Engine Speed increases when below 1/3 tank

[FlyBlueSkies]

Hi,

I have a Raptor 50 V2, with an OS 50 Hyper.

I noticed Friday, that when I get to about 1/3 tank of gas on the main tank (I have a header also), the engine seems to increase RPM when I come in for a landing.

So, from about 1/4 throttle to about 1/2 throttle, the engine seems to run at a higher RPM.

The engined does not seem overly hot. I can hold my finger on the back plate for about 5 seconds before I feel the heat. I can hold it there, but it is hot.

I have richened it by a couple of clicks, but it still increased RPM below 1/3 tank. However, it is running a little cooler.

The high speed is set to 2-3/4 turns out.

I am running Wildcat 30% helimix.

I did check the fuel lines, but they all looked fine. I did replace them for good measure, but I haven't flown it to see if it makes a difference.

I don't think this is normal, do you?

If not, what might it be?

[OHIOBOWHUNTER]

hey jeff..it may be because your real rich on the high and lean on the low..so when your lower on the stick the engine is actually running leaner (more rpm ) then as you approach mid to high stick your moving into the rich zone (lower rpm ):thinking don't know if this makes sense but its all I can think of ?

[vandelescrow]

I heard from a very reliable source this is very common on 30 and 50 size machines. The engine is going lean on the bottom half of the tank.

The reason is fluid dynamics, liquids tend not to compress as much as air does, in this case, exhaust gasses. At the top of the tank you do not need as much pressure from the muffler to keep a consistent steady supply to the carburetor. As more of the space in the fuel tank gets filled with air (exhaust gas) the more it can compress needing more muffler pressure to provide the same amount of fuel as it did at the top of the tank. Unfortunately, that extra pressure needed is not available on smaller engines.

For example, remember playing with those water filled rockets you pump up with air? The less water you had in the rocket, the less altitude you got no matter how much you pumped air into it.

What you can do: Unfortunately there isn’t much. I’ve tried putting a Perry pump on mine with a one way valve for extra fuel that the engine could not use, but no matter how lean I set the carb, it was still to rich. I was told the carb was not designed for that much pressure, only way to fix this scenario is find a carb that will fit an OS 50 hyper that has a regulator to handle the extra fuel / pressure.

Another thing you can do is set your needles for a happy medium where it is not to rich on the top half yet not to lean on the bottom.

Lastly and I know none of us want to do this, fly only half a tank at a time.

[FlyBlueSkies]

"hey jeff..it may be because your real rich on the high and lean on the low..so when your lower on the stick the engine is actually running leaner (more rpm ) then as you approach mid to high stick your moving into the rich zone (lower rpm ) don't know if this makes sense but its all I can think of ?"


I was thinking the same thing. I do have the idle mixture slightly lean. So, if the new tubing does not work, I'll try setting the idle mixture a little richer.

I'd like to fly today if I can. Have to wait and see what the weather is like.

[archiebald]

Unless you run a YS style pressurised system with a pressure regulator, then some leaning near the end of a tankful is inevitable.

[Helio Addict]

Or you could purchase a 60K carb and send it to OMI and have a cline reg installed and also send them your backplate so they can install a pressure fitting unless you want to do it yourself. This will give you a 3 needle carb and you can run a pressurized system like YS does.

[brgsstm]

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[FlyBlueSkies]

Thanks for all the replies!

I appreciate all the input and it was very helpful!