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By Sooty25
just going to throw this out there in case anyone has the answer.

Rotax 582 Bluetop, 220hrs (ish)

Lovely and smooth upto 4,000 rpm

6,000rpm full throttle again nice and smooth.

Rough and lumpy, 4250 - 5750 range

Rear cylinder spark plugs nice "biscuit" brown
Front cylinder, both plugs black.
Front cylinder CHT and EGT slightly cooler, both in flight and ground running.

Float bowls are clean
floats all okay
fuel reasonably fresh, tends to fly every 1-2 weeks.
No real signs of wear on needle
Carb rubbers good
raised clip on front needle to top to try and lean front cylinder

Any thoughts chaps?
By cockney steve
Outwith my area of"expertise", but I'm a lover of two-strokes. ..Crankcase seals, induction-manifold gaskets (and porosity in the manifold is a possibility)

Induction disc-valve (which, AIUI , is the Rotax method) cracked/warped/worn/ displaced. reed-valves can crack and they would tend to have a resonant frequency. (lots of info about them on u-tube (see pulse-jets)and various other pulse-jet articles :)

Carburettor float-needle/seat leaking/worn allowing a high fuel level or a crankcase accumulation.
Carb. diaphragm, again, a bit of an outsider, but could it be pulling away from the piston/body and causing an enrichment.Main Needle may not look worn,but the profile is not usually a straight the case of the late SU's, the needle was spring- biased to deliberately press against the side of the jet-orifice.(to stop the needle fluttering and get a more consistent mixture) yes, that worked really well! needle sawed the jet oval whilst whittling itself away ...killed the idle and mid-range mixtures....set a decent tickover and get a huge flat spot trying to accelerate... set tickover so rich that choke was redundant and plugs sooted and you could see the fuel gauge dropping :eye: (but the top-end was weak-enough to perform)..finally, the ignition-system.....electronic modules can be swapped ,see if the problem moves with the module.....points system, cam secure, baseplate secure,gap accurate and consistent -a worn bearing can allow points to flutter wildly between oversize gap and no gap.

Having said all that, I'd put money on it being a mixture problem. I'd expect a module/plugs to break down consistently, not at one particular rev. range. hth.

PS be very careful, running big throttle openings/large loads, with a deliberately weakened mix like is a sure way to melt a piston-crown or the plug electrodes become incandescent and you then have a glowplug engine in runaway-mode...only method of stopping it is starve fuel ,or, preferably, air...if it doesn't throw a rod first!
By Aerials
Sooty, has that fault just appeared or has it been developing over time?
I suggest putting the needle clip back to where it was and start again from there. Always try one thing at a time as it's rarely a combination of things conspiring together to give you a fault.

Fault-finding at a distance is always problematic but have a look and feel to ensure the 'choke' cable has a little slack in it when 'off' on the carburettor of the affected cylinder and that it isn't stuck on. Like C Steve, I also suspect fuel mixture over anything else.

You could try and take the slide assemblies and choke pistons out, retaining them on their control cables, then swap the bodies between cylinders to see if the fault moves with them.

Here is a very good Rotax 2-stroke fault finding guide: and here are the other articles:

Please let us know how you get on............
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By Sooty25
Steve, I was kinda fishing for someone to come back with "I've had that!" rather than a general lesson in 2 strokes.

582 has no points and dual ignition, same fault, same cylinder, both modules?
Air leaks causing rich plug colour and lower temps?

@Aerials thanks for the links, there is some clues in there. Chokes are operating correctly.

It ran fine last time it was flown 2 weeks ago.

Carbs are currently off for a strip and clean. We know its the mid range circuit and mixture. When ground running at 4500rpm we closed off fuel and as it leaned, it ran smooth. Hence lifting the needle clip to lean that cylinder a touch.

will post more after reassembly and re-test.
By cockney steve
@Sooty25 I appreciate I wittered on, a bit, but many Forumites are not as well-up on the finer points of I.C. engine operating principles...just trying to broaden the reference-base for others, as well as try to maybe prompt a different line of investigation.....a blocked transition /intermediate-jet (if fitted) is another possibility, given the narrow rev-range it happens, as could vibration at that frequency-range cause flooding if the float-needle/seat to pass. have had a float wear through the pivot on one side...float would intermittently catch on float-chamber, tilt-over and the carb emptied, the float would drop, allow fuel in and self-right. -or it would flood and revving the taters off it would free the float. (vibration) A globule of water will block a small jet! it will not "suck" through! it can take days for it to evaporate.
clean brass jets by boiling in vinegar for 10 minutes....far better than poking gas-welding tip-cleaners in them. alloy bodies respond to Cellulose thinners (standard) or gunwash..both are predominantly acetone. use with a small paintbrush with bristles trimmed to about half-length and a toothbrush (some plastics will "melt" in the thinners.)