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Is this normal?

Front (flywheel end) cylinder of a fan-cooled Rotax 447 consistently 75F hotter than rear (gearbox) cylinder. This isn't an instrumentation problem as I can swap all components (sensors, wiring, gauges) and get identical results. I've even changed the horrible ring/crimped sensors everyone uses, for proper machined rings with thermocouple wires silver soldered into them (as described in the Rotax installers manual). (THAT was a surprise - new readings were 350/425, rather than the 325/375 I was getting with the old sensors. All of a sudden I wasn't comfortable with the higher reading.)

It's single carb (of course), and I've experimented with tilting the carb slightly, in case it's favouring one cylinder. Also raised the needle to maximum rich. The temperatures are highest at full throttle (6500), and stay the same until below 6000, when they drop to a more reasonable 300/350 in the cruise (~5300).

I've even swapped 447s! Exactly the same readings (same carb though).

Am now considering adding a diversionary baffle into the engine cooling cowl to direct some more air over the front cylinder. The only thing stopping me is why would I have to modify what the great Rotax designers deemed unnecessary?

Both plugs have identical good colours.

Any thoughts?

Dave
  • Re: 447 CHTs Different.

    by » 10 years ago


    Problem sorted (thanks Bob Hood and Nick Axworthy of the BMAA forum).

    Early 447s did not have a small baffle in the cooling cowl that diverts more air to the front cylinder. Without it, the rear cylinder gets more than its fair share of the forced air, and the front one is starved. Addition of a copy of the said baffle fixed the problem (more-or-less - the front cylinder now remains below 400F, but is still 25-50F warmer then the rear).

    Dave

  • Re: 447 CHTs Different.

    by » 10 years ago


    Hi Dave,
    Yes, the baffle addition does make a difference, but also check the belt tension to make sure the fan isnt slipping. Your temps are still a bit high.

    You might consider a slightly richer main jet. The additional fuel at full throttle will tend to cool the engine, and might allow you to lower (lean) the needle for good cruise temps. You didn't say anything about EGTs.

    There's not a lot you can do on a single carb engine to balance the temps. Remember, the Rotax 2 cyl engines are basically 2 engines running on the same crankshaft. Slight differences in the cycinders, compressions, timing, and crankcase areas make differences in operational conditions.

    Dave B.

  • Re: 447 CHTs Different.

    by » 10 years ago


    I've just replaced the belt and pulleys after a failure, and am pretty sure the tension is correct (it was showing the same temps before the old belt failed).

    I think the higher than optimum temps may be down to using a 'proper' thermocouple (copper ring with sensor silver soldered directly in), rather than the typical Westach crimped thin copper 'banjo' style everyone uses. I estimate there's a 25 degree loss on real temps when using those.

    I'll recheck the timing - last done on installation about 10 hours ago, after a many year layoff.

    No EGTs, unfortunately.

    I've already upped to a slightly richer main jet after swapping from the silencer filter to the conical K&N. Your idea is a good one though - it is only at full throttle that I'm getting the elevated temps. I'll try another +10. Might ultimately save me some expensive (UK) fuel...

    Best,
    Dave

  • Re: 447 CHTs Different.

    by » 9 years ago


    My 447 is showing same difference between the cylinders. Where can I get a drawing or something like that shows where to install the baffle ?.

    Thanks, rgds

    JM Maldague

  • Re: 447 CHTs Different.

    by » 9 years ago


    Hi Jean....I have the latter 503 top cowl with the modified air deflector....its in my loft so when I get a chance I will take some pictures of it for you...unless anybody else posts some of the 447 for you.

    Nick.

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