Rotax Rick

"CAUTION "  A rich mixture is safe !!!!!  You can not trust a EGT gauge in a RAVE Valve engine.  The only jetting change from a 582 to a 670 is a 15K2 needle set on the second clip from the top & a 180 Main jet.   As for plug colors Chocolate  Brown is NOT LIGHT TAN. That's far to  lean.  Remember the oil is in with the gas . 
Never think you can run a 670 below a 175 main jet.  Rarely on the low end some engines ( 5% ) need the 15E5U needle that leans the low end. 
I want to add one more thing.  Rotax makes 3 different angle Y pipes . ( Manifolds )   Straight, slight angled & a steep angle that has s a cut flange to slide over 1 bolt in each cylinder.  That gives you several options on installing your exhaust.  Keep in mind the 583, 617, 618, 643 & 670 all have interchangeable Y pipes.

I never ever promised to any of my customers that my custom 618/ 670  exhaust would fit in a Kitfox without some mod to either the exhaust or cowlings. 
There is 6 different Kitfox models & guess what, the exhaust needed for each model is different.  I sell  4 different styles of 670 exhaust systems. 
A side mount, Rear mount, ( Gyro ) a straight & a Kitfox.  Every 618 that was installed in a Kitfox had a re welded exhaust.  Rotax never made a Rear mount style exhaust for the 618 .  Only a side mount exhaust.  Were working right now on modifying the customers 582 exhaust with hopes to see 82 + HP. 
I'm sorry if some misunderstood but I never promised a exhaust that would not require some changes.

I prefer either Amsoil Interceptor or Quicksilver PWC ( made by Mercury & is a semi synthetic ) at 40 to 1 ratio.  Going from 50 to 1 to 40 to 1 ratio is only 3.2 oz's of oil in 5 gallons of gas. 
Fill a 5 gallon bucket with gas & pour in 3.2 oz's of oil. Tell me that is going to build to much carbon. Burnt bearings will stop the engine & I never saw a single engine out of over 850 engines I rebuilt that quit because of to much carbon. These new oils do not make carbon build up a issue any longer. Also 40 to 1 will help your pistons rings & seals last longer.  Using leaning ratios of premix will make more power & less chance of a plug flowing but WE HAVE DUAL IGNITION FOR THIS !!!  Plus power is never better than reliability in a Aircraft engine !!!  
I have rebuild the Rotax Ultralight crankshafts since Jan 1 2011 & have not had a single lower rod bearing fail on any 582 crankshaft that I rebuilt. That's unreal since 3 out of every 5 582's that fail are caused by the lower rod bearing failing.  Thats on over 650 crankshafts to date.   As far as 100 LL AvGas it is the very best fuel you can run in your Rotax engine.  If I lived in Alaska & had a place 25 miles from anyone I would be running 100 LL AvGas & 32 to 1 ratio. It does build a deposit on the piston tops by that does not effect anything.
I know & understand people try to help with the knowledge  they have in there limited experiences .  But I know & see what works & what doesn't work. 

I use the ceramic top pistons .  They expand at a far slower rate than Non ceramic top pistons . The steel cylinder liner takes several minutes to expand. The colder the tempeture the  longer it takes.  I recommend 2 minutes at 2200 rpm while taxi to the run up area.  Then at 4,000 rpm for 2 minutes. Then increase to 4500 rpm's for 2 more minutes. Then to 5000 rpm for the last 1 minute of your run up.  Now do your mag check & your fuel pump test.   Do not return to a idle if the take off is delayed by a landing Aircraft. Once on the runway use full throttle until airborne. If your clear of obstacles back the rpm  down 250 rpm or more.
 ( 90% of engines that fail the damage that caused the failure  happened while running at full power
You have only .003 piston to cylinder clearance. If you scuff a piston ( and  maybe not even knew that  you did )   and your cruising along 10 hours latter at 5,200 rpm 
and  the engine shuts down without know warning at all !!!!  EVERY GAUGE WAS READING PERFECT !!!   Once you scuff a piston please change it.
 If not it will do it again 100% for sure. 
3 out of every 5 582's that fail is the lower rod bearing. The second largest 582 failure is cold seizures.   It's not the engine's fault !!   WARM THEM UP PLEASE !!!
I have never had a single 670 seize & that's out of 250 + 670's that I have built.  .005 to.006 piston to cylinder clearance is the reason.
Every 582 or 670  I rebuild get ceramic top pistons. 
 

The aftermarket coils for $150 is a deal.  You have dual ignition so even if one fails you do not have  a engine out.
Just DO NOT  cut the spark plug wires off & try to replace them. There bonded in with no holding screw in the center to replace the wires.  I recommend CRC Die Electric grease & coat them with that at each annual inspection. The wires will last for many years doing this.  If you want a used OEM Ducati coil look up on 
Ebay  SkiDoo 550 Ducati ignition.  This sled uses the same coil, trigger & flywheel as the dual ignition engines.