fbpx

 

Roger,
I have recently removed the airbox with the carb warming bypass and substituted K&N cone air filters in place of the airbox airfilter installation on my 912 uls. the airbox intake was routed to the front of the cowling. I now experience significantly higher oil and cylinder head temps.(as much as 15 deg C) higher. I have also noticed that the minimum idle speed has increased about 500 rpm.. IE the lowest idle speed I can achieve is around 1900rpm. I then have added air vents to the top of the cowling just in front of the carb air filters to facilitate cooler air from over the top of the cowling directly to the carbs, .. these vents showed no reduction in the temps. It seems to me unlikely that the airflow supply via the inside of the cowling as opposed to the old airbox supply set up is the cause of the increased temps-- although I may be wrong.. Any Ideas Roger ? what have I missed?
  • Re: removal of airbox cause significant oiltemp inc

    by » 11 years ago


    Hi Dave,

    Was the airbox you took off a factory Rotax airbox? If it was it was setup just for the Rotax. The addition of the K&N's in a hot cowled environment has changed the air flow and temp. Where did you route the carb vent lines? If those are in the wrong spot it will affect the carbs. What are the actual oil and CHT temps? When you took off the airbox with the carb heat bypass you are now getting the effects of carb heat all the time. When you back off the carb throttle stop screw will the carbs close all the way? Sounds like you need to do a mechanical sync and a pneumatic sync again. Out of curiosity what is your WOT flat and level in flight rpm? When you make changes to one thing at times you may need to adjust another.

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


  • Re: removal of airbox cause significant oiltemp inc

    by » 11 years ago


    Roger, once again thanks for your interest. to answer your questions:
    1. yes - rotax manufactured air box
    2. new cowling vents only were added to the canopy covering the engine - just forward and above the new cone airfilters
    3. WOT is 5700rpm
    4. static full throttle 5500
    5. with the idle stops screwed all the way out- throttle arms all the way back- idle rpm is 1800
    6. actual temps are as follows in deg C: ambient 25: oil 115-120: water 80-90: 500'ASL: 5250 RPM:
    map 26: TAS 98kts: level flight time 20 min. 300' climb produced marked temp increase.
    7. Mech sync done .. air mix checked needle pos as per factory setting from new (actual unknown)
    8. engine runs very smoothly from idle to max rpm.. no tell tale signs of a carb mis match.

    PS . I have reinstalled the airbox and will flight test 01/09.. and will report back.

  • Re: removal of airbox cause significant oiltemp inc

    by » 11 years ago


    Hi Dave,

    Let's see, did you do a pneumatic sync on the carbs? I see you did a mechanical. The mechanical gets you in the ballpark for idle only and the pneumatic is very important.
    This almost sounds like the vent tubes are being influenced at their new location. How and where are the vent tubes located? Air flow around the air filters can make a difference, but maybe not as big a one than you report. We may be dealing with more than one issue and they are combining to cause this. Air flow, vent tube location, carb sync?

    I converted you temps. OAT = 72F, oil = 248F, Coolant = 194F. What is the CHT and EGT? The CHT should be in the general area to the coolant temp. The hot oil doesn't coincide with the lower coolant temp. Let me know what the CHT and EGT temps are.

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


  • Re: removal of airbox cause significant oiltemp inc

    by » 11 years ago


    Roger.. thanks The coolant temp I report is in fact CHT. I will report back with the exact EGT's.. however from memory they are low.. less than 600 on the dial.( deg F I think.) rotax dial.

    the Pneumatic sync has not been done , however visuals of the arm movements show the open throttle positions of both carbs being similar .. If they are out at all it will be by a very small amount.. given the visual on the arm positions and running smoothness up to full power.

    I think new senders through out are called for ..?

  • Re: removal of airbox cause significant oiltemp inc

    by » 11 years ago


    Hi Dave,

    There is no way to tell how far off or in sync the carbs are bt looking at them. You absolutely must pneumatically sync them. They could be off anywhere from 2"-6" of vacuum. This alonne can cause temp differences, but worse yet it will beat you engine to death. The balance tube can only do so much and it isn't a cure all for out of sync carbs. The right side of the engine may be trying to run at 5000 rpm while the left side is trying to run at 5100 rpm and are opposing each other.

    Everyone should pneumatically sync the carbs every 100 hrs. and or annual. They will always be off.
    Un-synced carbs are hard on an engine and makes it vibrate more too. put the Rotax airbox back on and sync the carbs and your problems may disappear.

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


You do not have permissions to reply to this topic.