rambler53

My Feedback: (494)

I am running a strong Rossi .45 with a 12 ounce tank on a profile plane and every time I fuel it up and run it, it seems to be too rich in the air for the first half of the tank. Then after about 4 minutes it leans out and runs great. This happens on every tank. If I lean it out in the beginning it will get hot after the first half of the fuel has been used. Should I get a pump on this for a consistent run, or is there another trick? Am I missing something basic with a flaw in the tank, pickup line? Would like to get some ideas. It's always harder to think when it's your own plane!

parrthd

most all engines are tuned a bit rich on the start of a tank due to this "leaning" process.

something else you might want to try if you haven't already. after a flight, while the engine is still hot, refuel and check your motor while the engine is still hot with a full tank and with a half tank. another thing to consider is tank placement in regards of center line of tank in reference of carb. and as for a pump, i don't think that's really needed unless you've got a really long fuel line. if problems continue and you've exhausted all other means you could always try a "header" tank. "heli" pilots use them to get a more consistent run of their motors through a full tank of fuel. no reason why plane pilots couldn't do the same however, this is a profile and weight needs to be limited so, you might have to go with a 4 oz header tank and an 8 oz fuel tank. or somewhere there abouts........

anyway, just a few things to consider, hope this helps

BillS

A 12 oz tank is a bit large for a .45 engine and the results you are seeing are to be expected. Usually .45 engines will have 8 oz tanks and sometimes a maximum of 10 oz.

Although a pump might be used to solve the problem they add cost and additional complexity.

As a first step I would seriously reconsider the rationale for the large tank.

Bill

rambler53

My Feedback: (494)

The tank is slightly higher than the engine, but the clunk is even with the center line. I use 12 oz tank because these engines are so thirsty, I like a longer run. This is my version of li-po power! I'm heading to the field now with a 8 oz tank, see how it goes.

MinnFlyer

My Feedback: (4)

Just for jollies, replace the line inside the tank that goes to the clunk. You might have a pinhole that allows air in the line once the fuel goes down below that level.

rambler53

My Feedback: (494)

The line inside the tank goes out with the new tank, so that shotguns the fuel tank issues I thought. With a 8 oz tank, I can be completely inline with the engine, although the plane is rarely in a inline level with all the climbs and falls I do, not to mention inverted flight, the reference point on the tank shouldn't be an issue within 1" total +/-?
Anyway, off to the field I go.

mscic-RCU

My Feedback: (6)

You answered your own question. You said the tank is high. Rule of thumb is the centerline of the tank, not the clunk, needs to be on the centerline of the carb. Fuel is draining to the carb when the tank is full, thus the rich run with a full tank. When you reach a half tank, the engine now has to suck the fuel from the tank, hence a lean run. With the centerline of tank on the carb centerline, the engine now has suck fuel all the time, thus you are able to set a reliable needle setting.

rambler53

My Feedback: (494)

With that, why doesn't it lean inverted? It's still rich and effectively, the center line is now low.

pt19 flyer

My Feedback: (1)

hi
from all I have read and experienced the centerline of the tank should be 1/4 to 1/2 inch below the needle valve on the engine. if it is higher or lower than this it will affect fuel draw and the way the engine runs.

good luck and happy flying

pt19 flyer

da Rock

krosypal,
Nowhere in your post do you mention muffler pressure.

Most modern glow engines' carburetors are sized to benefit from having muffler pressure help push fuel from the tank to the carb. If you're not using muffler pressure, you ought to try it before going to something else.

da Rock

Our engines today should run decently with the fuel tank within a half inch of the centerline of the carb. If you're using muffler pressure the system is closed and the tank location usually has more effect on setting the needle while sitting on the ground than you'd expect. Once the airplane is flying, with any sort of closed, pressure fuel system, the accelerations and g-loads in the air take over. And the centerline of the carb means almost nothing.

Often, when an engine runs for a predictable amount of time and then runs differently (and that difference is predictable), it's often a tank problem. There might be a hole in the pickup line inside the tank. The vent might be covered for the first 4 minutes and affecting the inner tank pressure..... something amiss inside the tank that is covered with fuel until the tank is drained to a point.

Simple test........
Try a different tank.

Simple test......
Change all the present tubing.

loughbd

Muffler pressure would make it run even richer on the ground. Most if not all engines will run fine without muffler pressure. The pressure from a muffler isn't all that much, less than one psi. All it does is keep a positive head on the fuel system.

