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Surging RPM Rotax 503dcdi

in 2 Stroke Technical Questions 11 years ago
Hi guys I wonder if any of you guys would like to comment on a problem I had when flying my trike a few days ago,

The 503dcdi fitted to my trike was brand new about 12 months ago one of the last available here in the UK,

When I first fitted it I was not happy with the EGTs running at 650c 1200f at cruise 5000rpm and 625c 1160f WOT with all the factory std jetting and needle positions suitable for use in my area...the agent I bought it off said to me it should be fine at those settings and to leave it at these std Rotax settings,

But I was still not happy running at max EGT...so I raised the needles one notch and this had the designed effect of getting the EGT down to 625c max 1160f at cruise 5000rpm,

I have ran it like this for the last 12 months totally happy with it...but around 2 months ago I thought the fuel burn was higher than normal so I dropped the needles one slot back to factory spec and test flew it...still hitting 650c or a tad more as it was when I first fitted the engine...so I lifted them back again,

A few days ago I had a flight in my trike on a very hot day over here for us anyway...it was 20c 70f

And had a prob I have not had before...the RPM would not hold steady at cruise power...it was ok at WOT and lower RPMs but the mid range it would surge from my normal 5000rpm to over 6000rpm,

I would close the throttle a bit to get back to 5000rpm only to have it drop to around 4700rpm, I would re adjust again to 5000rpm...only to have it back at 6000+ a short time later,

As I say it was an unusually hot day over here and the EGTs where quite low at around 560c 1040f...I suspect a rich mixture problem,

I have been reading this very interesting artical,

http://www.challengers101.com/ExhaustGasTemp.html

And was wondering what you guys think of the idea from the artical of giving my prop more pitch,

This will take my RPM down from the 6250 static 6500rpm climb out I am getting know which are in line with Rotaxes recommendations but I maybe able to get my needle back to std Rotax setting with more pitch if it works out...but I dont want to mess about in this area if its not a good idea,

Any ideas will be most welcome.

Nick.

  • Re: Surging RPM Rotax 503dcdi

    by » 11 years ago

    Nick,

    I would leave the pitch alone if you are climbing at 6500 with 6250 static.

    My experience has been that high or low needle settings will give you rich or lean mixtures but do not cause surging. It sounds more like something is out of kilter with your needle, needle jet, or maybe even the throttle cables or quadrant. Maybe one carb slide sticking or a cable not moving properly.

    If your present pitch and settings worked before,they should work now. Something besides settings would seem to be the problem. Sometimes that tiny O ring on the needle can split and make the needle sit at an angle. Check the enrichener circuit and make sure it is opening, closing, and sealing properly. A bad viton tip on the float needle can change the mix intermittently.

    My first thought still would be to check the needle, slides, and cables for free movement and sync.

    Bill.
      
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  • Re: Surging RPM Rotax 503dcdi

    by » 11 years ago

    Hi Bill thanks for your ideas...yes I think your right check the part of the carbs I was working on,

    The only reason I was thinking of giving the prop more pitch is because of the ideas the guy in the link has re over hot EGTs...and with me already being at a richer setting on my needle due my high EGT and the day being very hot...I thought the engine may not be able to cope if the mixture was just to rick...going to look at the plug colours as well,

    He goes on to say more pitch even if the max rec RPM is not adeared to...thinking about it my CHTs do seem low at 150c in cruise,

    Anyway before I think of messing with the prop I will have a good look at the carb.

    Nick.
      
    Report

  • Re: Surging RPM Rotax 503dcdi

    by » 11 years ago

    Hello again Bill....not had a chance to go over my carbs yet...going to give the air-filter a good clean as well as I have notice its moist with 2 stroke oil,

    Just trying to get my head around the theory in Brian Carpenters video,

    "Is your 2 stroke about to quit"

    http://www.eaavideo.org/video.aspx?v=626943072001

    I wonder if you have ever watched it Bill...just cant get my head around how lowering the RPM from the Rotax recommended 6500rpm to maybe 6000rpm with a pitch change...I would have thought the engine would be just running inefficiently at that RPM,

    This guy also goes on about the subject,

    www.challengers101.com/ExhaustGasTemp.htm

    Love to hear other's views on this subject.

    Nick.
      
    Report

  • Re: Surging RPM Rotax 503dcdi

    by » 11 years ago

    Nick,

    Increasing the pitch to lower the max RPM to 6000 will be putting your prop load very close to your torque curve. It Will also not allow you to reach the max. HP your engine was designed to deliver. The video was right in saying that increased prop load will lower EGTs, but it also limits HP production and increases CHT.

    Set the RPM to Rotax maximum allowable recommendations with prop pitch.

    Lower the EGTs with jet and needle adjustments. Main jet for WOT, needle jet and jet needle for midrange, and idle jet for idle.

    Lowering the EGTS with larger jets or needle adjustments will make the idle somewhat richer but that is a good thing. You need that extra richness for acceleration.

    Bill.
      
    Report

  • Re: Surging RPM Rotax 503dcdi

    by » 11 years ago

    Thanks Bill your words are reassuring...I remember when I watched that vid not being able to get my head around the fact that you would be taking the engine out of its power band,

    I think it was just that I also have lowish CHTs(150c) that made it look attractive to me.


    Nick.
      
    Report
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