Rotax 503 single carb jetting...

[Turbohoolik]

Hello!

On my Scat hovercraft I have Rotax 503 with single Mikuni 38mm.
I'm stuck with carb jetting.

It had 360 main and needle clip was on the middle. Engine run fine till 3/4 throttle but on WOT engine rpm dropped a little. I wouldn't say engine start to bog but definitely there was change in the engine sound.

I put in 330 main and lowered needle as low as possible. Now rpm are not lowering on WOT but engine doesn't sound happy yet... Still sound difference in 3/4 throttle and WOT.
Unfortunately I run out of jets right now, ordered smaller ones.

some jetting advice?

 

[Eric S]

Brian at Rainbow Aviation says when people bring their Rotax powered planes to him b/c they aren't running right, they've often been trying to cure symptoms with carb. jetting. He says set it to what the Rotax jetting chart says to use and leave it there, then look for other causes.
Bad gas? Prop/RPM setting accurate? Cracked rubber carb. boot? Insufficient fuel flow? It's something else, but it's not carb. jetting.

Eric

 

[Doug Riley]

To determine if mixture is the issue, run the engine up to WOT under normal load and run it at WOT for perhaps half a minute. Then shut it off WITHOUT reducing throttle. Then remove the plugs and examine their color. Brown is ideal, black is too rich, light grey/white is too lean.

As Eric S. mentioned, jetting is not always the problem. A restrictive air filter can cause mixture problems, as can the faults that Eric lists.

 

[Mayfield]

I agree with Eric. Brian Carpenter and Roger Lee know a lot about Rotax engines. They both teach trouble shooting procedures that emphasize returning the engine to a known state (factory standard) before beginning to throw parts or adjustments at the problem.

I can't resist a short example from flight test experiences.

When testing to the edge of the control envelope in propeller driven airplanes it is possible to depart controlled flight. In airplanes with high wing loading and/or loaded to a CG extreme, it is possible for the departure to be disorienting.

Reducing power to idle, moving the stick smoothly full aft, and smoothly applying full left rudder, should ultimately result in the airplane entering an upright spin to the left, a known state. After reaching the known state, normal recovery procedures are executed.

Does this procedure always work? No, but in most cases, it will result in the airplane reacting in a recognizable, and recoverable way.

The moral is, whether procedures for flight or for troubleshooting maintenance issues, Put the aircraft in a known state, then go from there.

Jim

 

[Gyro-nut]

I agree with Eric. Brian Carpenter and Roger Lee know a lot about Rotax engines. They both teach trouble shooting procedures that emphasize returning the engine to a known state (factory standard) before beginning to throw parts or adjustments at the problem.

I can't resist a short example from flight test experiences.

When testing to the edge of the control envelope in propeller driven aircraft it is possible to depart controlled flight. In high wing load aircraft and/or aircraft loaded to a CG extreme, it is possible the departure can be disorienting.

Reducing power to idle, moving the stick smoothly full aft, and smoothly applying full left rudder, should ultimately result in the airplane entering an upright spin to the left, a known state. After reaching the known state, normal recovery procedures are executed.

Does this procedure always work? No, but in most cases, it will result in the airplane reacting in a recognizable, and recoverable way.

The moral is, whether procedures for flight or for troubleshooting maintenance issues, Put the aircraft in a known state, then go from there.

Jim

100% spot on Jim.
I know when my wife is acting strangely, there are specific buttons I can push to get her back into a completely pissed off state.... then I know where to go from there

 

[Waddles]

The attached from a LEAF catalogue.

 

[Turbohoolik]

I chopped spark plugs, in this test I was running 290 main. It has been always that second cylinder is getting more fuel than first one. I'm running single carb. I have checked air leaks from intake many time. Thi is freshly rebuild engine. Before rebuild second cylinder was getting more fuel to.