Bad idle and fouled plugs

by Trevor Doig in General Tech Discussion 12 years ago

My 200hr 80hp 912UL Rotax has started idling badly and I found the cylinders 1 and 3 spark plugs were quite black. Not wipeable, quite hard carbon. The plugs had done 55 hours. I replaced the plugs but the idle did not improve and after half an hour flying the plugs were quite black again. I had a a compression and leak-down test done and all is fine. The engine runs smoothly and performs well. My Rotax mechanic says my idle at 1500 is too low and I should increase it to 1800, but that won't fix the dirty plus problem. Any thoughts? Trever, NZ.

Re: Bad idle and fouled plugs

by bill martin » 12 years ago

bill martin

10 757 0

Hi Trevor,

Your mechanic is right. You should idle around 1800. I idle mine at 1900.

If the plugs are black on one side of the engine (1 and 3), it sounds like you are too rich on that side. First thing I would try is a pneumatic sync to balance the carbs. That will probably cure the rough idle. If the plugs are still black after that, it could be the enrichener circuit not closing all the way, a worn or dirty float needle and/or needle seat, float arms out of adjustment, or a leaky float.

Up the idle and do the sync first; That may be all you need.

Bill.

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Re: Bad idle and fouled plugs

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.52k

H trevor,

I agree with Bill,

Even though the compression on the UL is lower than the ULS and can handle the lower rpm a little better it is still too low and will cause the carbon build up and a little rougher running verses a smoother idling run at 1700-1800. If the carbon is not the dry velvet black and it is hard and baked on you may have another issue. The dry velvet carbon goes away at the higher cruise rpms. Then if left to idle too long, improper idle sync or the air mixture screw is set wrong it can show up again, but hard baked on carbon may possibly be something else entirely. You should do a carb sync just to rule that issue out, but that isn't high up on my list of possible issues. The hard long time baked hard carbon is more concerning. Which plugs on #1-#3 are black, all or different ones? It would be interesting to know if it is only one on each and if it might be on one ignition or coil? An air imbalance or a partial air flow blockage may cause this to excessive carbon. You may want to investigate a little more. While your at it double check the mixture screw on the bottom of the carb so it is 1.5 turns out. Try to get back to us with a little more info.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Bad idle and fouled plugs

by Trevor Doig » 12 years ago

Trevor Doig

4 5 0

Thanks guys. As you both suggested, I will get the pneumatic sync done next week and see if that shows up anything. I will also check on the plugs condition and post that on this forum. Thanks again. Trevor NZ

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Bad idle and fouled plugs

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