Sport Pilot Talk

I've been having starting problems with my Sting, with a Rotax 912 ULS. We're now officially stumped. Here is the chain of events.

Airplane sat for about 3 months, waiting for various non-engine parts to arrive.

When we started it up, the idle seemed high and the idle was adjusted. Probably too low, I can get it below 1650 rpm's.

Next time I went to fly, it was uneventful, other than the low idle.

Then I started to experience starting problems.

At first we couldn't determine if we were missing "spark" or "fuel". After changing all the spark plugs, the airplane fired right up and ran great (with the low idle). The airplane has never failed to start after being started & run.

The last 3 times we tried to start it, no luck. Replaced the spark plugs again, no change.

We have fuel pressure and can smell fuel at the exhaust pipe, so one would assume it's getting fuel.

Used a battery boosters and the engine is spinning plenty fast on start ... it's not the battery.

Try it with "starter fluid" today and still no luck. If it won't start with starter fluid, we're assuming there's a lack of spark.

Anyone have any ideas?


Thanks!

Possibly bad ignition modules, a known 912ULS weakness. Throw a bag of ice on the modules for 30min and see if it starts after that. The running circuit is usually fine, the starting circuit is what commonly fails. If one fails the plane will still start, it's only when both starting circuits fail that the engine won't start.

Unfortunately this might get expensive, a pair of the newer modules is about $2k.

After troubling shooting your starting problem, get that idle set right. You wouldn't want the motor to quit at a critical moment...

designrs wrote:After troubling shooting your starting problem, get that idle set right. You wouldn't want the motor to quit at a critical moment...

Factory minimum idle is 1400rpm, it's not going to quit there or above. And once in the air with the prop loaded, you're not going to see much less than 2000rpm in the air at idle. If it were to quit, it would do so on the ground where it's a non-issue.

The main problem with low idle is that below 1800rpm the gearbox harmonics will cause chatter, which over time can cause wear. Just bump up the throttle and taxi at 1800rpm or so. My idle is set around 1600-1700rpm, I like it that way because there is less residual thrust at idle in the air and the airplane comes down better and steeper. I just use the throttle on the ground to stay out of the yellow rpm range.

Generically, Rotax recommends 1800-2200 idle RPM. Ref. Rotax carb adjustment video. You must have a Rotax owners membership to view the video.

With a free-castering nosewheel and differential braking my SportCruiser often taxi's at 2200+ RPM. Constant thrust manipulation is required with the differential braking... eg. add power while brake and turn.

It must be nice to just taxi with a steerable nosewheel. LOL

MrMorden wrote:Possibly bad ignition modules, a known 912ULS weakness. Throw a bag of ice on the modules for 30min and see if it starts after that. The running circuit is usually fine, the starting circuit is what commonly fails. If one fails the plane will still start, it's only when both starting circuits fail that the engine won't start.

Unfortunately this might get expensive, a pair of the newer modules is about $2k.

Brian -- the 'bad' ignition modules appear to be 06/07 vintage although, obviously, anything could fail. Does sound like ignition module issues.

Quickie...

When I went through this, it was recommended that I purchase an inductive timing light, which I did.

About $35 and will tell you immediately and safely is spark is being produced by each module.

My issue was in fact bad modules from 2007.

Hi Briant,

You most likely have one of two issues.

First it sat for three months and the fuel was most likely never drained from the cab bowls. Pull the carbs and clean them out with some spray can carb cleaner. Remove the idle jet and run a single strand of wire from a 16 ga. wire down through the center of the idle jet. You can spend weeks looking for the issue, but it's usually best to just do it and save all that time.

It could be the ignition modules so before you pull the carbs put a fairly good size ice pack on top and around the modules and let it sit for 30-40 minutes. Then try and start it. If it starts buy some new ignition modules.

If it happens to start before either of these then put a set of carb sync gauges (not an electronic one) and see if one carb is pulling way more vacuum than the other or both are pulling way more vacuum than normal and this will tell you the carbs need cleaning and have dried fuel blocking passages.

40 minutes of ice pack & it started right up.

I guess that means new Modules?

That's what it means. The starting circuit is gone.

roger lee wrote:It could be the ignition modules so before you pull the carbs put a fairly good size ice pack on top and around the modules and let it sit for 30-40 minutes. Then try and start it. If it starts buy some new ignition modules.

Could you explain (to a lay person/non mechanic) the physics behind the ice pack procedure?

Flocker wrote:

roger lee wrote:It could be the ignition modules so before you pull the carbs put a fairly good size ice pack on top and around the modules and let it sit for 30-40 minutes. Then try and start it. If it starts buy some new ignition modules.

Could you explain (to a lay person/non mechanic) the physics behind the ice pack procedure?

I was about to ask the same. Not doubting that it is a good diagnostic test, just curious how it works.

roger lee wrote:That's what it means. The starting circuit is gone.

Thanks Andy & Roger!

Just so I understand better ...

At the suggestion of Bill Canino, we also tried switching the ignition modules. Well, not exactly "switching" the modules, we just switched the connections. This didn't make any difference. Under normal circumstances, can't one properly operating module start the engine? So this would indicate that both modules are bad?

At least as I understood Lockwood Aviations Website, there's a replacement module that also contains the "Soft Start Module", that's priced at 50% of the cost of new standard modules. When I called them today, either the woman I talked with was confused or I misunderstood the description on their website.

http://www.lockwood.aero/p-3710-advance ... e-kit.aspx

Thanks again for the help, guys!

MrMorden wrote:

designrs wrote:After troubling shooting your starting problem, get that idle set right. You wouldn't want the motor to quit at a critical moment...

Factory minimum idle is 1400rpm, it's not going to quit there or above. And once in the air with the prop loaded, you're not going to see much less than 2000rpm in the air at idle. If it were to quit, it would do so on the ground where it's a non-issue.

That's exactly right, Andy. It did quit once after landing and starting to taxi. Of course when I'm in the air, I pay a little extra attention if it starts to sputter a bit