with my 55i US ..

After 1 year , after many fights over email with the guys from 3W ... after many modification of the cowl to overcome the airflow problem I decide to open the carburetor and see what the hell is going on. The caburetor is ok but the reed valves was .. see for yourself.

I've ordered from A.I. the new valves and i'm waiting for the flying season to begin .

I had a similar problem with a 3W 100 twin. The reeds looked physically fine (no cracks, curls, splits). I replaced the entire ignition and carburetor. I put a gas tank on the ground and ran the fuel directly to the carb. I start the engine, then pick up the fuselage (wings removed) and roll the fuselage sideways and upside down. The engine drops about 500 rpm or more (Sounds like it goes rich). I sold the engine.

Diaphram carbs are SUPPOSED to operate in any position, however, in my experience it depends on the pop-off pressure and whether the engine is over/under carbureted. If the engine is overcarbureted (too large a carb) the pop-off pressure will necessarily be low to allow the low vacuum signal at the venturi to draw sufficient fuel. While the engine will operate fine in one orientation, the mixture will change in another. Why? Gravity will cause a small force on the diaphram in all positions. If the needle-valve spring pressure (which determines pop-off pressure) is low then gravity will alter the vacuum signal required for a particular fuel feed rate. Although this will happen with ANY pop-off pressure engine/carb combination, using a smaller carb with a higher pop-off pressure will minimize the problem.

It makes sense that a higher pop off pressure on a smaller carb would make the effects of gravity and centrifugal force less of a percentage of the regulator's operation?