Homer712

My Feedback: (17)

Ordered a Magnum .91 4-stroke this afternoon and was reading the manual that's available on line. In the section on fuel tank selection they make the following statement:

The size of the fuel tank used should be 12oz. - 14oz., depending on the
model and the length of flights desired. Use of a 14oz. tank will provide
between 10 - 15 minutes of run time at full throttle. Use of a fuel tank any
larger than 14oz. can lead to excessive leaning of the engine during flight
and is not recommended.

Trying to think this through, the only thing I can come up with is that the engine may not be able to draw fuel correctly from a nearly empty fuel tank whose volume is larger than 14 oz. Am I on the right track? I tend to fly very conservative, usually land after about 9 to 10 minutes, and normally with at least 1/4 tank of fuel left. Would the 14 oz. limit still hold true in this case?

flybyjohn

My Feedback: (2)

Just my guess but with more than a 14 oz tank centered on the carb., when the fuel gets low, it will be harder to pull fuel up from the bottom of the tank. It would be like having the tank too low.

Sport_Pilot

Actually its the length that is the problem. Flying straight up on a nearly empty tank will be the leanest part of a typical run. The larger tanks are usually longer and will make this worse. Uniflow tank setup will not improve this, if you need a larger tank recommend a pump or regulator.

earlwb

Yes as Sport Pilot stated it is the length of the fuel line from the tip of the clunk to the carb on the engine.
The longer the fuel line the harder it is for the engine to draw fuel properly. So you wind up running the engine more rich to compensate, which may result in too much of a power loss at the beginning of the flight. But after the fuel tank gets down to around 1/2 full, then the engine leans out more and getting more lean as the tank empties.

Hobbsy

My Feedback: (102)

H, fourstrokes typically use about 1 oz of fuel per minute per cu in displacement. A 12oz tank will be plenty for a 10-12 minute flight. I used to get all enthusiastic and cram the largest tank that would fit and after the last flight I'd end up pumping half a tank back into the jug.

Homer712

My Feedback: (17)

Thanks for the input guys. I guess in this age of the ARF, sometimes it's hard (if not impossible) to control the fuel tank size/shape. I have a couple of Hanger 9 ARF planes. A Toledo Special, fuel tank on that one is only 11 oz. and I have a Saito 62 on it, so I'm safe there. The other one, a Pulse 60 XT comes with a "shaped" 17 oz. fuel tank that fits like a glove into a "egg shaped" space under the front hatch. There's nothing available from any of the fuel tank folks that will fit that space. Possibly, all I should do is open the tank up and shorten the fuel intake line. Or maby better yet, do nothing. There's a lot of these Pulse 60 XTs out there and I may be the only one in search of an issue to solve. It's the engineer in me. Just can't leave well enough alone.

1QwkSport2.5r

Shortening the fuel pickup line is probably going to complicate things needlessly, IMO. If it were me, I'd run the engine on the tank that fits the air frame. Check the engine's needling with the fuel level low - if the engine runs too lean at the bottom of the tank or doesn't needle well, install a pump or regulator. Though I dont know how well a regulator would work on a 4-stroke since a handful of them dont use muffler pressure at all. Keep in mind, I'm not a pilot... But I do consider myself an engine nut and have done many experiments with fuel tank sizes, tank placement, and using a pump. I have ran a 16oz fuel tank on a .29 2-stroke with the tank centerline being 1/2" above the spraybar with satisfactory results, but the tank was right behind the engine.

1QwkSport2.5r

Sorry for the duplicate post here... My cat decided to try her paw at posting on RCU... Instead she copied what I wrote.. go figure... [:-]

1QwkSport2.5r

My cat decided she wanted to post, but she isn't a very good typist.. LoL. Duplicate post, sorry..

Hobbsy

My Feedback: (102)

1Q, fourstrokes run regulators very well, here's a Saito 1.30 with two Cline regulators, since the check valve captures the peaks of the power pulses there is plenty of pressure. In fact there is enough to put a serious bulge in a stiff walled Sullivan tank.

1QwkSport2.5r

It looks like you have those turbo muffler adapters... right? I guess I was under the impression that most four strokes dont have/use pressure taps for tank pressure.. I know there is a lot of muffler pressure coming from a four stroke, so it makes sense that one could bulge a fuel tank.. Thanks for the information, I now stand corrected.

I stand by my thought that shortening the fuel pickup line is not going to cure the trouble of an engine running over-lean at the bottom of too big of fuel tank... If one uses a felt clunk in the tank, that will cause even more problems on bigger engines. My SuperTigre S90K will not run anywhere close to rich enough using a felt clunk, even on a 4oz tank. I normally use a 16oz tank on this engine which the engine can drain in about 10-12 minutes at WOT using a normal clunk.

Sport_Pilot

Shortening the fuel line pick up in the tank will not be a very satisfactory solution. This is because it is the fuel level not the clunk length that is the problem. If the fuel tank is too long it will have no effect till the fuel level of a verticle tank is below the minimum amount the engine can draw. Shortining the clunk would limit that perhaps, but it would also shorten the amount of fuel that can be used. Worse yet it would run normally until the plane pitch's up, causing instant starvation and a dead stick landing.

DaveFri

My Feedback: (7)

Sorry ..not hijack the thread but....if you had a 120 4-stroke...then how much flite time could one expect from a 12-14 oz tank...approximately..??

carrellh

Assuming the 1.20 FS uses 1.2 ounces per minute, which is a common assumption, about 10 minutes on 12 ounces and about 11.6 minutes on 14 ounces.

carrellh

My brother and I use a lot of .91 four strokes. We typically use 10 to 12 ounce tanks. We set the timer for 10 minutes and land shortly after it beeps. We never run out of fuel and are not carrying a lot of excess fuel weight around.