Two choices. Watch another two episodes of The Wire off DVD tonight or think about why my 503 has been trying to tell me it needs help.

Lately it has begun to start on one cylinder sometimes, and once or twice very briefly run on one cylinder while in the air. I've had it 11 years and never seen this problem before. The miss-fire problem began after the latest 7 month lay-up in March this year. Done 20 flight hours since, with said occasional warnings.

Carb float-bowl always correctly full - it's part of my starting and shut-down ritual - no primer bulb on this aircraft.

The electrical and mechanical connections to the coils and plugs look good as far as I can see while the engine is still in the aircraft.

Trouble is I can't run it up on a test bed and make measurements. In the Minimax it's tightly cowled so even doing substitutions or swap-over of the coils is difficult.

Plugs were renewed half-past March (old plugs looked normal). Still the same. After that and throughout this season, each time I arrived at the field I pulled alternate plug caps off [using a dummy plug to save stress on the coil]. Engine started reasonably well on whichever remaining live cylinder every time. Put the plug cap and cowl back on, started and flew. Occasional miss-fires during taxying out. Starting for the return flight and taxying back not a problem.

But today the engine started but the problem didn't clear fully. I didn't fly.

I deliberately let the engine run at abt 2500 RPM for a while mostly on one cylinder, I wanted to know which cylinder was not firing. The front plug looked perfect but the rear plug looked grotty and dark - but serviceable if you know what I mean.

True, it's a points-ignition job and due for another ignition re-time / fan belt service anyway. It needs to come out. I could replace everything electrical but that's expensive and it's not easy to find trustworthy parts. But I don't want to have to take it out again and again if I haven't solved the problem.

How do I calculate what needs to be replaced? What is the most common failure mode?

Antoni,

Try replacing the stator plate. It's becoming a regular problem on our Rotax 2 strokes. The older the engine the more likely the stator coils/plate will begin to fail. If you don't have another you can substitute then at least try to replace the two sets of points and the condensers as well. I know the points shouldn't be a problem if they're kept clean and gapped properly, but the condensers can start to break down over time, and personally I'd replace them as a set.

Regarding the stator plate failures. I've read various accounts of difficult to pin down problems with the Rotax engines, and the bloke with the 582 powered Rans S6 at our field had to have his stator plate replaced very recently due to very odd problems with it. His problems were similar to yours, with an onset of difficult starting, misfires, less power, etc. In the end I happened to read somewhere of someone who had the same problems, and they all went away as soon as his stator was replace. So I told our Rans bloke about this and he approached the UK Rotax main dealers. They replaced the plate under warranty, as his engine was less than 18 months old! Now it's all good, and the engine's back to easy starting and full power.

Rotax's shake so much they Solder on the Stator Can start to come loose too....

I would definately be looking for a Stator plus points and condensors. condensors go bad over time, and don't cheap out on any of that stuff, it's your A$$ on the line.

Cheapo chinese condensors aint worth the paper box they're put in.