912ULS Rough Start After Warm-Up and Shutdown

Sounds like hot fuel/vaporisation.

If you have a fuel return system, run the boost pump until a steady pressure is achieved. This should reduce the amount of vapour/bubbles in your fuel delivery system. Start engine. There may still be a short rough period.  Leave pump on until you reach a safe altitude.

😈

I can't see this being vapor lock for the short run, but mainly a fully exposed engine to the outside air.

I live in Tucson, AZ too and fly and work out of Ryan Airfield.

Sounds to me like one of the Ignition modules has gone bad in the start mode (3 degrees ATDC) and the engine is only starting on one module. Then after it gets out of the soft start mode in a few seconds it switches over to the 26 degree BTDC where they are both running okay. Do you know someone with another Rotax where you can borrow a module and troubleshoot yours?

One thing to give a try is go ahead and warm the engine and shut it down like you normally do. Then place a flat 1 gallon bag of ice around the two modules. Don't overfill the ice bag because you want to stuff it around both modules for about 25 - 30 minutes. Then try to give it a start. If it starts right up then you have a bad module.

"Sounds like hot fuel/vaporisation.

If you have a fuel return system, run the boost pump until a steady pressure is achieved. This should reduce the amount of vapour/bubbles in your fuel delivery system. Start engine. There may still be a short rough period.  Leave pump on until you reach a safe altitude."

That's my standard process.  I always start the pump, wait 5 seconds or so, and start.  I leave the pump in until cruising altitude.  I flew yesterday and let the pump run for 15 seconds and I don't think it made a difference.  But thank you for the input.  Keeping it in the back of my mind.

The current government has passed "wavers" for states to sell winter gas into the hot summer months.  (oil supply issues) I would try some avgas, up to 50%, mixed in.  This should minimize any vapour lock issue if that is the problem. 

Cheers

Roger Lee wrote:

I can't see this being vapor lock for the short run, but mainly a fully exposed engine to the outside air.

I live in Tucson, AZ too and fly and work out of Ryan Airfield.

Sounds to me like one of the Ignition modules has gone bad in the start mode (3 degrees ATDC) and the engine is only starting on one module. Then after it gets out of the soft start mode in a few seconds it switches over to the 26 degree BTDC where they are both running okay. Do you know someone with another Rotax where you can borrow a module and troubleshoot yours?

One thing to give a try is go ahead and warm the engine and shut it down like you normally do. Then place a flat 1 gallon bag of ice around the two modules. Don't overfill the ice bag because you want to stuff it around both modules for about 25 - 30 minutes. Then try to give it a start. If it starts right up then you have a bad module.

I flew last night but I left the ice bag I'd filled at home like an idiot (no "take ice bag" on my checklist!).  Of the 3 starts yesterday, the first was kinda sputtery but with a noticable timing advance.  Warmup was minimal as engine temp was 113 at start.  Second start at takeoff was sluggish and didn't smooth out until I gave it some throttle, no timing change discernable, but it was probably 4-6 seconds of sputtering so it could have occurred during that.  Engine during flight was nominal.  Third start to taxi from runway to ramp was the best, almost completely normal with just a small sputter that I wouldn't otherwise have paid much attention to, and a clear timing change as expected.  

So, more of the same basically, and I didn't do what you suggested, but will.  I've no one to swap modules with, but it could possibly be done next month during the annual (at the manufacturer, Powrachute).

I appreciate your input Roger.  Thank you.