912 ULS and 912iS Sport gearbox difference

My Opinion...but others know more that I do.

The IS engine was upgraded to the "Sport" that increased the torque in the midrange and the gearbox was not really up to it. There have been upgrades to the IS gearbox including an oil spray system. Therefore any New IS engine will have the latest version and should be very reliable. The MM "says" 1000 hr between inspections which is better than the 600hr on the ULS...and who knows with the latest versions it will likely meet that.

What I like about the IS is that it starts like a car, easily and therefore there is little strain on the clutch, a well tuned ULS with a good battery starts well too, but sometimes the carbs are not balanced well and sometimes the battery is too small/low and it kicks back....causing bad things on the clutch

I have both, (2015 year IS Sport and 2017 year ULS) they are both well maintained and have excellent batteries, therefore I have had no issues with either one...with 850 and 620 hr so far.....

The 912ULS gearbox has a 30-degree dog hub, and the 912iS has a zero-degree dog hub, so there is no friction TQ inspection on the iS Gearboxes. You can't put a 30-degree dog hub in a 912iS gearbox, as it would create starting issues due to the two crank positioning sensors being so close to each other.

Both the 912ULS and the 912iS come equipped with an overload clutch. Click on the below link to see the video that provides a visual and more explanation of the dog hub and friction torque.

https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/60-friction-torque29

Glenn Martin wrote:

My Opinion...but others know more that I do.

The IS engine was upgraded to the "Sport" that increased the torque in the midrange and the gearbox was not really up to it. There have been upgrades to the IS gearbox including an oil spray system. Therefore any New IS engine will have the latest version and should be very reliable. The MM "says" 1000 hr between inspections which is better than the 600hr on the ULS...and who knows with the latest versions it will likely meet that.

What I like about the IS is that it starts like a car, easily and therefore there is little strain on the clutch, a well tuned ULS with a good battery starts well too, but sometimes the carbs are not balanced well and sometimes the battery is too small/low and it kicks back....causing bad things on the clutch

I have both, (2015 year IS Sport and 2017 year ULS) they are both well maintained and have excellent batteries, therefore I have had no issues with either one...with 850 and 620 hr so far.....

You mention kickback. This is mostly a problem with the older ignition systems which fire a few degrees *before* TDC at cranking rpm. The newer "soft start" ones fire a few degrees *after* TDC to address this problem. I have the old system, but a "fix" was suggested to me in an iMRT class. The starter drives the flywheel via a sprag clutch, pushing on the flywheel. But if the flywheel is moving faster than the starter can spin it, the connection releases. In every other vehicle, we are trained to release the key or starter button the instant the engine fires. With the ULS however, hold it down "too long" until the engine is solidly running. It won't hurt the starter and greatly reduces kickbacks. In fact, I haven't had a single one since I began starting up this way!