kkling77

My Feedback: (16)

I have a 3W 106 TOC that has sags considerably when the airplane is rolled inverted. Throttle setting does not seem to be an issue, as it happens at half and full throttle. The engine probably has 15 gallons run through it. I was running lawnboy ashless oil at 50:1 and just changed to Amsoil Sabre at 100:1 last night. I have tried leaning the high and low end needles but have not seen any improvement. It almost seems like the engine is richening up considerably when inverted. Anybody seen this before or have any suggestions on what to try?

davewallace

My Feedback: (29)

77,
The most common cause of is pressure changes in the cowl around the carb pressure regulator. Do you have your pressure regulator vented to the fusalage interior? This usually the reason for this.

Dave

rmh

Tank location can also cause this -
lower your tank as low as you can
The pressure head -- relative vertical height PLUS carb setup (regulator pressures ) can and do sometimes cause this
alway do easiest things first -if no canisters are in the way
put the tank on floor of model

Geistware

My Feedback: (16)

Make sure that the carb opening is not in the airstream.
When I have had this issue on my planes this was the problem.

kkling77

My Feedback: (16)

I have not tried running a vent line to the inside of the fuselage yet, but the more I read it seems like this is the thing to do.

Unfortunately the tank is already as low as it can go in the airplane.

The carburetor opening is in the slipstream as well (the airplane is a 2.6m Comp Yak).

My next question is, what changes when the aircraft is inverted to cause the engine to run this way, yet be solved by running a vent line inside the fuse or taking the carburetor out of the slipstream? Just trying to gain a little more insight into what is actually causing this. Thanks for all the advice so far.

pe reivers

How did the engine behave before you switched oil? Why not stay at at least 2% in the mix. It takes a long time (hours) of running, before the new oil has replaced all of the old. Until that time, your engine is at risk if the oils are not well compatible. Have you allowed for that?
Inverted and knife edge running problems are a common issue with the use of diaphragm carbs. The issue is well discussed on RC-Universe, including the causes and possible solutions.
I suggest you do a search first.

JoeAirPort

My Feedback: (41)

One thing I did to trouble shoot a problem was this: I had three of my buddies turn my 30% 50cc Edge 540 over inverted while it was running at idle. I wanted to see if it would die. It did drop RPM's a little. When pitched forward and down (inverted) it almost died but recovered. I think it's because the tank was lower than the engine upright, when we rolled it over, the tank was higher than the engine making it run rich. Jerking the plane forward and down simulated a low G manuever. The MFG of my engine said "the engine does not know what attitude it's at". Well apparently mine does. So mine was not due to the air flow/pressure in the cowling. It was a gravity/G thing. When the carb sees head pressure on it from the tank, it passes that pressure onto the engine making it run rich. In my case only the low end was a problem. I can't detect any issues at all at mid to high RPM's.

If I were you I'd shield the diaphragm hole with the bottom of a pop can. Cut it to size, drill a couple of holes and make a little dome over the hole. Let air get to it, but not blown directly on it. I believe this might be your problem. If not that, it's air blowing into the carb air inlet. The diaphragm hole draft block is the easiest to try.

flyinrazrback

My Feedback: (69)

In my comp arf 2.6 yak and 3w 106, I took my snorkel off the carb and just ran the line back in the fuse, ran flawlessly. I have never been a fan of 100:1 in any engine, run Bel Ray MC-1 at 50/1 in all my 3Ws.

kkling77

My Feedback: (16)

The fuel tank is actually probably a bit higher than the engine normally and slightly below when inverted. The engine was acting the same regardless of the oil used. I think I will end up attaching a fitting to the carb. and running a line back into the fuse. This seems like the most widely used remedy to solve this problem. Thanks again for the help.

flyinrazrback

My Feedback: (69)

Does your carb have a nipple on it for a vent line coming out of the front of the carb? If not, you can use something like this:

http://www.troybuiltmodels.com/CarbPlate.htm

davewallace

My Feedback: (29)

77,
Please make sure that the tube or fitting doesn't contact the diaphram as this will cause problems. Most remove the cover and solder a tube to it as in the picture.

Dave

Tired Old Man

My Feedback: (1)

The suggestions about raising and lowering the tank height are rather humerous. Rolling the aircraft upright or inverted simply relocates the problem when the tank heights are changed. If the isssue is "head pressure", by changing the tank height all you will be doing is moving the problem from one side to the other.

davewallace

My Feedback: (29)

77,
The answer is angle of attack. When inverted or upright the airplane has a slight positive angle of attack. This creats a slightly differntly airflow through the cowl in each aditude. The amgle is need to generate lift with a symetrical air foil.

Dave


[quote]ORIGINAL: kkling77

My next question is, what changes when the aircraft is inverted to cause the engine to run this way, yet be solved by running a vent line inside the fuse or taking the carburetor out of the slipstream?

Tired Old Man

My Feedback: (1)

Dave,

I'm impressed!! You're one of the few that understands the relationship of angle of attack and lift with a symetrical airfoil.

kkling77

My Feedback: (16)

Dave,
Thanks for all the helpful tips and information. I am going to solder a tube on and run a line inside the fuse and hope that works.

davewallace

My Feedback: (29)

Surfer,
Now if I only could learn to type.

Dave

J Grav

Hi guys,

I also have a 3w 106 with this issue. I have the CS model that comes with the right angle velocity stack. The carb equalization tube runs into this stack and is supposed to eliminate any pressure/airstream issues. The engine runs perfect on the ground. Good transition, good high end and good idle. But, when taking off, I hit the throttle and it just doesn't want to get any revs. It uses the whole field to take off. Once in the air, it seems fine. I have seen a little throttle change when inverted, but not when vertical. So if it was the tank position, I would think it would have a problem in a hover. The tank is pretty low in the airplane anyway. As far as airstream, the engine runs great when teathered. There is still a lot of air going past that engine. I seem to have the problem when going very slow on take off. Probably less then 20mph. I will make a belled velocity stack like on my other engines and move the carb plate equalizing tube to inside the fuse, but Has anyone had issues with their right angle velocity stack?

Thanks,

Jim

wingburner

My Feedback: (17)

Phone Gerhard at AI and buy the new aluminum reed cage housing block ($35) and your troubles will be over....These engines are famous for fuel puddling in the stock housing. Go inverted or knife edge and guess what...Yup, richening of the engine...I've been through it all..people telling me it's angle of attach, airflow etc etc etc...tried all the supposed cures...Enlarging holes, building air dams and reflective pieces and put the tube everywhere but up someones $$$...The new one cleans that up....You can see it in the pic

RTK

My Feedback: (1)

kkling77-- You can also try leaning the high. Go easy at it but that should help too. If it were me I would not run any oil at 100:1, I prefer the 32:1-50:1 range.

ciressarg

My Feedback: (14)

I've seen this problem with two 106's both were too rich. They both seemed to be running very good on the ground but inverted or KE they sounded like they were going to quite. The high needle has a very fine adjustment. You may need to turn it farther than you think to get results. Use a tach.

Eric

Mitsu1

My Feedback: (13)

Wingburner gave you the answer in post #18. I've researched this and all the other modeling message boards, and confirmed by the consensus. Save yourself the headache. Your answer is detailed in post #18.