BillS

You are correct the muffler pressure is not very large. Muffler pressure is sufficient to lift fuel about 9� at WOT on most engines. At idle the pressure is essentially zero.

However muffler pressure does have a large effect on Hi and Low needle settings.

Bill

da Rock

That's not completely true nowadays. They will run, but often not "fine" or reliably. A great number of engines nowadays have intake openings in the carb that're sized as large as the engine can use. The needle valve can supply that opening without having muffler pressure's help simply by backing the needle out far enough. No problem..... That is, no problem when the airplane is sitting on the ground with a unchanging G-load.

But you're going to have to set the needle for flight loads. G-load on your fuel is measurable. It does change how hard it is to move an adequate amount of fuel into the carb. And no matter where the needle is set, when the venturi opening is as large as possible, the suction isn't going to be enough to adequately supply the engine. Especially when that engine is working to pull the airframe while the airframe is pulling extra Gs. Just when the engine needs the most power is when the fuel is hardest to pull.

Setting the needle for flight loads is harder when the setting has no help from muffler pressure to deal with flight loads. It's not only harder to get a good setting, but with some engines, you ain't gonna git 'er done.

Now, if all you do is take off and fly around and land, you might not actually need muffler pressure. And if you've got an engine that's not advertised as "strong", you also might not need it. But truth is, it's so dead simple to do....... run a fuel line from the existing vent on your tank to the existing pressure tap that came on your muffler....... why not?

da Rock

If that Rossi has a lot of hours on it, and especially if you've been using high castor oil fuel, it might be time to "decoke" the sucker.

Does the side of your piston look black?

pt19 flyer

My Feedback: (1)

hi
the problem you are having is almost always due to improper tank placement. if too high it will siphon until about half tank and then tends to lean out. try to relocate tank centerline about 1/4 to 1/2 inch below air intake on throttle and see if this doesn't correct problem. also try to avoid any upward loops in feed line. try to keep as straight as possible. this however is not as important as tank centerline placement

good luck and happy flying
pt19 flyer

themanimal

My Feedback: (1)

I had a similar problem and I fixed it by using a pressurized, one way reserve "tank" that I amde in about 20 minutes. There is a thread here somewhere but in essence, this is what you need.

1. One syringe (medium sized)
2. one short piece of fuel tubing
3. sheet metal screw
4. fuel proof silicone.

Here is how it is done.

Unscrew the needle of the syringe and throw the plunger away. Cut the body of the syringe down so it is about 1-1/2 " or 2" long. Take a 3 inch piece of fuel tubing and make two slits in it near one end. Screw the sheet metal screw (or any screw large enough) into the end with the slits. Now, place the tubing (screw end first) into the syringe and silicone it in place leaving a little space between the screw and the syringe nipple where the needle was. Let it dry.
Connect the fuel tubing from the engine to the "needle" end of the syringe- there should be a little nipple where the needle was. Connect the line from the tank (I use a fuel filter) to the line coming out of the syringe and "tada" pressurized inline tank.

The pressure and fuel from the tank will expand the two slits you made in the fule line and fill the syringe. If for whatever reason your tank pressure drops (like it is getting near empty) or you cut the throttle then the slits in the fuel tubing close and prevent a loss of pressure.

I didn't invent it but it works pretty damn good.

Jim Thomerson

Make a uniflow tank and your engine will run fine. All these posts and no one has told you the answer.
http://www.fraserker.com/heli/uniflo...flow_works.htm

camdyson

Re muffler pressure: Having seen what happened to the running of 2 of my (admittedly 4-strokes, an OS70 and OS90) engines when the pressure lines came off the mufflers, I can assure you it makes a BIG difference - at least if tuned with them on!

Why not try just do everything "right" before deciding solutions don't work - use an 8oz tank, get the CENTRE of the tank at or just below the spraybar, and use muffler pressure if available. Replace fuel lines and plug if in doubt. I agree tank position isn't all that critical in theory, but may as well get it right and eliminate the possibility.

THEN we'll scratch our heads if it still doesn't hold a decent tune all flight. Remember K.I.S.S. How big is the carb venturi by the way? - this can effect fuel draw too. Some hi-po engines can be pains due to this (ie big venturi means powerful but temperamental).......

Happy flying,

Cam