Engine shakes violently upon start.

"I have a Rotax 912 ULS that sometimes when it starts it shakes violently. I can shut it off and restart sometimes successfully. At first I thought it may be the gearbox clutch slipping but after overhaul I still have the same problem. The Sprag clutch was also thought to be the problem, however I do not get any kick back upon start. I am starting to think that it could be faulty ignition. Any ideas on how to troubleshoot this problem would be appreciated.

Ernie"

Hi Ernie,

How are you starting your engine and when warm what is the idle rpm?

This may be just about your starting procedure. If you have a low idle rpm and choke it, but have the throttle all the way back then the start rpm may be just too low and it will shake. Have you tried choking it and cracking the throttle about 1/2"? When was the last carb sync? What are the plug gaps and when were they replaced? Is the mag check normal?

I have a similar thing, although I don't think its the same.

Mine, 912ULS, when I start it, for the first 5 seconds, it sounds like it's running on 3 cylinders, the RPM is low and then it kicks in to be normal.

Is that normal?

Roger Lee suggested it might be the carb gasket leaking and sucking air. They came out with a new rubber gasket to replace the cork one. Mine had the correct gasket but found one float heavy in one carb. Replaced the float and solved the problem. Thanks to everyone for the great support.

John Fleming wrote:

I have a similar thing, although I don't think its the same.

Mine, 912ULS, when I start it, for the first 5 seconds, it sounds like it's running on 3 cylinders, the RPM is low and then it kicks in to be normal.

Is that normal?

That rather sounds like the normal "soft start" to me.

Hi Tyler,

Good point about the soft start on startup. I got locked into violent shake.