Scale RCM 1/3 Scale Champ Build

[Snoopy1]

Agree with on the amount of air it needs per revolution. But the diameter of the prop is determined by the cam timing and as you said time will tell. My flying buddy has on of these new radials 90cc four stroke and the carburetor on it was surprisingly small but it runs great.

 

[49dimes]

The venturi is rather small on the carburetor, looks like something from a 30 CC string line trimmer. I guess it has to handle 42 1/2 cc per revolution. When I look at the carburetor it is hard the image this engine will swing a 25"-27" propeller. we shall see. I'm guessing the small venturi is need to develop sufficient pressure pulses? Can you explain why it loads at idle?

NO . I believe your engine is timed as any other boxer style twin. And I mean the mechanical timing built into the camshaft.
The duration of the intake valve timing will have an effect on the pulse's the carb sees. Even though it is a twin four stroke it is going to spark like a twin 2 stroke. At idle they just don't burn all the fuel as efficiently as a 2 stroke. So when you come off the throttle you have a bit of loading.

 

[49dimes]

Here is a short vid I found of the Roto 85FS. Note the rpm drop after prolonged idle. Spool up is marginal but it swings a decent size prop with good torque and the sound is very nice. And also looking at this vid it appears that Roto has changed the way the carb is pulsed. Looks to be coming from the crankcase. Is yours done this way ? If so my mistake. I was thinking of their earlier engines.

 

[49dimes]

And here is an 85FS that Adrian has done. Bigger carb and his custom timed ignition system. Note that the pulse line from the crankcase is not hooked up to the carb .

 

[TonyHallo]

My carburetor is pulse ported to the intake in the fiber insulator between the carburetor and manifold.

 

[49dimes]

My carburetor is pulse ported to the intake in the fiber insulator between the carburetor and manifold.

Thank you. Your engine is still going to perform ok but I would consider a little bit bigger carb or pulse it from the crankcase.
I don't know what carb Adrian used but I imagine it is similar to the Walbro HDA-48.

 

[acerc]

My carburetor is pulse ported to the intake in the fiber insulator between the carburetor and manifold.

As a fellow modeler that runs four stroke gas engines I would not touch anything. These engines, same as two strokes, are designed and tested around the way they come and perform great. As for sag on prolonged idle most any engine, be it two or four stroke, will either sag or climb depending on tune, fuel mix, atmospheric conditions, prop size, and degree baffling. I currently have a number of the Valach engines and everything is set up appropriately and there is no sag or climb on a normal amount of idle time. A prolonged idle will cause a minor sag because with proper baffling the engines will actually cool down. This cooling down causes the mixture to become very slightly rich. And we don't want to tune that rich away because in flight when the engine warms up it would go lean. There are a lot of variables in the tune, don't over think it.

 

[49dimes]

As a fellow modeler that runs four stroke gas engines I would not touch anything. These engines, same as two strokes, are designed and tested around the way they come and perform great. As for sag on prolonged idle most any engine, be it two or four stroke, will either sag or climb depending on tune, fuel mix, atmospheric conditions, prop size, and degree baffling. I currently have a number of the Valach engines and everything is set up appropriately and there is no sag or climb on a normal amount of idle time. A prolonged idle will cause a minor sag because with proper baffling the engines will actually cool down. This cooling down causes the mixture to become very slightly rich. And we don't want to tune that rich away because in flight when the engine warms up it would go lean. There are a lot of variables in the tune, don't over think it.

He has a version of engine that is known to have carb problems (by design) and overheating even with proper cooling (Baffling). The video I first posted shows how ROTO made an improvement to the "pulsing" of the carb to get more reliable needle settings and then Adrian's video showing how you can hop up the performance a bit more and not worry about ruining your engine.
So you would have him just leave it as it be and possibly have a dead stick when he is ready to maiden a plane he will probably have a few hundred hours making? Let's just ….

 

[acerc]

I guess having dealt with only two of these engines I may have some shortcomings in how they operate, all I do know is they they ran fine.

 

[TonyHallo]

The venturi is rather small on the carburetor, looks like something from a 30 CC string line trimmer?

The carburetor is a Walbro WT962, Used on a DLE 20? Even smaller than i throught