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I'm trying to confirm that I'm doing the carb sync correctly and have some basic questions:

Question 1:  The Rotax videos (part 2 and 4) on the mech carb sync and idle set respectively stated that the next step after adjusting the throttle lever idle stop screws clockwise by 1 1/2 turns was to then adjust the throttle quadrant stop in the cockpit so it hits its stop at the same time the carb throttle levers hit their stops.  I have a McFarland twist-to-lock throttle cable in my Kitfox that doesn't appear to have any external adjustable stop (unless I'm overlooking some hidden feature).  However, it does only pull out so far and reaches some sort of internal stop for min idle.  If I use that cable min idle position as the “stop” and then lock the cables to the lever ends at that position won’t the min idle be set high at that point in the process?  If I then try to reset the idle lower by adjusting the lever stops on the carbs won’t the internal throttle cable “stop” prevent any lowering of the idle back down to the desired level?  What’s the correct procedure to overcome this dilemma or am I missing something?

Question 2:  During the pneumatic sync phase of the process the Rotax video (part 3) and the maintenance manual both state that the idle sync should be accomplished at 2500 rpm and checked at higher speeds as well.  In some older Rotax forum discussions it was suggested to do the idle sync at 3000 rpm or higher (4000 rpm).  What is the latest thinking on the idle rpm level?  

Question 3:  I’m using a CarbMate to accomplish the sync and hooking it up per option 2 in the maintenance manual.  The CarbMate has LEDs on both sides of the zero point but how do I tell which carb needs to be adjusted up or down at its respective Bowden cable?  Is this dependent on which connection tube I hook up to which side of the manifold cross over tube?  If so, is there a right and a wrong orientation to accomplish that hook up?

  • Re: ULS Carburetor sync

    by » one month ago


    1) The "min ide" at the carbs themselves can go very low, possibly less then the 1400 min (which could cause your engine to stop).  The 1.5 turns should put you comfortably above that "absolute" minimum, which is probably close where you want your idle to be (if you routinely idle for long at less than 1800rpm, you are stressing your gearbox).  You then want your cockpit idle "stop" to basically match that position. In essence, you want to hit that stop before you hit the ones on the carb, as those tabs can bend or break, depending on your config.  Once that's all set, you can use the screws at the carb to fine tune your idle, but that should be much less than 1.5 turns, obviously.

    2) The latest thinking is that 2500 is rather low.  I would do the p.sync. closer to 3000.  Roger has written about that extensively in some other threads.

    3) You can adjust either one, till they match, but if you adjust the one down it will tend lower your idle.  If you adjust the other up, it will tend to raise your idle.  That's why when you are (almost) done, you need to re-check your idle speed.


  • Re: ULS Carburetor sync

    by » one month ago


    Thanks for the clarification.  Q4: So, if my idle setting is too high after a p-sync and I don't have an adjustable idle stop on my throttle cable then am I forced to back off both bowden cable clamps by equal amounts to get the idle down and not put any stress on the carb idle stops?

    Q5:  I attempted to do the p-sync today with the throttle level set at 3000 rpm (set with the throttle cables' friction lock) and  the left carb shook so bad with the CarbMate hooked up that it apparently overcame the spring force on the float valve needle that fuel vented again.  When I hooked the manifold tube back up the shaking stops, as does the fuel venting but the engine still doesn't seem to run as smooth as I've heard most do at idle.  Both carbs tend to vibrate more than it seems that they should, although I'm not exactly sure how much vibration is normal and acceptable.  I got the most progress in getting the engine to run smoother by lowering the float levels and I've installed the level 3 carb overhaul kit, cleaned the carbs multiple times, and redone the mech-sync again and again with no further improvement.  How smooth should I expect the engine to run at idle before I can accomplish a good p-sync?  Should it be rock steady typically or something less?


  • Re: ULS Carburetor sync

    by » one month ago


    You can usually fine tune the idle using the stop screws.  A little pressure is going to be OK.  That's why the screws/stops exist.  If you have to actually move the cable clamps, you are basically starting over from the beginning.

    The fact that the engine shakes badly when the compensating tube is disconnected is an indication that the carbs are grossly out of sync with each other even after your mechanical sync, which they should not be.

    When you say the carbs are vibrating, of course it's not the carbs themselves, but the whole engine.  I have never ever had shaking like you describe, or fuel coming out of the vent lines.  I think it's an indication of a larger problem.  I have a suspicion that when you overhauled the carbs, something did not get put back together exactly right.  It's very easy to miss some tiny little things.  If I were you (which I'm not), I would send both carbs out for an independent inspection/repair, and then try doing the whole sync process once they come back and you know that the carbs are good-to-go.  Right now, they're your biggest question mark.

    The guy that gave me my PPL checkride told me a story of how he had an engine out with his Rotax, right after he had attempted to rebuild his carbs himself.  Fortunately he was able to make a successful emergency landing.  I forget the details of what he had done  wrong, but he told me, "That's the LAST time I'm messing with carbs on my own!"


    Thank you said by: Larry Olson

  • Re: ULS Carburetor sync

    by » one month ago


    When I overhauled the carbs I didn't touch the main jet needle and clip because I assumed that it had nothing to do with the idling of the engine and the clip must have still been in the slot as installed by Rotax when the engine was new (no one ever moved it since).  Is it possible that it could impact the engine at the lower rpm range that I'm running at and is the clip position critical to this whole discussion?  I don't remember now which slot the clip was in when I replaced the rubber diaphragm.

    If this main needle and clip isn't associated with the idling issue I'm now seeing I will no doubt take the carbs into my nearest overhaul facility and have them look at it (LEAF is within 85 miles away).


  • Re: ULS Carburetor sync

    by » 6 weeks ago


    I agree. I would not expect needle clip position to have much affect on idle.  The throttle butterfly is basically closed at idle, and fuel should not really be coming through the needle jet.  

    But what about the starting carbs (aka the choke)?  What position are they in when you start, and then when you're just idling?  Are they in the same position on both carbs?  Did you take them apart (replace the gaskets, etc)?  Are you sure they went back together properly?  


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