Use of 100LL fuel in 914

Rotax approves the use of 100LL in the 914. More frequent oil changes are required. Most technicians recommend using a lead scavenger such as DECALIN to minimize lead depositing. Rotax has approved using Decalin, although they do not specifically endorsed it.

Stick & Rudder flight school in Boise has been running 914's exclusively on 100LL for many years.

You can use 100LL, but you're far better off in the long run with auto fuel. Nothing low lead about 100LL. It has far more lead in it to raise the octane to 100 than our old time cars ever did. It's not about the short term it's the long term leading you need to look after. 

Aviation (AV Gas) fuel of 100LL : Pro: Starts fast, found at all airports (manned airports anyway) never has an octane issue. Airports have very efficient fuel filtration for 100LL. Con: More expensive (although fuel is the cheapest running cost on the Rotax), Lead reduces the time between oil changes to 25 hours, and gray nasty soot gets on the fuselage and in the engine. You have to add an additive to prevent lead from plugging everything up on the inside. On about every 150 to 200 hours you have to clean the lead out of the oil can. Gearbox oil contamination reduces the gearbox overhaul time a bit. Valve seats can get sticky without the TCP additive, so add it religiously.

Selection of suitable operating fluids for ROTAX® Engine Type 912 i, 915 i, 912 and 914 (Series) ATA System: 12-10-00 Operating fluids.SI-914-019 explains your problem.

Excessive engine vibration, particularly at low idle speeds, can impair the carburetor fuel metering system leading to a too rich mixture condition. This rich mixture condition can further lead to rough engine operation and excessive carbon and lead deposits if using 100LL fuel.

so avoid avgas 100LL, use Mogas 912 S/ULS - 914 F/UL Min. RON 95

You might check into swift fuel.