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Thread: Need advice on engine roughness in my Model 4 912 UL

  1. #1

    Join Date
    Sep 2017
    Location
    Raleigh, NC
    Posts
    31

    Default Need advice on engine roughness in my Model 4 912 UL

    I need some advice. On my model 4 912UL, I've got 914 carbs that have been re-jetted for 912 and the throttle position sensor has been removed. For all intents and purposes it has been running fine for 200+ hours except for within a narrow rpm range of 2100-2700, especially when it's cold. When i'm warming up, i try to avoid that range. Once it's warmed up, it can hover in that area OK, but it's not as smooth as the rest of the RPM range.

    This annual i'm determined to lick this problem. Working with an A&P, we've tried all sorts of things. We've...


    • checked float weights
    • checked float levels by measuring the bronze fork thingy to verify it's around 10.2mm when carb upsidedown
    • i've replaced all the rubber
    • upgraded to the new 893 115 fuel pump (originally had 893 110)
    • added a 4-way splitter on the pressure line to add an oraphice (tried .035 and .040)
    • added a pressure gauge to see what the fuel pressure is
    • tried a pressure regulator and set pressure to ~5psi instead of spec 7+ of the new fuel pump


    The pressure regulator seems to have made the biggest difference. It runs fairly smooth (with mechanical sync only and no throttle cables attached yet) in the 2400 rpm range, which it has never done. My question is why? Why would Rotax spec the new fuel pump to put out 7.2 PSI when the Heavy Maintenance manual specifically states that the carbs expect no more than 5.8 PSI? Even with the return line and .035 oraphice, we're seeing pressure at the carbs of 6+ PSI. Our theory is that the carbs not holding back the fuel pressure from the pump, being forced to run rich, vibration/shaking ensues making the problem worse (especially on a model 4). LEAF tells me that they have an $85 treatment that they've had to do even on NEW carbs to make sure they can hold back the pressure of the new pumps. What the heck?

    I don't like the pressure regulator solution, but it seems to be my only solution at the moment. If i stick with the pressure regulator, would you put it before or after the 4-way splitter and return line, or add 2 regulators after the 4-way splitter between the splitter and each carb?

    Thanks in advance.

    [EDIT] OH, before you tell me to just live with the shaking in the "bad" RPM band... I just put in an order for a new exhaust from Kitfox because of a crack i found due to this shaking. We've also found other signs of damage due to shaking that I've replaced or repaired. So it's just not OK for this engine to shake at any RPM band.

    Brian Cass
    KF Model IV Speedster
    Rotax 912UL

  2. #2

    Join Date
    Jan 2018
    Location
    Vernon, BC
    Posts
    59

    Default Re: Need advice on engine roughness in my Model 4 912 UL

    you said you replaced all the rubber, but just checking, did you swap the o-ring on the enricher plunger? a friend found that was a common problem when his customers couldn't get rid of a new running rough situation ...
    did your motor mount get any cracks?

    regards

  3. #3

    Join Date
    Sep 2017
    Location
    Raleigh, NC
    Posts
    31

    Default Re: Need advice on engine roughness in my Model 4 912 UL

    My engine uses the primer kit that was a sold a long time ago. Enrichening circuit is safety wired closed.
    Brian Cass
    KF Model IV Speedster
    Rotax 912UL

  4. #4
    Senior Member Dusty's Avatar
    Join Date
    Dec 2012
    Location
    Christchurch, New Zealand
    Posts
    378

    Default Re: Need advice on engine roughness in my Model 4 912 UL

    If your engine has more than 400 hrs (and a heavy prop) I would get your gearbox checked.
    The ul isn't as prone to rough idle as the uls but Hal Stockman at zipper big bore ? Sells a larger balance tube, the 4 uses a longer tube than stock so may not be as efficient.
    cheers
    Dusty

  5. #5
    Super Moderator Av8r3400's Avatar
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    Jan 2009
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    Merrill, WI
    Posts
    3,044

    Default Re: Need advice on engine roughness in my Model 4 912 UL

    Quote Originally Posted by skyguy04 View Post
    My engine uses the primer kit that was a sold a long time ago...
    Just because the enricher circuit is wired closed does not mean the o-ring is not dry-rotted or otherwise damaged causing a problem. It's probably worth disassembling them to be sure.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  6. #6

    Join Date
    Sep 2017
    Location
    Raleigh, NC
    Posts
    31

    Default Re: Need advice on engine roughness in my Model 4 912 UL

    Ok, I highly doubt that’s the case because they were just rebuilt 2-3 years ago by Lockwood, but I will definitely pull them apart and take a look.



    Sent from my iPhone using Tapatalk
    Brian Cass
    KF Model IV Speedster
    Rotax 912UL

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