912ul woes

[egp8111]

Anyone have any thoughts on this: The engine starts fine and runs smooth for about one minute then it develops a miss and runs rough from idle until about 3000 rpm. Above 3000 it smooths out and developes good power . Runs fine in cruise until you pull it back for landing when it starts running rough again. I've had the carbs rebuilt, the gearbox rebuilt and have syn'ed them to death but I still can't get it to run smooth below 3000rpm. I've changed the carb boots and checked for induction leaks. I'm running out of ideas
Thanks
EG

[kitfox5v]

EG
I believe you have an ignition problem. Check the coil connections and ground straps. I live fairly close and could fly up and give you a hand if you can't find it.
Eddie

[Av8r3400]

I've had this problem as well. This is, IMO, 90% carburetor related. These ignitions will either work or not, like a lightbulb.

Who rebuilt your carbs? I have seen carbs done by LEAF, Lockwood and CPS that were not done right/well. One of the first things to check is that your enrichers are both operating smoothly and completely opening when released. (This was my problem, I believe)

How long ago were the carbs done? Do you have the updated carb floats or still the "old" ones? Heavy floats can cause rough running. The needle valves also can be incorrect. There are different color neoprene tips. (I'm sorry I don't remember the correct color)

Because of the design of the early Kitfoxes and reversing the intake manifolds, this makes this installation tricky at times to get right. The carbs need to be setup just right, then they work like a dream.

[egp8111]

Roger Lee in Az rebuilt the carbs. He is one of the best. New floats ,diaphragms,needle valves etc. I've been through them as well. Nothing seems to make a change. Is it trying to tell me something by running smooth for a minute or so before it starts missing. Is it becoming too rich as it warms up? I've been through the enricher circuits numerous times and can find nothing wrong but it still seems somehow it rich. The rpm drop is the same on both ignition boxes , wouldn't you expect a difference if one of the ignitions systems had a problem? Still I'm running out of ideas on the carb.
Thanks
EG

[avidflyer]

Have you tried putting on the chokes/enrichers to see if the miss gets better or worse? If engine speeds up, it's to lean, if it gets worse, it's to rich. Jim Chuk

[HighWing]

Years ago I rebuilt my carburetors with new everything. Problems similar to yours developed, but mostly when going from run to idle - landing. Wisdom then suggested it might be a float bowl problem and went back to the old float needles and it was like magic. Used the same needles for 900 hrs. Like was mentioned, Rotax makes three versions of the float needles.

[DesertFox4]

I had my carbs recently sonically cleaned and rebuilt. Installed them and one carb would overboard fuel and ran so rough we could not sync the carbs. Turned out the low speed curcuit still had debris blocking a tiny passage. Took carb to the bench, disasembled it and cleared with carb cleaner and compressed air to dislodge tiny spec of something. Reinstalled and runs as new.

Ignition problems are much rarer than carb problems on the Rotax four strokes.

[WWhunter]

Curious as to what you ended up finding? Did you get the issue resolved, and if so, what was the problem?

[bogmonster]

Mine was just like that. Turned out to be my wiring to the ignition modules. Wire had some breaks in it, so it ran well at idle and must have vibrated enough at high speed to work well. From 3-4k it was rough as could be. Finally showed itself on run up when testing the L/R ignition.

Anyone have any thoughts on this: The engine starts fine and runs smooth for about one minute then it develops a miss and runs rough from idle until about 3000 rpm. Above 3000 it smooths out and developes good power . Runs fine in cruise until you pull it back for landing when it starts running rough again. I've had the carbs rebuilt, the gearbox rebuilt and have syn'ed them to death but I still can't get it to run smooth below 3000rpm. I've changed the carb boots and checked for induction leaks. I'm running out of ideas
Thanks
EG

[egp8111]

No answeres yet. Doing some test this week suggested by Lockwood. I'll keep thread posted of the results.
What's the best way to check for wiring faults?
Thanks
EG