Hard starting Rotax 912 ULS

Need more info.

It may not be just one thing. Several things may contribute here. Cold outside temps are not helping your cause. Do you have a big heavy prop like a long Warp Drive or a lighter composite prop? First you found the plugs that were too wide a gap. I don't know how old they are, but you may want to replace them. Then it could be the carbs contributing to the issue. Have they been recently synced and last time they were overhauled? Do you have the choke setup or the primer setup? It can be your starting technique. If you have a fairly low idle rpm you'll need full choke and crack the throttle. If you have a high idle rpm then no throttle with the choke. When you turn the key does the engine continue to turn over or does the prop stop on its own?

The carbs were rebuilt at Lockwood 4 years ago. At that time they had to replace the floats. That's the last time my A&P synced them. Going forward I'm going to have him do it every annual. It's a Sensenich prop. I usually use choke when I start it throttle all the way out. I change the plugs at every annual which would have been last May. I plan to put new plugs in now regardless of what else has to be done. The prop continues to turn but when it tries to start it sometimes stops.

Here’s some simple things to consider. How old is the fuel being used? Auto gas degrades and should be dumped if it’s several months old because it looses its octane making for difficult starts and rough running. Cold weather starts are more problematic even at 50 degrees without engine preheating. Check that the battery is fully charged, even a few tenths of a volt drop from full charge is a big difference especially in cold weather.

Hi Charles,

You need to remove the carb bowls,

Weigh the floats as soon as you remove them from the carb bowls and dry them off. Don't let them sit around for an hour before you weigh them or the fuel will evaporate and the weight reading will be off. At four years old they can be the issue. They had a couple of years trying to get the float chemistry correct so the floats wouldn't absorb fuel. If your floats are too heavy they are flooding the carb which makes it hard to start and usually has rough idling because of the flooding.

Carbs definitely need to be synced. After that much time they are definitely out of sync. They should be done at least every annual and one thing to always check if you are having issues to at least rule them in or out of the problem. This could be your issue. Once warm what is the idle rpm? If it's low down around 1650 - 1800 then crack the throttle on start. If it's higher around 2K rpm at idle then leave the throttle closed. Drop the carb bowls and unscrew the idle jet right next to the main jet. It takes just a straight screwdriver. My bet is the jets are restricted or clogged. Use a a single strand of wire from and 18 -20 gauge wire to rod it out. It's a very tiny hole. Then flush the jet in both directions with carb cleaner followed by some high pressure air. DO NOT use a drill bit or welding torch tip cleaner. 

Doing these three things may easily fix your issue. Even if they don't these need to be done so they don't contribute to the issue and you can rule these things in or out..

Thanks Rodger. It usually idles at 2000 thanks to some creep in my throttle. I verified that they were serviced 4 years ago. I'll have a look at the floats but I suspect I may just send them out to be rebuilt. I doubt my A&P will be happy to hear we should check the sync every annual. I'm also going to replace the plugs and fuel filters. I usually hump mogas out to the field but based on some other posts I think I'm going to stick with a 50/50 mogas/avgas which will make my A&P happy since he always tells me to do that. I'll report back once I do some more testing.