Engine shakes violently upon start.

You would have to have a fault on both ignition circuits for it to cause such symptoms. Do you have a soft-start device fitted which could be setting the wrong ignition timing?

Hi Norvel

First, you say it is a 912ULS therefore it may have an overload clutch or may not.  Check your engine to confirm one was fitted.  The overload clutch has very little to do with the gearbox/engine shaking on startup, that is more likely the shimming of the preload springs internally (Belleville washers) as they control the torsional wind up between the prop and the crankshaft pulse of the pistons.  

I have seen problems on engines where the shock mounts, the rubber bits that damp the shake on your engine mount, are loose or failed.  Check to see if they might have damage and are no longer working.  If they are damaged the pulse on start up can cause the fuel in the float bowl to splash violently and it will perhaps even start to push fuel out the vent lines.  

I would suggest you find a Rotax technician who if familiar with your installation.  (aircraft make model) 

As for an ignition problem try this.  Turn on only 1 ignition and try to start.  If you have toggle switches this is easy, if it is a rotary key type then that might make it impossible to check easily.  If you have toggle for each ignition try one side start and then stop and try the other.  Unless you have been working on the wiring for some reason it is less likely this is the issue however. 

If the engine has sticking or contaminated carb bits it may be part of the problem.  Sometimes just fresh gas can help a lot.  If it has been using auto fuel try some Avgas and clear out the old fuel.  Water contamination might also be a cause.  

Cheers

If it shakes on startup, I can usually shut it off and start again with no problem (not always). Both mag checks at less than 100 drop. After getting it started it runs perfectly. I have checked all wiring and found nothing wrong. This is a 2015 engine with a soft start module. Less than 500 hours total.

You mentioned carb jets. That is something I have not considered. I may just overhaul both carbs just to see if it will solve the problem. Problem is that it runs so well once it is started. I will try and take a video of the bad start and post.

Engine S/N 6784976

Thanks for the replay.

If it shakes Only at the first start and then clears up and runs fine all day, the problem is probably fuel-related.
Assume it starts with fuel in only one carb.
The next time it shakes on start-up, Shut it down and check the temperature of all 4 Exhaust Header pipes.
If one side is hot and the other side is cold ... There's the problem! 

Is your local weather hotter than it has been before the problem? (Carb Drying out between flights?)
Do run an electric AUX Fuel pump for ~ 30 seconds before the start attempt to assure both carbs are full?
Are you starting with the Choke ON?
When it shakes, Does Advancing the throttle settle it out?

I’m with Bill here.  Sounds like you are starting lean.  Inspect your cold start (“choke”) cabling and make sure it fully opens when you pull the knob.