I made the mistake of installing an AFR meter on my 912UL, to try to understand the vibration issue I had at cruise.   Engine does not have an airbox, but KN-filters because of space issues.  The AFR meter shows a very lean condition at cruise afr 15.5

I tried rising the needles to richer and it made it better below maybe 5000 but still very lean running at cruise.   I then tested with larger needle jets and found that with 2.80 jet needle it is fine at cruise, but too rich further down in the register.  I am happy the main jet is correct as I get AFR 12.5 at WOT climb out.


With that I concluded the issue is the CV carbs don't work properly without an airbox.   I have btw had the same with motorbikes. Remove airbox and it never runs good with CV carbs.  Google is full of people discussing this issue.   


So I tried cutting down the slide springs and that helped.  I have done it in increments and today I was back at 2.74 needle jets (just one above stock) and needle in richest position. It still runs lean at cruise at 15.2+ AFR which is not acceptable, but it only takes a tine bit of throttle and it starts richening up.   I


It used to be lean all the way then apply full throttle and suddenly with a marked change AFR drops to a correct 12.5.    With the gradual cutting back on the spring the transition has gotten progressively smoothed out and now, with a full 4 spring windings removed the mixture drops to below 15 with only slightly more throttle.  Tmrw I am thinking take off another 1/2 coil.   Anyone who has tried this.   No comments please on put it back to stock. It is worth exploring

  • Re: 912UL CV carbs without airbox-what is the fix for the lean running ?

    by » one year ago

    Look at a few things 

    1float weight

    Type of fuel valve needle there are 3 2 work 1 doesn't silver gold and black blacks don't work 

    Vent carb vent into air filter out air flow

    No exhaust Leaks    as air gets on and causes lean readings 

    13 2  to 13.4 flat line makes best power  

    13.00 to 1 WOT  to prevent detonation 

    The idle screw alters the whole range 

  • Re: 912UL CV carbs without airbox-what is the fix for the lean running ?

    by » one year ago

    floats are new last r revision and level at 12-13 mm

    vent hose as it always was, under the float bowl clip

    No exhaust leak

    I have the power settings nailed, just don't like the AFR15.2 (today) at cruise. Think it is on the lean side.

    Idle screws are at 1.75 now, for best idle but will play with them again

    After cutting back the slide spring it now behaves better and I get more fuel when throttling up.- Feels responsive and powerful.   It is just that lean condition  at cruise I can't seem to get rid of.  Of course made worse by the cold.    A bigger needle jet 2.76 fixes that but then it gets rich around 3000 rpm.  Maybe I just need to accept that.  A different needle taper maybe, longer taper section, with same end diameter as now.   Good thing is much improved already in the way it reponds to throttle on final

  • Re: 912UL CV carbs without airbox-what is the fix for the lean running ?

    by » one year ago

    I am thinking I should try a different needle. But the needles I have are not marked.    Are they Rotax part, and not a Bing part ?   I would like to try a longer taper, to see if I can get slightly more fuel at cruise. It is fine under load, but at cruise nothing I have tried works.   I am still at AFR 15.5. What to do ?   15.5 is too lean and engine is not running well until I apply more throttle and climb. Then it drops to more reasonable numbers and runs fine.  

  • Re: 912UL CV carbs without airbox-what is the fix for the lean running ?

    by » one year ago

    Take the AFR out and it won't bother you anymore. LOL

    I've never had a plane in the shop or even seen one with an AFR. Unless you have some factory numbers to compare with I don't think I would make too many changes.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

  • Re: 912UL CV carbs without airbox-what is the fix for the lean running ?

    by » one year ago

    How dare you apply 21st century technology to a carburetor :-) 

    Other Bing carbs had jet needle numbers that were a mix of numbers and letters, which told you whether it was richer or leaner at the upper or lower ends.  In other words, it more or less described the taper.  I've never found anything for the CV carb that gave more than one option, and it doesn't conform to the number and letter system they used on other carbs.  The only number I ever found other than the Rotax part number "961-215" was "252", which may be "46-252".  The old needles I have in a bag downstairs certainly don't have any numbers, and I doubt they ever did.  Of course the Bing site is no help, because alarms go off and your IP is reported to their lawyers if you mention aircraft :-) 



You do not have permissions to reply to this topic.